tag:blogger.com,1999:blog-29144932453442657272024-03-13T19:22:01.368-07:00Tube Amps with a TwistHeath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.comBlogger10125tag:blogger.com,1999:blog-2914493245344265727.post-2936278808420266852020-08-05T11:01:00.001-07:002020-08-05T11:01:34.391-07:00My 6384 SE ExperimentsIn this post I'd like to detail a series of experiments that led to the design of the 826 amp that I wrote about in my last post. I investigated several circuits using a 6384 output tube, with the intention of eventually subbing in the 826 after I figured out what worked the best. The 6384 is basically like a 6L6GC, but built to survive the apocalypse and with a different pin out and a little more sensitive screen grid. For all of these experiments, I ran the 6384 at ~70mA into a 5K output transformer at 395V. Clipping occurred at a little over 10W.<div><br /><div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiwMIbUbZcVVSehN2_1nTMKNZwxmOldXUNV-ksGzd5rNVpelF0J_rZGtiIf_JNhtO35tBDS0-MI0WCdbietxAg9yQ0yDnhOBylmF4eACJYRK40Zc8lGTY0GXbkEShzA9WUpd4w3l4aEolg/s2048/IMG_20200623_200759842.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiwMIbUbZcVVSehN2_1nTMKNZwxmOldXUNV-ksGzd5rNVpelF0J_rZGtiIf_JNhtO35tBDS0-MI0WCdbietxAg9yQ0yDnhOBylmF4eACJYRK40Zc8lGTY0GXbkEShzA9WUpd4w3l4aEolg/w512-h384/IMG_20200623_200759842.jpg" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">The Breadboard Test Amp</td></tr></tbody></table><br />Below is the first circuit I tested. The idea here was to drive plate-to-grid output tube shunt feedback with an ideal transconductance amplifier (very high impedance output). Taken to its extreme, this approach can give up to 100% local feedback to the output stage. In this implementation, the output tube grid bias resistor spoils the feedback scheme, resulting in only ~30% output tube local feedback. <br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghn-rzN0DTFuOblIbLbDnLS3JrijwKo5Dm_F8RSHDnSgn9CdYpRmdyrSOmNYoRB76RQC_eQRqVFClcwutU4Yg22ZCYUGrdZwxBjJva3AlsW6Iz0LqFyfGPzT2a7LEDwDQcAqxEDw1ngYg/s2048/Circuit+1.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghn-rzN0DTFuOblIbLbDnLS3JrijwKo5Dm_F8RSHDnSgn9CdYpRmdyrSOmNYoRB76RQC_eQRqVFClcwutU4Yg22ZCYUGrdZwxBjJva3AlsW6Iz0LqFyfGPzT2a7LEDwDQcAqxEDw1ngYg/w512-h384/Circuit+1.jpg" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">First Test Circuit</td></tr></tbody></table><br />Below is the resulting distortion spectrum at 1W into 8 Ohms. <br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQZCRkqRB_BSybHwEDDwJxM_OoqQZsO4Nnuf0d__Yt-oC1xjf-J7DMMQb58nc-1s_xR8CXJD63LzceycYoz5q5NsWGa6v-Slrfs9crMH2IQyEyK9WAZi9LJAvZyt0Uhu3bsTNhfqLPhvE/s1803/1W.png" style="margin-left: auto; margin-right: auto;"><img border="0" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjQZCRkqRB_BSybHwEDDwJxM_OoqQZsO4Nnuf0d__Yt-oC1xjf-J7DMMQb58nc-1s_xR8CXJD63LzceycYoz5q5NsWGa6v-Slrfs9crMH2IQyEyK9WAZi9LJAvZyt0Uhu3bsTNhfqLPhvE/w512-h261/1W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.70% Distortion @ 1W</td></tr></tbody></table><br /></div><div>Amplifier Zout was measured at 1.67 Ohms. This seems like a low-cost, simple SE amp with decent performance, but I wanted to see if I could fix the feedback current leak and get better results.</div><div><br /></div><div>The next step was to add a mosfet grid driver for the output tube, since my eventual tube choice was going to need one to operate properly. This allowed me to use a much higher-valued bias resistor since mosfets have very-low gate leakage. The 100k resistance that was pulling current from the feedback is now a 3M resistance, which is more than an order of magnitude larger than the feedback resistor. This gives ~94% local feedback if my calculations are correct.<br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxkWdOvV-_BhJuZr5ff7nKI33nqEyZowtj0shIHYqQLXKUGOKAzHFdH950XuQS6cD-nFSX1SfTEY-z2NRGuWSIg1HiCszE0F6BA0rKXYYI36Qmxvk_AjzryP0Q2_F4XSvULADPmRoigjI/s2048/Circuit+2.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxkWdOvV-_BhJuZr5ff7nKI33nqEyZowtj0shIHYqQLXKUGOKAzHFdH950XuQS6cD-nFSX1SfTEY-z2NRGuWSIg1HiCszE0F6BA0rKXYYI36Qmxvk_AjzryP0Q2_F4XSvULADPmRoigjI/w512-h384/Circuit+2.jpg" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Second Test Circuit</td></tr></tbody></table><br />Below is the resulting distortion spectrum at 1W into 8 Ohms. <br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgHUayk3uQBeKWESfQmR9nbJdnWPhX_VIflWK-bb7tFadWuHZY4m2GSIrWox4CFB58I3FGB4zxEDNDZQrycVOz2z_BDGtRtuohhY7k72MvLm7NJLk92hnDIEuQxEiK3Jf7XQUMaw5a8sLM/s1805/1W.png" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgHUayk3uQBeKWESfQmR9nbJdnWPhX_VIflWK-bb7tFadWuHZY4m2GSIrWox4CFB58I3FGB4zxEDNDZQrycVOz2z_BDGtRtuohhY7k72MvLm7NJLk92hnDIEuQxEiK3Jf7XQUMaw5a8sLM/w512-h261/1W.png" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.25% Distortion @ 1W</td></tr></tbody></table><br />Amplifier Zout was measured at 1.1 Ohms. Things definitely got better.</div><div><br /></div><div>At this point, I had results that represented something close to the best one could get with a feedback loop that only involved the output stage. I wanted to do better. I wanted 0.1% distortion at 1W or better. I wanted less than 1 Ohm Zout. I knew that I'd have a harder time once I subbed the 826 since it has less transconductance than the 6384. I therefore abandoned the idea of output tube only feedback and decided to involve the driver stage as well in the feedback loop. </div><div><br /></div><div>I decided to try feedback from the output tube plate to driver cathode. Below is the circuit:</div><div><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiYof3StXMgiRazYgBXkpjhlqVLcxEdm0l7GRCTAy6S2r2I1pvibxPRQhfcZRJ7xoz6S11Ya_8iJKF0cZ11ZMQ9Q5FBZJJnlEP9w_GEmTcODZBFaJsES1NysS_0v9QA4-abA9SQGZdPUzQ/s2048/Circuit+4.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiYof3StXMgiRazYgBXkpjhlqVLcxEdm0l7GRCTAy6S2r2I1pvibxPRQhfcZRJ7xoz6S11Ya_8iJKF0cZ11ZMQ9Q5FBZJJnlEP9w_GEmTcODZBFaJsES1NysS_0v9QA4-abA9SQGZdPUzQ/w512-h384/Circuit+4.jpg" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Third and Fourth Test Circuits</td></tr></tbody></table><br />I used 6EW6 and 6BN11 (they are equivalent but I switched over to the 6BN11 because that's what I eventually plan to use) as driver tubes. I loaded it with a CCS in parallel with a resistor. The resistor reduces gain a bit with the goal of reducing plate voltage drift. </div><div><br /></div><div>In my third test, I used a 440k resistor in parallel with the CCS. The schematic shows 1M there, because that ended up working better but just imagine a 440k there for now. This gave the driver a gain of ~360 (measured with output tube removed). 1W distortion result below:<br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjV0wQHQ7Q7WcA3Q0Kg73yYUaz_yakTOouX4Vro2rjIYBX0LtzQus21ywfQXu4VT4j0HBL7q6n5hIKiFR0txwz61L13rlPckMUjlg79k3c6XN1dmUhfFxZYj7gW6sLqIob41nblju38vs8/s1920/1WSec.png" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjV0wQHQ7Q7WcA3Q0Kg73yYUaz_yakTOouX4Vro2rjIYBX0LtzQus21ywfQXu4VT4j0HBL7q6n5hIKiFR0txwz61L13rlPckMUjlg79k3c6XN1dmUhfFxZYj7gW6sLqIob41nblju38vs8/w512-h288/1WSec.png" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.27% Distortion @ 1W</td></tr></tbody></table><br /></div><div>This ended up being very close to the same result I got with the second test circuit above. In fact, I though I had some measurement setup issue since results at every power level were very similar. Then I though about it more. The gain of the driver was equal to the overall gain I was trying to set with my feedback divider. This means that the excess gain (applied as feedback) was equal to the gain of the output stage. All of the output stage gain was going to feedback, which is very similar to the configuration in the second test setup. Ok, so that kind of makes sense.</div><div><br /></div><div>I decided to see what I could do to increase gain a bit further and I ended up replacing the 440k resistor with a 1M. This increased driver gain to ~560. 1W distortion below:<br /><br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgTYwS48VflZAIsVLJcKXbly0j4szXnI562qfMQ5wKknYdaKJCToof3_kvXnrS_7NHJ56br-BJq-tuqwRirv9KgGDw9WoqQv9o4XY_639t8kTuMMS2-vBOBqbd770Ua2jAb1-Wq2fcQTHg/s1801/1WSec.png" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgTYwS48VflZAIsVLJcKXbly0j4szXnI562qfMQ5wKknYdaKJCToof3_kvXnrS_7NHJ56br-BJq-tuqwRirv9KgGDw9WoqQv9o4XY_639t8kTuMMS2-vBOBqbd770Ua2jAb1-Wq2fcQTHg/w512-h260/1WSec.png" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.14% Distortion @ 1W<br /></td></tr></tbody></table><br />On this one I stopped to measure Zout which came out to 0.8 Ohms. This is really starting to get into the ballpark of the performance I was looking for. Part of me worried that the 826 would distort more and so I wanted some more margin if I could get it and I had one more idea to try.</div><div><br /></div><div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgGbega91AulRFQ8hVIghQbRdmIPzTp_gNVdizEkCdEtHqMk-NhaSneahAm5azzqtid7XfyNGnZ9OQd_CAedc2gyy4lCRoHxJRTmqla_-elCjPO3mwyZKwhZPEL56ca1J5KAHkS2lZzzNk/s2048/Circuit+5.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgGbega91AulRFQ8hVIghQbRdmIPzTp_gNVdizEkCdEtHqMk-NhaSneahAm5azzqtid7XfyNGnZ9OQd_CAedc2gyy4lCRoHxJRTmqla_-elCjPO3mwyZKwhZPEL56ca1J5KAHkS2lZzzNk/w512-h384/Circuit+5.jpg" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Fifth Test Circuit</td></tr></tbody></table><br />This circuit was very similar except it has a p-channel FET that buffers the feedback network from the cathode of the 6BN11. AC cathode current variations in the 6BN11 (which include plate and screen current variations) end up making their way into the feedback network in the third/fourth test circuit. This was my attempt to separate them and provide a low impedance point for the 6BN11 cathode to avoid cathode degeneration in that stage, which reduces gain.</div><div><br /></div><div>This change resulted in a gain of ~2600 in the input stage (quite an increase!). One thing to note here is that plate voltage drifts with time in this configuration due to the high gain. A bias servo circuit will probably be necessary to build this into a working amplifier. 1W distortion below:<br /><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgMEPlD2CfSpJEsYEI_QC3jg9RdTavlWJTjVi5hDnZhbMZlzkvB634Yxb2uK3ZFUBsbOeWjo3PUuOa0Ds1BVJw7SPOKLQGWP47bQs4X7vu25Q6gNHIiDf1IeUZrAhP_5irEG008sO659lk/s1804/1Wp-chanbuffer.png" style="margin-left: auto; margin-right: auto;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgMEPlD2CfSpJEsYEI_QC3jg9RdTavlWJTjVi5hDnZhbMZlzkvB634Yxb2uK3ZFUBsbOeWjo3PUuOa0Ds1BVJw7SPOKLQGWP47bQs4X7vu25Q6gNHIiDf1IeUZrAhP_5irEG008sO659lk/w512-h261/1Wp-chanbuffer.png" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.046% Distortion @ 1W</td></tr></tbody></table><br />Zout was measured at 0.6 Ohms. This is very low, considering copper losses account for ~0.55 Ohms and cannot be corrected for by the feedback, since the output transformer is outside the loop. At this point, I considered the circuit plenty good enough. <div class="separator"></div></div></div><div><br /></div><div>I then substituted the 826 output tube and results can be seen <a href="https://tubeswithatwist.blogspot.com/2020/08/an-ultra-low-distortion-826-se-amp.html" target="_blank">here</a>. Distortion went down even further (to 0.027%), presumably due to the great linearity of the 826 (thank goodness). The 826 only has 2/3 the transconductance of the 6384 and 1/3 the gain in-circuit so I was worried it might go up. The Zout with the 826 was ~0.7 Ohms, which makes sense with the lower transconductance. </div><div><br /></div><div>Anyway, I hope that explains the progression I went through to arrive at this approach.</div>Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-20057415375901705602020-08-04T11:36:00.001-07:002020-08-05T13:03:52.300-07:00An Ultra-Low Distortion 826 SE Amp<div class="separator" style="clear: both; text-align: left;">And when I claim ultra-low distortion, I'm getting 0.027% THD at 1W. As of the time of writing this, I'm unaware of any other series-feed SE tube amp that does better.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">It's currently a breadboard amp with a very sophisticated forced air cooling system. ;)</div><div class="separator" style="clear: both; text-align: center;"><br /></div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvS-l29OPTuOW-a_F8WmUNqig-3XUwiFljHjlf03RH09EAuv0ixSm9H7S2r34PgEdR26BZRDO7Yro11GD9ra_ItmLaSmlDmfWpStRS4ljK4Wn3PGbpDbqlU72siEpUtJSxFPzDaGpJGOM/s2048/IMG_20200717_111056362.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1536" data-original-width="2048" height="384" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvS-l29OPTuOW-a_F8WmUNqig-3XUwiFljHjlf03RH09EAuv0ixSm9H7S2r34PgEdR26BZRDO7Yro11GD9ra_ItmLaSmlDmfWpStRS4ljK4Wn3PGbpDbqlU72siEpUtJSxFPzDaGpJGOM/w512-h384/IMG_20200717_111056362.jpg" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">The Amp</td></tr></tbody></table><div><br /></div><div>Here's a simplified schematic:</div><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiR0Jhtpp1PpFuigXadp9GfOOdiQcEnf2FJ89KVd9rYRsa-1V6wdZ6ksav8fRP6fv5NIdb_BLXv3KJXwA7TYpKg2F21_0nGyLdt8g7q0xmEbFyepCU4vd04EKpW6-IJYvES9Sjqf8Vs2eU/s2048/IMG_20200718_084319441.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="1536" data-original-width="2048" height="384" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiR0Jhtpp1PpFuigXadp9GfOOdiQcEnf2FJ89KVd9rYRsa-1V6wdZ6ksav8fRP6fv5NIdb_BLXv3KJXwA7TYpKg2F21_0nGyLdt8g7q0xmEbFyepCU4vd04EKpW6-IJYvES9Sjqf8Vs2eU/w512-h384/IMG_20200718_084319441.jpg" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Simplified Schematic</td></tr></tbody></table><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;">I'm currently running B+ at 520V. Output transformer is an Edcor 5k:8. Nothing exotic. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I'll walk through the theory of operation. I chose a 60W high-impedance directly-heated transmitting triode. It needs a positive grid bias and a low-impedance grid drive. This is provided by the N-channel mosfet. I'm running the 826 at 100mA (52W dissipation). When I get a new power transformer, I will try a bias point with a little more current and voltage, closer to the 60W limit. As it stands now, the 520V, 100mA operating point clips very symmetrically. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The input stage is designed to provide maximum gain and take feedback from the plate of the ouput tube. This allows us to reduce distortion a lot, but since the output transformer is outside the feedback loop, an expensive, exotic, high bandwidth output transformer is not needed for us to apply a robust feedback factor. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The 6BN11 (dual version of the 6EW6) has decent transconductance and develops a gain of ~2600 in this circuit. I'm sure there are many other small pentodes that would do just as well in this position. The p-channel FET source follower drives the cathode of the 6BN11. This isolates the feedback divider from the AC cathode currents of the input stage. The resulting low impedance drive of the feedback to the 6BN11 cathode results in over a 3X distortion reduction over tying the 6BN11 directly to the 520R feedback resistor. The traditional approach of connecting the cathode to the feedback network allows AC plate and screen currents in the input stage to corrupt the effectiveness of the feedback. A drawback to the high gain of the input stage is that bias stability isn't very good. The final version of this amplifier will probably require a bias servo on that stage. I have observed 100V drift in plate voltage in a listening session. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Sensitivity of the overall amplifier is set to ~0.7V. This can be easily changed by altering the 520R/220k resistor ratio. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I calculated ~23W output with a lossless transformer. In actual testing I got 19W when it started to clip. Seems about right for real-world loss. I mean, I really wish I had gotten to 20W, because that sounds a lot bigger than 19W, but I'll have to live with that disappointment, I guess.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">See below for distortion measurements at different power levels:</div><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxzgRAwGRmzgePcRNmhlwWLEZMRbM_U7LEHAewhA_5jfcJAxjiQ_9iYCgjAJ5MyGF-re2SzSwi_Tm3ZT7_ERKMADrJ7SVWml8gAwmYf-DsuHPPgBn0sbhubNPKJhGFURfzRaM-67UbzT4/s1808/001W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="918" data-original-width="1808" height="260" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjxzgRAwGRmzgePcRNmhlwWLEZMRbM_U7LEHAewhA_5jfcJAxjiQ_9iYCgjAJ5MyGF-re2SzSwi_Tm3ZT7_ERKMADrJ7SVWml8gAwmYf-DsuHPPgBn0sbhubNPKJhGFURfzRaM-67UbzT4/w512-h260/001W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.014% @ 100mW</td></tr></tbody></table><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGfqCICf0a8soBRIBbTOBqMWzd0wwrXMrFP848x9xcI762_rd4kuBFDgl7NdQhAUrxTdd0MI2GwmAHKtSDsxi-nuFWeXdimqbWKRQGcWTo_9JfXKwxVOMkAdnBLGzmj_302ZGNmZaIEkQ/s1808/005W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="922" data-original-width="1808" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGfqCICf0a8soBRIBbTOBqMWzd0wwrXMrFP848x9xcI762_rd4kuBFDgl7NdQhAUrxTdd0MI2GwmAHKtSDsxi-nuFWeXdimqbWKRQGcWTo_9JfXKwxVOMkAdnBLGzmj_302ZGNmZaIEkQ/w512-h261/005W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.017% @ 500mW</td></tr></tbody></table><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgswpOwH9QHbR_ym3xGChEXgS_ws4q6JyFzHoSnQ3F_eeVHzZnjxG4KqoJYwnknKIPDxNg5YlWWw0IcSYgLvT9gLLs_RzJpob4QPSIE1G4Tqov_RaMbXvSGJd2Vw_2VCiT3LlaxFroORKg/s1808/010W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="919" data-original-width="1808" height="260" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgswpOwH9QHbR_ym3xGChEXgS_ws4q6JyFzHoSnQ3F_eeVHzZnjxG4KqoJYwnknKIPDxNg5YlWWw0IcSYgLvT9gLLs_RzJpob4QPSIE1G4Tqov_RaMbXvSGJd2Vw_2VCiT3LlaxFroORKg/w512-h260/010W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.027% @ 1W</td></tr></tbody></table><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOPnN082VpBwTJwBvE1DouWn4_cn21mdVuGTqbQMXasxVuKkmasIaWw-91az08tsAHH5tOjFTVjC26UIUfcM7JWlN2_7NPmH1HxaBxRWj71nTIJZBYVUqsQVNrc4BYflaRPT_DkEmNXIM/s1805/020W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="920" data-original-width="1805" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOPnN082VpBwTJwBvE1DouWn4_cn21mdVuGTqbQMXasxVuKkmasIaWw-91az08tsAHH5tOjFTVjC26UIUfcM7JWlN2_7NPmH1HxaBxRWj71nTIJZBYVUqsQVNrc4BYflaRPT_DkEmNXIM/w512-h261/020W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.040% @ 2W<br /></td></tr></tbody></table><br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibuzJoPdEAjC8As_AS9ciCm5QGbK7DhZsC2oDrp7uRwJlz-m9OSdcrECvGQOi0QMN2vkUFm3C8EH1M0pO3qWuiX0-9PmrE6F1Iyaqqzl_XJ_vzSer7E1R5LPuZ0zQJS17rrpfthGgZpb4/s1805/050W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="920" data-original-width="1805" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibuzJoPdEAjC8As_AS9ciCm5QGbK7DhZsC2oDrp7uRwJlz-m9OSdcrECvGQOi0QMN2vkUFm3C8EH1M0pO3qWuiX0-9PmrE6F1Iyaqqzl_XJ_vzSer7E1R5LPuZ0zQJS17rrpfthGgZpb4/w512-h261/050W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.063% @ 5W<br /></td></tr></tbody></table><div style="text-align: center;"><br /></div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJFzOkHbvHuOzOTTccZW3tXiB9APKFWxBERewSm6o6I7h1KBwBPDEV-WI_gGeyntwlVaVui3osTwdiOXUivcdbUHLwQAbjCo0DeilxS6I2N4OdA8sClC9MsuAFh66u1-sOusizN8eJvu8/s1804/100W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="920" data-original-width="1804" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJFzOkHbvHuOzOTTccZW3tXiB9APKFWxBERewSm6o6I7h1KBwBPDEV-WI_gGeyntwlVaVui3osTwdiOXUivcdbUHLwQAbjCo0DeilxS6I2N4OdA8sClC9MsuAFh66u1-sOusizN8eJvu8/w512-h261/100W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.063% @ 10W<br /></td></tr></tbody></table><div class="separator" style="clear: both; text-align: center;"><br /></div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhllSwZWL22wOzTUfS7TRJputx1Kk_bbf21InSCcBr_zqS9FHvDsQcVXtOSvMPIgCzDvkf2CAJk0QBqFhMiVNpcpDcHqIEyXrTqf55BQ0y6LPoxAx0YZlmqwaLyifNPsgVh9KZXBqHJOps/s1807/190W.png" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="920" data-original-width="1807" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhllSwZWL22wOzTUfS7TRJputx1Kk_bbf21InSCcBr_zqS9FHvDsQcVXtOSvMPIgCzDvkf2CAJk0QBqFhMiVNpcpDcHqIEyXrTqf55BQ0y6LPoxAx0YZlmqwaLyifNPsgVh9KZXBqHJOps/w512-h261/190W.png" width="512" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">0.30% @ 19W</td></tr></tbody></table><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;">I measured Zout at 0.7 Ohms, so damping factor is over 10, which is probably pretty unusual for a SET amp. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Here's a 10 kHz square wave:</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgA2i6GgZjJGETwOEyctMWOyuu-oGNV1bGIgJXFz5k9z7uu5iPdPhgPEdnwEIiPRnSi2p9AZ7XyvuOeHZkzpG4v5zpAKZCoR_QhwVm-D9nKMgI4ZIoQ3t4d5uahJ6e_ii8gM34mdOGDrkc/s480/10kHzSquare.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="234" data-original-width="480" height="234" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgA2i6GgZjJGETwOEyctMWOyuu-oGNV1bGIgJXFz5k9z7uu5iPdPhgPEdnwEIiPRnSi2p9AZ7XyvuOeHZkzpG4v5zpAKZCoR_QhwVm-D9nKMgI4ZIoQ3t4d5uahJ6e_ii8gM34mdOGDrkc/w480-h234/10kHzSquare.jpg" width="480" /></a></div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Blue trace is the output transformer primary, yellow is the secondary. Both have a nice shape, but you can see how the output transformer affects the signal.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Here's a screenshot I took of the input tube plate waveform at 19W output:</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAcBRbm878UqY5705WXaVE1z8b3hmMounQ9xZddmgf7P0_32PNBaZ0Fgk7OTVKdFrLT06Gp7ZOYQll1VAexAM00M_AMCHv0T0da5QLjfbWLWqctw-KCk9FUTsityEB-AzwF_PRAeMlH2w/s480/InputPlate19W.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="234" data-original-width="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAcBRbm878UqY5705WXaVE1z8b3hmMounQ9xZddmgf7P0_32PNBaZ0Fgk7OTVKdFrLT06Gp7ZOYQll1VAexAM00M_AMCHv0T0da5QLjfbWLWqctw-KCk9FUTsityEB-AzwF_PRAeMlH2w/s0/InputPlate19W.jpg" /></a></div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">You can see the hump forming on the positive-going side where we are starting to hit saturation in the output tube. The negative side is also forming a "point." I didn't capture an image, but further increases in level cause sharp spikes to form on the positive and negative sides, indicating symmetrical clipping. It's a good operating point for this supply voltage. I didn't dwell too long in that state for fear of over-stressing the output tube grid.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I think I covered everything. To close things out, I'll attach some pictures of the 826 and the 6BN11 operating in the dark. One of the requirements for this amp was to have tubes that look good. I think I succeeded on that.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhkHWBIH-ME7_DSuEeixvpUFydAZnUnxi3IuYDLGuyeqDFlr6EgJ6sNO1ntYGaxNSSlQwsj9VgB2uiUdmFAr-zOX42Oe96pNot80CIZuA0zr3zyI7cD_AZf59tHpb0m1LYibjX-d0oEc9E/s2048/IMG_20200718_201812832.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="384" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhkHWBIH-ME7_DSuEeixvpUFydAZnUnxi3IuYDLGuyeqDFlr6EgJ6sNO1ntYGaxNSSlQwsj9VgB2uiUdmFAr-zOX42Oe96pNot80CIZuA0zr3zyI7cD_AZf59tHpb0m1LYibjX-d0oEc9E/w512-h384/IMG_20200718_201812832.jpg" width="512" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMf0XV9VddMHi18q-s6xN1A3_z7gPsBAMphdhkXFPwyZ2Nx242brz4pDBjRherR9DIBLdOu1V_VwcGMiAxf-7xr6TrPj_HiO_3zjfxHlwQvDpyN5bhiQKrnO7zQ-Ei9bxfLX41RvxfIlA/s2048/IMG_20200718_201524749.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="384" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMf0XV9VddMHi18q-s6xN1A3_z7gPsBAMphdhkXFPwyZ2Nx242brz4pDBjRherR9DIBLdOu1V_VwcGMiAxf-7xr6TrPj_HiO_3zjfxHlwQvDpyN5bhiQKrnO7zQ-Ei9bxfLX41RvxfIlA/w512-h384/IMG_20200718_201524749.jpg" width="512" /></a></div>Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com1tag:blogger.com,1999:blog-2914493245344265727.post-88791601819906770742020-08-03T16:17:00.003-07:002020-12-12T19:58:43.449-08:00Idea for Ultra-Low Distortion Unity-Coupled Amp<br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj_B_167mYg7fzKMbW1ohudnGws96OdZUkHP850mdGXtMXy2JYDjH4o483K_5VmZjbmOD65VHRO3Gc_HQonGm7v-Sn2RCdkmbQaq2CFk0J-qoBU27n4q6kaZEW92WzOd8qrzlxeVPIgU1Q/s868/UC3.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="679" data-original-width="868" height="313" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj_B_167mYg7fzKMbW1ohudnGws96OdZUkHP850mdGXtMXy2JYDjH4o483K_5VmZjbmOD65VHRO3Gc_HQonGm7v-Sn2RCdkmbQaq2CFk0J-qoBU27n4q6kaZEW92WzOd8qrzlxeVPIgU1Q/w400-h313/UC3.jpg" width="400" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;">I had some great results in experiments with an SE amp design. In that amp, I took feedback from the output tube plate and applied it to a high-gain pentode/p-channel FET input stage. This gave fantastic results. The amp had very low distortion and Zout. (0.027% THD @ 1W and 0.7 Ohms)</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">This got me thinking about applying this approach to push-pull amps and here is what I came up with for a Unity-Coupled amp. A couple of things stand out:</div><div class="separator" style="clear: both; text-align: left;"><ol style="text-align: left;"><li>Feedback from the output stage plate can be taken from the output tube cathode, since cathode and anode swings are equal in magnitude. The great thing about this is that the cathode sits near GND potential at idle so you don't have a bunch of power dissipated in the feedback resistor at idle. </li><li>Creating a feedback network that works to restore balance in the amp is easy since the feedback network idles at a low voltage. The feedback network can be direct-coupled without needing to be pulled down to something near the input stage grid or cathode potential. It's already there. This makes the input stage work similar to the input stage of an instrumentation amp.</li></ol></div><div class="separator" style="clear: both; text-align: left;">I drive the cathode of the input tube with a p-channel FET here and use the grid as the feedback node, and I configure the feedback network like the input stage of an instrumentation amplifier to correct imbalance in the amplifier.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The source follower drivers for the output tubes aren't really necessary. They simply reflect the way the output stage in my Unity-Coupled amplifier is currently configured. I like the immunity to blocking distortion that they offer so I will keep them there. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The Plitron transformers have pretty low DCRs in the windings so I expect that this amplifier would deliver very low distortion and a Zout of less than 0.4 Ohms, even without feedback around the output transformer.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Some day I'll work something like this into my Unity-Coupled amplifier.</div>Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-45384381183810784122020-07-30T17:09:00.000-07:002020-07-30T17:09:18.073-07:00If UNSET and the RCA50W Had a BabyWhat's UNSET? See <a href="https://www.diyaudio.com/forums/tubes-valves/356896-g2-using-mosfet-ultralinear-feed.html#post6267790" target="_blank">here</a>. It's a clever way of wrapping series-applied voltage feedback around an output stage <b>and</b> sharing some of the idle dissipation in the output stage between a mosfet follower and the output tube (which can get you more power if you increase B+ accordingly), apparently recently discovered by Mr. Tubelab and Mr. Smoking-Amp nearly simultaneously. <div><br /></div><div>What's the RCA 50W amp? See below:</div><div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGgUZ87rc5VcWpIA-1Pcqeois0YRABPZMTTGA2YamflPZzINDeEccHhfV0398zh5NTXweIhaOToKyT41LznILfBVAL5YzT0l0TgLwlITF9OzvRIl2vVsPkSQoutYl4dAuLGThXtEzOmsY/s926/RCA+Original.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="896" data-original-width="926" height="485" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGgUZ87rc5VcWpIA-1Pcqeois0YRABPZMTTGA2YamflPZzINDeEccHhfV0398zh5NTXweIhaOToKyT41LznILfBVAL5YzT0l0TgLwlITF9OzvRIl2vVsPkSQoutYl4dAuLGThXtEzOmsY/w500-h485/RCA+Original.jpg" width="500" /></a></div></div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">It's a 50W push-pull amp with three nested feedback loops that apparently can do 50W@0.1% THD, which ain't too shabby. The innermost loop is parallel-applied voltage feedback from plate to grid of the output tube. Surrounding that is series-applied voltage feedback from plate of output tube to driver cathode. And then there is the global loop that goes from amplifier output to input tube.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I've always had mixed feelings about this amp. It has a lot done right, but my back-of-the-envelope calculations show that the ouput tube plate to driver cathode feedback can't be very effective due to the fact that the low impedance load at the plate of the driver spoils a lot of the potential driver gain. Driver cathode degeneration lowers gain further. I'm not even totally sure that it has more gain than the gain that is trying to be set with the resistor ratio that is there.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Anyway, I've been playing with output tube plate to driver cathode feedback and have been having exceptional results with high gain drivers. It makes me wonder what would happen if we fixed the RCA amp a bit and used series-applied voltage feedback around the output tube instead of parallel-applied feedback, so I decided to run with that idea and here is a simplified conceptual schematic:</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh74HbE_apu_mOMUFSU7HAWJRNCvvxc_TQqN_C-sKnerkOxHCOZzJevBd-tqvNQAIWi15pe_HO2cTtZWOCo2CocvcokJMa2afFVNhYm2qHOrKpY8ucArY4iENQVDvQ2X8BIyj3Ud9ZpGFg/s871/RCA_50W_CCS.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="820" data-original-width="871" height="470" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh74HbE_apu_mOMUFSU7HAWJRNCvvxc_TQqN_C-sKnerkOxHCOZzJevBd-tqvNQAIWi15pe_HO2cTtZWOCo2CocvcokJMa2afFVNhYm2qHOrKpY8ucArY4iENQVDvQ2X8BIyj3Ud9ZpGFg/w500-h470/RCA_50W_CCS.jpg" width="500" /></a></div><div class="separator" style="clear: both; text-align: left;">I made the output tubes two KT88 UNSETs (UNPPTs?), which frees up the driver to use a very high impedance load and develop some serious gain. Oh yeah. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">This is pretty similar to my driver for my <a href="https://www.diyaudio.com/forums/tubes-valves/357559-corona-ultra-low-distortion-a2-dht-se-amp-prototype.html" target="_blank">recent 826 SE amp experiments</a>, only I reversed the input and feedback connections to make the phasing correct for negative feedback. Bias on the 6BN11 stage is not very stable over time due to absurdly-high gain, so a bias servo is probably mandatory on that stage. Open-loop gain of that stage is ~2600. This provides a lot of feedback and I expect resulting driving impedance on the primary of the transformer will be somewhere between 10 and 20 Ohms. Using a Hammond 1650R (or something with similar low copper losses) will result in a Zout of ~0.5 Ohms or so. Distortion will be extremely low. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">In my mind, at that point, global feedback is optional, so I have omitted it. It could be added back in with another gain stage. I also like input transformers for the immunity to ground loops that they offer so I included one. Obviously, I have omitted some necessary components such as stoppers, protection diodes, and something to tie input transformer secondary to some level other than what the leakage currents pull it to.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The UNSET output configuration offers the opportunity to share some of the idle dissipation between the mosfet and the power tube, so I have increased B+ to 530V. This still puts 450V across the output tubes like in the RCA amp, but now we can hit over 75W with the same class of output tube, and have plenty of idle current to keep idle output tube transconductance high.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">I decided to pull the output tube plate to driver grid feedback network down near GND with a CCS. My intuition tells me that having a high AC impedance at this node would have a balancing effect on the amplifier. I'm not sure how well this would work (haven't simmed it or anything). The other option would be to ground the center point (maybe with a trim pot in the center to adjust out AC imbalances in the two halves).</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Anyway, I think it would make a good amp that could make your ears bleed with low distortion.</div>Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-22467045995239789522020-07-29T20:06:00.000-07:002020-07-29T20:06:52.617-07:00Unity-Coupled Plitron Amp RevisitedI decided to re-work my Unity-Coupled amplifier that I wrote up <a href="http://tubeswithatwist.blogspot.com/2015/01/a-unity-coupled-kt88-amp-with-plitron.html" target="_blank">here</a>. <div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEic1kchBb3EtHKOm1QvwXjqJDpDdxfmt9Z6vw-oepzA0xtGvZaw_u0jBFKsvFPEY-4kdV1peFw7gzdm_dVkYxeE6z1uW1CmYsHT3j5eIEPDIx5bqTvqqf4t2-p7PlZM8QxtnSmuC9C3WFI/s2048/IMG_20200729_195748833.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEic1kchBb3EtHKOm1QvwXjqJDpDdxfmt9Z6vw-oepzA0xtGvZaw_u0jBFKsvFPEY-4kdV1peFw7gzdm_dVkYxeE6z1uW1CmYsHT3j5eIEPDIx5bqTvqqf4t2-p7PlZM8QxtnSmuC9C3WFI/w500-h375/IMG_20200729_195748833.jpg" width="500" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlVcdqbNVBHercU4Lg5yI1jkvpZSZzeCnCQifP3GGPojGwxmey6t5hXkP9j_QheZ4aJJ4c9-_6hx9M2L03xGr7pmwEFIro150b3vtii7mylvEKTIl-tqMJD1pgj4NtnqLJaiB2Pw2oJns/s2048/IMG_20200729_195823226.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlVcdqbNVBHercU4Lg5yI1jkvpZSZzeCnCQifP3GGPojGwxmey6t5hXkP9j_QheZ4aJJ4c9-_6hx9M2L03xGr7pmwEFIro150b3vtii7mylvEKTIl-tqMJD1pgj4NtnqLJaiB2Pw2oJns/w500-h375/IMG_20200729_195823226.jpg" width="500" /></a></div><div><br /><div><br /></div><div>One thing that always bothered me was that I used a split-rail supply (-300V/+500V) to get 800V total supply voltage for the 841 tubes, which left the grid circuit of the 841 referenced to a negative supply and coupling any power supply noise directly into the 841 grid circuit. The supplies were Maida-style regulators so are pretty good but are not perfect. </div><div><br /></div><div>Another thing that bothered me was ground loops. I find background hum/buzz to be more objectionable than distortion, quite honestly, and I was having difficulty getting rid of it all. </div><div><br /></div><div>I also had some random rustling sounds in the amp, which seem to be most commonly a symptom of a bad solder joint. Unfortunately, this was a very complex amp so finding a bad solder joint is not easy, but it did make me want to get in there and change things with the hope of finding and fixing that issue.</div><div><br /></div><div>I settled on a few things to change:</div><div><br /></div><div>1. Reference 841 stage to GND.</div><div>2. Re-do grounding. This amp has seen too many changes and grounding scheme has suffered.</div><div>3. Input transformers to totally prevent ground loops from ever happening in equipment interconnects.</div><div>4. New input board due to space constraints and the fact that I want to put some things I'd learned using op-amps into practice. </div><div><br /></div><div>Below is the new configuration for the tube section:<br /><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjZ9NLnzm-NCJg9bBbQryMUIB58InGQuelu81DMU2-BKzJJP419v3YeOHt-BaJhnf81rkd6nMWz-O8gnBa4FKLdoZW1IKrQGZ9fhua6boYSlqiYfeHO-aTwmJBelCWW9Yk1r7xVFgWDJgU/s922/UC_841.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="832" data-original-width="922" height="451" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjZ9NLnzm-NCJg9bBbQryMUIB58InGQuelu81DMU2-BKzJJP419v3YeOHt-BaJhnf81rkd6nMWz-O8gnBa4FKLdoZW1IKrQGZ9fhua6boYSlqiYfeHO-aTwmJBelCWW9Yk1r7xVFgWDJgU/w500-h451/UC_841.jpg" width="500" /></a></div><div><br /></div><div>The 841 stage cathode is now tied to GND, rather than a negative supply. The CCS plate load is supplied from the opposite side output tube plate. In McIntosh amps, this resulted in a little bit of positive feedback to the driver. It increased gain by giving the plate load a higher dynamic impedance. In this circuit, the driver gain is unchanged because the driver already had a CCS plate load. The CCS impedance probably goes up some, but I don't expect that it makes any difference in the gain of the driver. In this amplifier, it's just a free HV supply that always stays 60V above the driver plate voltage.</div><div><br /></div><div>The 841 is now driven by a solid-state op-amp circuit detailed below. The 841 requires the grid to be driven positive with low distortion (Class A2 operation) and I wanted to try a new approach and see if I could reduce distortion.</div><div><br /></div><div>The rest of it is a pretty straightforward implementation of a Unity-Coupled output stage with a source follower driver and a mosfet that drops the screen voltage. 460V is a higher screen voltage than necessary for the load that the PAT-1070-UC presents, so it is beneficial to lower it.</div><div><br /></div><div>Below is the solid-state op amp input section of the amplifier: </div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhUi0LvBw-OjsexGEzYEO_XlVo4tcQsdOwaTseLx6PuZDI6l2sn4S0HM-eeOcTWZ1NYI51AzKffvrjUb0o4d1h1HPlJpcb8mBl3jt6rao-QPWeYQS3C89TAtFjedPApNMV5XCoRq0X8ob4/s2048/IMG_20200715_124223144.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhUi0LvBw-OjsexGEzYEO_XlVo4tcQsdOwaTseLx6PuZDI6l2sn4S0HM-eeOcTWZ1NYI51AzKffvrjUb0o4d1h1HPlJpcb8mBl3jt6rao-QPWeYQS3C89TAtFjedPApNMV5XCoRq0X8ob4/w500-h375/IMG_20200715_124223144.jpg" width="500" /></a></div><div><br /></div></div><div>All op-amps used were LME49860s powered by +-18V. The LME49860 is very similar to the LM4562 but is a little more expensive and can handle a little higher supply voltage, which I thought would be beneficial.</div><div><br /></div><div>A Jensen JT-11P-1 input transformer with recommended snubber network feed opposite phase non-inverting amps. This feeds a follower that drives the grid of the 841. Bias is presented to the non-inverting input and the feedback scheme around the follower is designed to help the op-amp deal gracefully with the ~300pF load the 841 grid presents. </div><div><br /></div><div>I added an AC balance adjustment (the 10R pot) to allow adjustment for gain mismatches in the two phases of the amplifier. I didn't expect perfect matching of the 841s and made an adjustment to compensate. </div><div><br /></div><div>Here are pictures of the op-amp boards:</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhF6QJ8_ljJpUnTui8GXTzhi2My78A9MAW3Gr_5OR1NuPrKj0-7kt_P4Ogbb4jEhX1T075F_Pz53hCUlzrutaQoNmKeySsc9GItniibYXu5r1Xciweb7BETthw1t8r3MVY6xGYFMiII6pk/s2048/IMG_20200328_155006949.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhF6QJ8_ljJpUnTui8GXTzhi2My78A9MAW3Gr_5OR1NuPrKj0-7kt_P4Ogbb4jEhX1T075F_Pz53hCUlzrutaQoNmKeySsc9GItniibYXu5r1Xciweb7BETthw1t8r3MVY6xGYFMiII6pk/w500-h375/IMG_20200328_155006949.jpg" width="500" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhXyvuHnBo6ugDWETTNtlSJout_5MKpBwD12EdUIMPeojN_j0XMvLzS5kA1BJUljGtuzsMULAC629u85aLP3gmUrz5uJrJWIm5pnqb988vWgXB2P3hqfASukKkdqPU40Jvs5y6sC03KdUQ/s2048/IMG_20200328_155015668.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhXyvuHnBo6ugDWETTNtlSJout_5MKpBwD12EdUIMPeojN_j0XMvLzS5kA1BJUljGtuzsMULAC629u85aLP3gmUrz5uJrJWIm5pnqb988vWgXB2P3hqfASukKkdqPU40Jvs5y6sC03KdUQ/w500-h375/IMG_20200328_155015668.jpg" width="500" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3p6xxVcuSiadm7wfv2KZVO658CqCoGxyorYILgZ_4HmCeFYhpkVp4fLu4O9B1FUBteqkgI3yk0Tgir8FlUSGFOOd0qsII3hyJ-oh9_shb6YJjPXxQZIbK-oPbEdW-LIsfRPh4pCEnKao/s2048/IMG_20200321_162805654.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="2048" height="375" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3p6xxVcuSiadm7wfv2KZVO658CqCoGxyorYILgZ_4HmCeFYhpkVp4fLu4O9B1FUBteqkgI3yk0Tgir8FlUSGFOOd0qsII3hyJ-oh9_shb6YJjPXxQZIbK-oPbEdW-LIsfRPh4pCEnKao/w500-h375/IMG_20200321_162805654.jpg" width="500" /></a></div><div><br /></div><div>The first stage is on the manufactured board. The second stage is on the prototype board. I wasn't likely to ever need something exactly like this again so I just hand wired it. Each 841 has its own bias adjustment. </div><div><br /></div><div>I used cheap AC-DC 18V isolated output switching supplies to generate the +-18V. I constructed a CLC filter to clean up the ripple on the output and got great results. Below is a screen shot of filter input and filter output:</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNUYadJpdhZ9t_zIWjrnv1XuvRlrQe4oPonxtLuw_28irLxRSPPkQ0WeAqj3wljtltYZ3TsRvV0u-k_-iPecq9hEcnIhGYXW2ro84kTy-gzbXMdl0dYgDCO-yGb0Syscy9183fqRR_DFE/s480/SDS00004.BMP" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="234" data-original-width="480" height="244" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNUYadJpdhZ9t_zIWjrnv1XuvRlrQe4oPonxtLuw_28irLxRSPPkQ0WeAqj3wljtltYZ3TsRvV0u-k_-iPecq9hEcnIhGYXW2ro84kTy-gzbXMdl0dYgDCO-yGb0Syscy9183fqRR_DFE/w500-h244/SDS00004.BMP" width="500" /></a></div><div><br /></div><div><br /></div><div>After implementing the new input stage and the changes to the tube section, all of the background buzz was gone from one channel. That both excited me and disappointed me. I was hoping it would be gone from both. </div><div><br /></div><div>So next I set out to redo the grounding. I re-did everything with a nice bus bar, but still there was buzz in one channel with a 120Hz ripple waveform visible on the output of that channel. I couldn't tell where exactly it was originating from but I isolated it to the output stage and just decided to build two new output tube boards. I replaced them both and the buzz went away. Not sure what was bad, but problem is now gone and no more random rustling sounds, either. </div><div><br /></div><div>Below are distortion results:</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjB05M04J8-JdUfstau-ty06PgwqOkRSBEHuWxGeiXs9hkPwaTC2MWPOwNnV2_j5DyLZcy8hxIwAeu_IuCHYMUfpGAiRDUYE9sd8Pmc1rYj4f751p3cmEY29rYsZLA9awvIrTtyfKDsOKI/s1803/R100m.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="920" data-original-width="1803" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjB05M04J8-JdUfstau-ty06PgwqOkRSBEHuWxGeiXs9hkPwaTC2MWPOwNnV2_j5DyLZcy8hxIwAeu_IuCHYMUfpGAiRDUYE9sd8Pmc1rYj4f751p3cmEY29rYsZLA9awvIrTtyfKDsOKI/w500-h255/R100m.png" width="500" /></a></div><div style="text-align: center;">100mW</div><div style="text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgWNAus90KfDAQHsTEQf9rLXHO9LUDTH-potgove02v-yANwipB6zocLnn0dIA2ZVQDgSaG5K7-WBseFAZOWQEgvWy1WaxMkMHyN8b7aWixlgxTvq2X5hXb-WDUeWvNxjcSaRck5t0bVko/s1802/R500m.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="919" data-original-width="1802" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgWNAus90KfDAQHsTEQf9rLXHO9LUDTH-potgove02v-yANwipB6zocLnn0dIA2ZVQDgSaG5K7-WBseFAZOWQEgvWy1WaxMkMHyN8b7aWixlgxTvq2X5hXb-WDUeWvNxjcSaRck5t0bVko/w500-h255/R500m.png" width="500" /></a></div><div class="separator" style="clear: both; text-align: center;">500mW</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDWrCr6oqhYwnD2a1EKxJMlZJ_HapJGXY3AScbSLmcWunttrmRdA_F4C9YApfFV4WbjwmWlFggLuecFHeN6R3zinNjK4K9mqsM0Bh8z_7mgjQYiaV9BIWpAEN4wZL0wmvQ6euVBL8U5F0/s1803/R1W.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="920" data-original-width="1803" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDWrCr6oqhYwnD2a1EKxJMlZJ_HapJGXY3AScbSLmcWunttrmRdA_F4C9YApfFV4WbjwmWlFggLuecFHeN6R3zinNjK4K9mqsM0Bh8z_7mgjQYiaV9BIWpAEN4wZL0wmvQ6euVBL8U5F0/w500-h255/R1W.png" width="500" /></a></div><div class="separator" style="clear: both; text-align: center;">1W</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmhlJ3dDIkIHTaHsdoS1HHYEJ1L97vqActJ5fKC_r1CRrW3qL_dKlRu-Jrg5DqUPnmCLxyogHR8PwItoVk5RTQcnftTwglwOtte-lElr-9zVGC_46c1zhL4wzuloq5HSInIS4DVwWFmw0/s1804/R2W.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="921" data-original-width="1804" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhmhlJ3dDIkIHTaHsdoS1HHYEJ1L97vqActJ5fKC_r1CRrW3qL_dKlRu-Jrg5DqUPnmCLxyogHR8PwItoVk5RTQcnftTwglwOtte-lElr-9zVGC_46c1zhL4wzuloq5HSInIS4DVwWFmw0/w500-h255/R2W.png" width="500" /></a></div><div class="separator" style="clear: both; text-align: center;">2W</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj93uB7P3_qN1eAja2VSJqdL36e94CXK1GirDG7q1B7AhfK1R6LyTv5JI67ch-RRNap-m0NBMtUklewhuPSPvVTgg7QFNWNRYR7olsc570aY_qkZhUDqxfxrvFuSO4vVf0X2Z1H6HBZv_w/s1806/R5W.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="920" data-original-width="1806" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj93uB7P3_qN1eAja2VSJqdL36e94CXK1GirDG7q1B7AhfK1R6LyTv5JI67ch-RRNap-m0NBMtUklewhuPSPvVTgg7QFNWNRYR7olsc570aY_qkZhUDqxfxrvFuSO4vVf0X2Z1H6HBZv_w/w500-h255/R5W.png" width="500" /></a></div><div class="separator" style="clear: both; text-align: center;">5W</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6PrgtH_sYWRwt72hxPG3tfEBrXG8jYwGUgk6zShLkcNBYcjaFoKNSbvlwzW0yLL4IvtGMHckYtLF4BUZakgOTm-PITTEjbZca2QGupfnCFIe0wjHBj0Fz93hYchArfyVBuLyMM_OZ-uw/s1802/R10W.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="920" data-original-width="1802" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj6PrgtH_sYWRwt72hxPG3tfEBrXG8jYwGUgk6zShLkcNBYcjaFoKNSbvlwzW0yLL4IvtGMHckYtLF4BUZakgOTm-PITTEjbZca2QGupfnCFIe0wjHBj0Fz93hYchArfyVBuLyMM_OZ-uw/w500-h255/R10W.png" width="500" /></a></div><div class="separator" style="clear: both; text-align: center;">10W</div><div class="separator" style="clear: both; text-align: center;"><br /></div><div>Distortion is pretty good for an open-loop amp, but I've got plans to make it much better when my 841s wear out some day. For now, I just like the way they look too much to get rid of them.</div><div><br /></div><div>I didn't measure Zout this time, but I didn't change anything that should affect that, so it should still be ~1.1 Ohm.</div><div><br /></div><div>The amp clips at about 40W. I was using the balanced output from my USB soundcard to drive the amp for the distortion test and I didn't realize that it only puts out .7Vrms max, so it didn't have enough output to drive the amp all the way to clipping. I designed to a 2Vrms sensitivity since the source that I use with this amp puts that out with no trouble. I'll have to wire this up so that my Pete Millett soundcard interface can drive it to clipping when I get some more time and update this post with more measurements.</div><div><br /></div><div>The amp is so much more satisfying to listen to now. I mean, I can't say that I notice a difference in distortion but having the background be totally silent is such an improvement.</div></div>Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-14857718713287705242017-04-24T17:17:00.000-07:002017-04-24T17:17:12.983-07:00How to Make a Pentode Voltage Amplifier That is More Linear Than Any TriodeIt is generally accepted that pentodes are on average a good bit less linear as voltage amplifiers than triodes. Today I'm going to show how to take a pentode and make it more linear than any triode, or at least any triode that I have any experience with (if there is one that beats this performance, I wouldn't mind knowing about it).<span id="goog_544982081"></span><br />
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I've previously written on this blog about highly-linear power stages using KT88s. The approach here will be similar, using voltage feedback that is parallel applied and driven with a p-channel FET, so the overall input impedance of the circuit will be very high and easy to drive. The p-channel FET makes things work out with a minimum of components. There are other variations of this circuit that would be possible with n-channel devices or tube followers/active loads should one desire to keep it all tubes.<br />
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Conceptually, this is kind of what I'm attempting to do as far as feedback scheme goes:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjd-h3byd3GNPPVcJ6Ht_P_yqOwdvkRPJSAUT2bnV679N7uL9tKW8jo42mawJMbqVLjOs9pgcITbfVGB_Atc3FzZBcEfKY-ZlcILL9ODf9608M44F72gdpLnY86zdADqpJx2DQK64VIyl4/s1600/IMG_20170424_170325079.jpg" imageanchor="1"><img border="0" height="100" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjd-h3byd3GNPPVcJ6Ht_P_yqOwdvkRPJSAUT2bnV679N7uL9tKW8jo42mawJMbqVLjOs9pgcITbfVGB_Atc3FzZBcEfKY-ZlcILL9ODf9608M44F72gdpLnY86zdADqpJx2DQK64VIyl4/s320/IMG_20170424_170325079.jpg" width="320" /></a><br />
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And here is a more fleshed-out conceptual drawing of the actual circuit:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUEr_2eueb2oiQrypqcUeNhBugcxzLxlKuwdVwoZyZHKatMsFRGMg2mKr1eO3XfMzMxK60fJB7-ar_rfULKUuDYioCjD1Enxop_X0bmsClyjgwnOqAbPXbpWKpNxv_jBok8eiCejOmuKs/s1600/IMG_20170424_170333652.jpg" imageanchor="1"><img border="0" height="331" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUEr_2eueb2oiQrypqcUeNhBugcxzLxlKuwdVwoZyZHKatMsFRGMg2mKr1eO3XfMzMxK60fJB7-ar_rfULKUuDYioCjD1Enxop_X0bmsClyjgwnOqAbPXbpWKpNxv_jBok8eiCejOmuKs/s400/IMG_20170424_170333652.jpg" width="400" /></a><br />
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What I've done is make a very high gain amplifier by putting an active load on a pentode, then I am using local feedback to reduce my distortion down to minuscule levels. I drive the feedback network from the source of the mosfet in the active load (a low impedance drive point) to keep the feedback network from loading the high-impedance point at the pentode plate. The circuit works very well and is very simple.Feedback-phobia is unwarranted as will be shown in the distortion spectra, which reveals very little distortion and what is there being low-order.<br />
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However, before I get too far perhaps I should take a slight detour and explain why I did these experiments with big power pentodes like the EL84 and EL34. I started this experiment trying to find a replacement for the driver stage in my Unity-Coupled amp. It requires 160Vrms to drive the output stage to clipping so it requires a tube capable of idling at 350 or more volts. I also wanted to develop a driver for an SE amp with a follower output stage, which would require a tube that can idle at 500V or more on the plate. EL34 was the cheapest tube I could find that fit the bill. It is overkill but it was the cheapest one. I also had an EL84 laying around so I ran some tests with it. I also had several beam tubes around and tested some of them but the results were not as good as the true pentodes. This makes a bit of sense, since the characteristics of beam tubes get kinky at low plate voltages and low currents. The performance of the beam tubes were still quite good, just not as good as the pentodes.<br />
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These concepts could be applied to small signal pentodes as well. I haven't tried it yet, but if/when I get the chance I will.<br />
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Here is a simplified version of the practical circuit (I have left out gate protection diodes and gate stoppers):<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgN8JT6wmdeuUlEZSQozx36wipMC6oQ1RicRanWDxpuuIt9vxKf0RrmdC8xTU4XNVjCQ1P7bPUH-v_PLv7aMWWbJdtO8-xDD_8RVH7KY2cRs0_-n_B4fTsiIWNI-1Di5Xyt4q24S_iIifE/s1600/IMG_20170424_170312395.jpg" imageanchor="1"><img border="0" height="208" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgN8JT6wmdeuUlEZSQozx36wipMC6oQ1RicRanWDxpuuIt9vxKf0RrmdC8xTU4XNVjCQ1P7bPUH-v_PLv7aMWWbJdtO8-xDD_8RVH7KY2cRs0_-n_B4fTsiIWNI-1Di5Xyt4q24S_iIifE/s400/IMG_20170424_170312395.jpg" width="400" /></a><br />
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With these feedback resistors I get a gain of a little over 13. I spent a lot of time optimizing plate voltage and screen voltage and found that the higher the plate idle voltage, the lower the distortion. The opposite was true of screen voltage, the lower the voltage the lower the distortion. Of course, with the screen you can only go so low until you clip at the Vg = 0 line so you have to find that point and back off a little. For the EL34, this was 35V at a 10mA operating point.<br />
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I also had a bit of peaking in the frequency response that showed in square wave testing. I solved this with adding 12pF in parallel with the feedback resistor. This also limits bandwidth to 200kHz, which is really plenty. Earlier tests showed the response to be 3dB down at 350kHz. With a 10M90S as the upper device in the active load, there was no peaking in the response but with a 1700V IXYS part, there was peaking and ringing in the square wave before adding the capacitor.<br />
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Here are the distortion results I got with the plate set at 600V and 35V on the screen:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgMM22E2WoCI2EPOKsVsq6poMn6sLoZq1HDvkCzdJM0vnNDrSJLE2bHpvLbZoNjaHO19MMZvD6gV8aARWbEowvcWKss69oMPF7yh6_qN77L0s8Lgoh-ZyS_U551fKrJSU_QfIs2qLgq7zU/s1600/10Vrms.png" imageanchor="1"><img border="0" height="207" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgMM22E2WoCI2EPOKsVsq6poMn6sLoZq1HDvkCzdJM0vnNDrSJLE2bHpvLbZoNjaHO19MMZvD6gV8aARWbEowvcWKss69oMPF7yh6_qN77L0s8Lgoh-ZyS_U551fKrJSU_QfIs2qLgq7zU/s400/10Vrms.png" width="400" /></a><br />
<b>10Vrms</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhZkJ-M7zeoR3k0EOtQxhUBD7ZpiMLvU6aa4jjYO9b6S9J4TZDo8Mk9Ogjz9uXDFpvXvoBYYBw4ZZP7STXmts0kg8TsM1PT69YHaaNRTYKQnSXKDTOJdUWoFTU9_KoBEkyXZX-LJGCy8AU/s1600/20Vrms.png" imageanchor="1"><img border="0" height="205" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhZkJ-M7zeoR3k0EOtQxhUBD7ZpiMLvU6aa4jjYO9b6S9J4TZDo8Mk9Ogjz9uXDFpvXvoBYYBw4ZZP7STXmts0kg8TsM1PT69YHaaNRTYKQnSXKDTOJdUWoFTU9_KoBEkyXZX-LJGCy8AU/s400/20Vrms.png" width="400" /></a><br />
<b>20Vrms</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvDXySkd5em7i03DCA-UYg8TT_fDMInuwffZWQ-PhIu9pdc0WZPFbpD-ScPbrhfcnidAxevzCl6cQRw_tvWdvNNvPixxRXuOakvP-Hu6YYNRLvkDMbv4E5v0qks9oU8Aes9jBZMiRcR9g/s1600/30VrmsB.png" imageanchor="1"><img border="0" height="207" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvDXySkd5em7i03DCA-UYg8TT_fDMInuwffZWQ-PhIu9pdc0WZPFbpD-ScPbrhfcnidAxevzCl6cQRw_tvWdvNNvPixxRXuOakvP-Hu6YYNRLvkDMbv4E5v0qks9oU8Aes9jBZMiRcR9g/s400/30VrmsB.png" width="400" /></a><br />
<b>30Vrms</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgEeapp2TNTSU4L5H2YpC5mnUtWmCGkfngZBBBUteuRK3rcf4nlZpPsa_qJBpP442f8Ch8BDG_W8Iewz4GwbJnBCJNzfX9qYpm73dSlic6ruXWAReyndpcxVZFvKKysqVwRfuWFjwcNSTk/s1600/50VrmsB.png" imageanchor="1"><img border="0" height="206" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgEeapp2TNTSU4L5H2YpC5mnUtWmCGkfngZBBBUteuRK3rcf4nlZpPsa_qJBpP442f8Ch8BDG_W8Iewz4GwbJnBCJNzfX9qYpm73dSlic6ruXWAReyndpcxVZFvKKysqVwRfuWFjwcNSTk/s400/50VrmsB.png" width="400" /></a><br />
<b>50Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjGWgM_qdCcAuBe3jjyyuRVHfZJZ9JvxtaR4-z5ZJJPd_3OzXN88IBdueiUyS0iXPcrQgtuA8oczaVjUeMyBRsTR1ABPHEk6dbPjS-jPRP4e5DJQA5mNJ6tC4zixqdKHy37Fj2Fd1zOigU/s1600/100VrmsB.png" imageanchor="1"><img border="0" height="206" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjGWgM_qdCcAuBe3jjyyuRVHfZJZ9JvxtaR4-z5ZJJPd_3OzXN88IBdueiUyS0iXPcrQgtuA8oczaVjUeMyBRsTR1ABPHEk6dbPjS-jPRP4e5DJQA5mNJ6tC4zixqdKHy37Fj2Fd1zOigU/s400/100VrmsB.png" width="400" /></a><br />
<b>100Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjYBuyKAxvhxqFIYtjXSEz4MqZNqA2W68ChwoGJBKkhMpIEAhP6ft5QlB_7ehrmyksEE4TQAyx7UiDMhBVUp5RAuZBRm99R1ofkcqqUO3Gw8PJnK-KgS-bnrVcVLwWud0XcJQ1-Zdm3_E/s1600/160VrmsB.png" imageanchor="1"><img border="0" height="206" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjYBuyKAxvhxqFIYtjXSEz4MqZNqA2W68ChwoGJBKkhMpIEAhP6ft5QlB_7ehrmyksEE4TQAyx7UiDMhBVUp5RAuZBRm99R1ofkcqqUO3Gw8PJnK-KgS-bnrVcVLwWud0XcJQ1-Zdm3_E/s400/160VrmsB.png" width="400" /></a><br />
<b>160Vrms</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUc_j07l8UAL6Q0wCmhtbTyA-KLdtGAcLjT5sg6DOzYuuHB19_uvMWjnGR0AeNViOvOReqxRB9DDZB37vSR3ld4wEcr9x-WRh-8omQc6MlqaLGaibccxemqm8UP4eYiO0xwxjYuH5jABw/s1600/200VrmsB.png" imageanchor="1"><img border="0" height="206" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUc_j07l8UAL6Q0wCmhtbTyA-KLdtGAcLjT5sg6DOzYuuHB19_uvMWjnGR0AeNViOvOReqxRB9DDZB37vSR3ld4wEcr9x-WRh-8omQc6MlqaLGaibccxemqm8UP4eYiO0xwxjYuH5jABw/s400/200VrmsB.png" width="400" /></a><br />
<b>200Vrms</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiawo3Afk0HNAJT4S0uBmtGAILBn4mgFUxA5pO9rnto2M4vnCuY2-za_rPSH6ftOMlBIdPYPQlnQlC79hW0fm8ALelyapsCcVNc-8xsCBYEMM3K8MiDnfbI6K5AwYuZbpDM86FVPXIelnE/s1600/250VrmsB.png" imageanchor="1"><img border="0" height="207" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiawo3Afk0HNAJT4S0uBmtGAILBn4mgFUxA5pO9rnto2M4vnCuY2-za_rPSH6ftOMlBIdPYPQlnQlC79hW0fm8ALelyapsCcVNc-8xsCBYEMM3K8MiDnfbI6K5AwYuZbpDM86FVPXIelnE/s400/250VrmsB.png" width="400" /></a><br />
<b>250Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkWUr_eSRcT-OZNRcBzx7xAN0T0WF0ggcmoE5sBnI_9VjRC1gWW_1V4mxJq3uYMymYfUvB61HujTys3DWjLkfhI1gIQuOdKQTOQcUpdgXLwaQ12CsXp_RIAXuLaVRMKiYpf72dtID88B0/s1600/300VrmsB.png" imageanchor="1"><img border="0" height="207" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkWUr_eSRcT-OZNRcBzx7xAN0T0WF0ggcmoE5sBnI_9VjRC1gWW_1V4mxJq3uYMymYfUvB61HujTys3DWjLkfhI1gIQuOdKQTOQcUpdgXLwaQ12CsXp_RIAXuLaVRMKiYpf72dtID88B0/s400/300VrmsB.png" width="400" /></a><br />
<b>300Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiWo20LVUE4_PErhkRhaigGM8_66HjGNKO4mV6r0VrhB3CAVGDmSPELZvQJBz9uyoLzcLGi2CjjC7dHW4Cd657V5pcbKoyU5xUy-0qkxiRiNI7lFKYpmWN2HoVVZ9YU_o0i_Sku_qtHyxM/s1600/323VrmsB.png" imageanchor="1"><img border="0" height="205" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiWo20LVUE4_PErhkRhaigGM8_66HjGNKO4mV6r0VrhB3CAVGDmSPELZvQJBz9uyoLzcLGi2CjjC7dHW4Cd657V5pcbKoyU5xUy-0qkxiRiNI7lFKYpmWN2HoVVZ9YU_o0i_Sku_qtHyxM/s400/323VrmsB.png" width="400" /></a><br />
<b>325Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUpcE9d3yiFF51TZ5KmRNtjKPC-Iczj8BICivSSyyq_W3MqlNvctzJCqepRvs07Kyxu6o39hcG1_I-xcE9htWGf_qyXn-zxNzChi-EwuAW2rSynVzj6UtZbTwF3mQi6xTYBKzbPz_jnPk/s1600/sqw12pF.jpg" imageanchor="1"><img border="0" height="195" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUpcE9d3yiFF51TZ5KmRNtjKPC-Iczj8BICivSSyyq_W3MqlNvctzJCqepRvs07Kyxu6o39hcG1_I-xcE9htWGf_qyXn-zxNzChi-EwuAW2rSynVzj6UtZbTwF3mQi6xTYBKzbPz_jnPk/s400/sqw12pF.jpg" width="400" /></a><br />
<b>10kHz Square Wave</b><br />
<br />
Here are some earlier tests on an EL84 with the plate idle voltage set to 225V and 100V on the screen:<br />
<br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgROR9LWnbUmukdClaLSO6K87j7iKvrAyzK1c4sz7HgMbhbFT53B9hZ3j2PQVsJexi8pWlijB3pp8UdFSkrzUJ1tNrqF4rWBnb0fRaDsc8FlwXcgWQCbujl8bq5r5GYgzbbyx1HwhK9rUA/s1600/EL84_10.7VrmsOut.png" imageanchor="1"><img border="0" height="197" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgROR9LWnbUmukdClaLSO6K87j7iKvrAyzK1c4sz7HgMbhbFT53B9hZ3j2PQVsJexi8pWlijB3pp8UdFSkrzUJ1tNrqF4rWBnb0fRaDsc8FlwXcgWQCbujl8bq5r5GYgzbbyx1HwhK9rUA/s400/EL84_10.7VrmsOut.png" width="400" /></a><br />
<b>10Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgTYmxERhospqlhVLFT7IHceUXg5xlyXZtc8ogVJR-PJGEaxqQebp2ER0aKF0GR_RWGdEgtZBrLpqkMRrPpY9VA4wDQMePfTumzmOSs-9IHJbNFaZ0w-z6TSCD-ackUmYgCmUbVbKstIWg/s1600/EL84_21.6VrmsOut.png" imageanchor="1"><img border="0" height="198" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgTYmxERhospqlhVLFT7IHceUXg5xlyXZtc8ogVJR-PJGEaxqQebp2ER0aKF0GR_RWGdEgtZBrLpqkMRrPpY9VA4wDQMePfTumzmOSs-9IHJbNFaZ0w-z6TSCD-ackUmYgCmUbVbKstIWg/s400/EL84_21.6VrmsOut.png" width="400" /></a><br />
<b>20Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEio-TjENtYdeZ1cD9bxBWeUPotgfVGCBa-WF55v0rw-vwiHhMEK5g0pvbtXemKseDNBAIverus8uNPQQdZS2cEIT-5dQY-Gt4Pbjxh53OyMDTDQ4EIvW3WG0vUU0QR2TrciDWA3oVuxVGc/s1600/EL84_30.6VrmsOut.png" imageanchor="1"><img border="0" height="198" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEio-TjENtYdeZ1cD9bxBWeUPotgfVGCBa-WF55v0rw-vwiHhMEK5g0pvbtXemKseDNBAIverus8uNPQQdZS2cEIT-5dQY-Gt4Pbjxh53OyMDTDQ4EIvW3WG0vUU0QR2TrciDWA3oVuxVGc/s400/EL84_30.6VrmsOut.png" width="400" /></a><br />
<b>30Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdwE4KRIf_XYpIGbzrvyYvYZ_W9VexMUTYT8nZEWBLfbAHLKrLM35hnYb09qYbwsi5p5MD7BbhZ5vSeg1aknaygtGi7ZGYni2mVzRfLpDiv_IyrA25Khrdy6pOrwSFX2C3HlQJaV207-M/s1600/EL84_50.7VrmsOut.png" imageanchor="1"><img border="0" height="198" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdwE4KRIf_XYpIGbzrvyYvYZ_W9VexMUTYT8nZEWBLfbAHLKrLM35hnYb09qYbwsi5p5MD7BbhZ5vSeg1aknaygtGi7ZGYni2mVzRfLpDiv_IyrA25Khrdy6pOrwSFX2C3HlQJaV207-M/s400/EL84_50.7VrmsOut.png" width="400" /></a><br />
<b>50Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0BmVabAa7Rr1vm-VMBCaxL5Ag-ahLKF_D4Bp4kI1T7dkgg3pQB_Fgf8SkPnKJNZQrxEwVGmC0II47EYnHHPFdFdew1FmAT6PbHMtce-vErIjuQhaOGN0Hd_6_blDQ1UmuvWME65MbV98/s1600/EL84_75.4VrmsOut.png" imageanchor="1"><img border="0" height="196" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0BmVabAa7Rr1vm-VMBCaxL5Ag-ahLKF_D4Bp4kI1T7dkgg3pQB_Fgf8SkPnKJNZQrxEwVGmC0II47EYnHHPFdFdew1FmAT6PbHMtce-vErIjuQhaOGN0Hd_6_blDQ1UmuvWME65MbV98/s400/EL84_75.4VrmsOut.png" width="400" /></a><br />
<b>75Vrms</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPl_TZbU4ELNNr4n2noIqwx2vStUrsHXna4d7f1zYqxQKsVSP3_Q1gkf5JUnsA0HKZEQH0C_11H0X0YAPJacsheN8PeSt31LAP8rylxCAPVCGJIJudQpiUOhHthH0viNtyivVZtGNqocI/s1600/EL84_101.1VrmsOut.png" imageanchor="1"><img border="0" height="196" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgPl_TZbU4ELNNr4n2noIqwx2vStUrsHXna4d7f1zYqxQKsVSP3_Q1gkf5JUnsA0HKZEQH0C_11H0X0YAPJacsheN8PeSt31LAP8rylxCAPVCGJIJudQpiUOhHthH0viNtyivVZtGNqocI/s400/EL84_101.1VrmsOut.png" width="400" /></a><br />
<b>100Vrms</b><br />
<b><br /></b>
I think that I could have gotten the EL84 distortion down more. I performed the EL84 testing before I discovered that lowering screen voltage reduces distortion. I will revisit that testing when I get a chance.Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-58950012069632704972017-03-24T15:29:00.000-07:002017-04-24T17:19:44.386-07:00P-Channel Shunt Feedback KT88 Amp RevisitedThis is Part 2 of a series on an amp I designed and built. Click <a href="http://tubeswithatwist.blogspot.com/2015/05/a-push-pull-amplifier-with-simple-plate.html">here</a> for Part 1.<br />
<br />
It has been a couple of years since I built and delivered an amplifier to my brother. I thought it was a pretty novel, simple and effective circuit. I was in a rush to give it to him as a Christmas present, so I didn't get all of the time that I wanted to explore and optimize the topology.<br />
<br />
Well, it seems that it broke and so I took it back to repair it. The problems were determined to be inadequate heat-sinking on some SMT power resistors. I fixed the problem and decided to spend some time testing, optimizing, and upgrading. For reference, here is the schematic of what I initially delivered:<br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhzgwwCHQV8E4kIoEWZN9jUGSe1ABUw3b7qRud8YKZhbnRkBvNZNqS8OdGowIo5mGc-DL9IhBOKLPRsG8HLsM3CeFnC5s6O0ncytMeJLHdtdxrb8y5agKoFK0Ns4AFZBeQvT58p7ww8K0U/s1600/AudioClark.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="255" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhzgwwCHQV8E4kIoEWZN9jUGSe1ABUw3b7qRud8YKZhbnRkBvNZNqS8OdGowIo5mGc-DL9IhBOKLPRsG8HLsM3CeFnC5s6O0ncytMeJLHdtdxrb8y5agKoFK0Ns4AFZBeQvT58p7ww8K0U/s400/AudioClark.png" width="400" /></a></div>
<br />
The first measurement I made was output impedance at a few different output tube idle currents (450V B+). Here are the results:<br />
<br />
3.7 Ohms @ 50mA<br />
3.3 Ohms @ 60mA<br />
3.1 Ohms @ 70mA<br />
<div>
<div>
<br /></div>
<div>
Those are a bit higher than I liked. I also noticed that the 10k SMT resistor in the output tube feedback network was probably running a little hot for the heat-sinking that the board offered. I had also experimented with similar feedback on EL34s and had found that a 100k feedback resistor should be more than adequate in this position, saving some heat, so I did some analysis and decided to decrease the current in the feedback network and increase the feedback from 20% to 30%. The -388V rail for the p-channel fet should have no problem supporting that and the 6BL7 should have adequate voltage swing as well. The amp also has more gain than desired so this increased local feedback should be good all around. Hopefully, this would get output impedance down somewhere close to 2 Ohms as well. The Hammond 1650R output transformers have a primary DC resistance of 97.8 Ohms and a secondary resistance of 0.21 Ohms, so amplifier Zout is still dominated by effective output tube rp and not high DCRs in the output transformer. </div>
<div>
<br /></div>
<div>
Other changes include modifying the input to accept balanced signals and adding a bias servo to keep output tube idle currents precisely controlled at all times. Here is the modified schematic:</div>
<div>
<br /></div>
<div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi40zS-7wm9HQjgNbSfatTVGi7uLe-ZiQQ0KHF6C19FrrN7km2Spdrj6iXbWmguYYRlvzBboZoSxkl7h6o8YR643erAZnEpSKVsEJSjwJ_vIrySTixoVdH8gTE_Ciyx6iey6CDfapSGCPg/s1600/ClarkAmp2.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="227" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi40zS-7wm9HQjgNbSfatTVGi7uLe-ZiQQ0KHF6C19FrrN7km2Spdrj6iXbWmguYYRlvzBboZoSxkl7h6o8YR643erAZnEpSKVsEJSjwJ_vIrySTixoVdH8gTE_Ciyx6iey6CDfapSGCPg/s400/ClarkAmp2.png" width="400" /></a></div>
<br />
After changing the feedback resistors, the Zout were measured as:<br />
<br />
2.9 Ohms @ 50mA<br />
2.4 Ohms @ 60mA<br />
2.2 Ohms @ 70mA</div>
</div>
<div>
<br /></div>
<div>
I took detailed distortion measurements at many power levels at all three idle currents and was surprised at how little variation there was, so I decided to bias the output tubes at 60mA. It seemed to be a good balance of lowering Zout but getting long output tube life. Here are some of the distortion spectra:</div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgUsVs717jg-qJogFITPjQeqYTFWWyl5HwGoTz84zxH2lz_OGSXAvBHN7H1EybCoAduLp0r7ChZAj1_pJ8mbMH8xlGx4RcwbIB75OpGgO9xOXqb4EYwe6uLKqoedE94sOg6-RNO9eVHisU/s1600/100mW60mA.png" imageanchor="1"><img border="0" height="192" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgUsVs717jg-qJogFITPjQeqYTFWWyl5HwGoTz84zxH2lz_OGSXAvBHN7H1EybCoAduLp0r7ChZAj1_pJ8mbMH8xlGx4RcwbIB75OpGgO9xOXqb4EYwe6uLKqoedE94sOg6-RNO9eVHisU/s400/100mW60mA.png" width="400" /></a></div>
<div>
<b>100mW</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizosaEJtZHpFKA1XqFV8ezwQqvua8bUE3IIqiW823Wr5B9KTHwW9SmiZUQVmDmhFkCYLi_2piyw0XJrBCE0IUSKrzZi-luI2nXUmC2xhyphenhyphenHqW2W0-a7gMN1AisbR4ab4RbEit-BGyQ-_aQ/s1600/500mW60mA.png" imageanchor="1"><img border="0" height="191" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizosaEJtZHpFKA1XqFV8ezwQqvua8bUE3IIqiW823Wr5B9KTHwW9SmiZUQVmDmhFkCYLi_2piyw0XJrBCE0IUSKrzZi-luI2nXUmC2xhyphenhyphenHqW2W0-a7gMN1AisbR4ab4RbEit-BGyQ-_aQ/s400/500mW60mA.png" width="400" /></a></div>
<div>
<b>500mW</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifRmZ-aiPFgEBxYp-SqFiK09XYrt-UsoWNwLq_NGnLR7PSNLu30SAqqwhonUPpdkuO_lcbyRqWxBgDhXNbCi0DzKmjEbAZsHKLLIJXC6kOs8tTbf30c7nNArghmMYz8UeqVY1XXp0MQow/s1600/1W60mA.png" imageanchor="1"><img border="0" height="190" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifRmZ-aiPFgEBxYp-SqFiK09XYrt-UsoWNwLq_NGnLR7PSNLu30SAqqwhonUPpdkuO_lcbyRqWxBgDhXNbCi0DzKmjEbAZsHKLLIJXC6kOs8tTbf30c7nNArghmMYz8UeqVY1XXp0MQow/s400/1W60mA.png" width="400" /></a></div>
<div>
<b>1W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgN3G5ELu3TDtY3nu3dbfbpQtpxxz0lF7E-VYGhkpNu11861eIM4Ua9PUVH4v3Rz0OMSYZ8-nQDud8F_55HvswWTXIVpJ9GJ6oJtKgre6iSizfQaBMHTj2SC_5u_3gxYrWTcJN30OZ3e8/s1600/2W60mA.png" imageanchor="1"><img border="0" height="191" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhgN3G5ELu3TDtY3nu3dbfbpQtpxxz0lF7E-VYGhkpNu11861eIM4Ua9PUVH4v3Rz0OMSYZ8-nQDud8F_55HvswWTXIVpJ9GJ6oJtKgre6iSizfQaBMHTj2SC_5u_3gxYrWTcJN30OZ3e8/s400/2W60mA.png" width="400" /></a></div>
<div>
<b>2W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiAJLQ-X1HPw6Xv98h4HAA8PPT8BQiQHJH9fxogRHXFadzcN7UO636bnePi5JZEuDSqldEA7ACMMTMB4B0CNcQGskBy5-mKBNNISPKwmdhWMdSNi5YdHtjd0mKAr29gyZWlAXkV5oMSq4g/s1600/5W60mA.png" imageanchor="1"><img border="0" height="190" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiAJLQ-X1HPw6Xv98h4HAA8PPT8BQiQHJH9fxogRHXFadzcN7UO636bnePi5JZEuDSqldEA7ACMMTMB4B0CNcQGskBy5-mKBNNISPKwmdhWMdSNi5YdHtjd0mKAr29gyZWlAXkV5oMSq4g/s400/5W60mA.png" width="400" /></a></div>
<div>
<b>5W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0HF7In2D_sQwjmqtMPT1R6138de1EB1gn_mKqZeWXHsMvmwCANVORRoAqUShYOzK51xWzdMu8y2Dh0t80DDNmT7HqBQXnXc9AOtOTObqdWJ4vgZB83uGvar4SaNrYl_xBP-QKBQnyBjY/s1600/10W60mA.png" imageanchor="1"><img border="0" height="190" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0HF7In2D_sQwjmqtMPT1R6138de1EB1gn_mKqZeWXHsMvmwCANVORRoAqUShYOzK51xWzdMu8y2Dh0t80DDNmT7HqBQXnXc9AOtOTObqdWJ4vgZB83uGvar4SaNrYl_xBP-QKBQnyBjY/s400/10W60mA.png" width="400" /></a></div>
<div>
<b>10W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhE8ph-3eeqa48lVSv9y5jSoGqJOtWn7jvEOK3VI0GYUYVlQ96-DZ6Sxqd-NOVyr6v2_JzWGTlhY5YGzVFfLf8qgMaC4XsMKIkfI8ah8ql5yDKzE2NcD9uuAMpidtJRGRrkwJ6GJSOgwyk/s1600/20W60mA.png" imageanchor="1"><img border="0" height="190" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhE8ph-3eeqa48lVSv9y5jSoGqJOtWn7jvEOK3VI0GYUYVlQ96-DZ6Sxqd-NOVyr6v2_JzWGTlhY5YGzVFfLf8qgMaC4XsMKIkfI8ah8ql5yDKzE2NcD9uuAMpidtJRGRrkwJ6GJSOgwyk/s400/20W60mA.png" width="400" /></a></div>
<div>
<b>20W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjfhK_cKUToSCDwnWH0nKtnF_fHk3R2LL_D9QpU7UCELvGyOo8OObP9AI9MEUDkkVQ8pOz-FhY0Y9_alGaPjKsXyYcjMJQO7kvv6876uk8nNMH25TKD5pC91zbQ0l7vgwdhsVfAuUmbLM8/s1600/40W60mA.png" imageanchor="1"><img border="0" height="191" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjfhK_cKUToSCDwnWH0nKtnF_fHk3R2LL_D9QpU7UCELvGyOo8OObP9AI9MEUDkkVQ8pOz-FhY0Y9_alGaPjKsXyYcjMJQO7kvv6876uk8nNMH25TKD5pC91zbQ0l7vgwdhsVfAuUmbLM8/s400/40W60mA.png" width="400" /></a></div>
<div>
<b>40W</b></div>
<div>
<br /></div>
<div>
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwaDhIE5E6S9t3lSwiaEGt7g2LAQwoiueQbGlTvVZvyS6R3Qqmws9VXbOAFBLnxqfSmLVuK6R74J49VTqCsq7O3uLp13I9dRm4x5qSlsvc_C-EEXMYMv72138nN4IVp-pcSvTs9HBTUlI/s1600/50W60mA.png" imageanchor="1"><img border="0" height="191" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwaDhIE5E6S9t3lSwiaEGt7g2LAQwoiueQbGlTvVZvyS6R3Qqmws9VXbOAFBLnxqfSmLVuK6R74J49VTqCsq7O3uLp13I9dRm4x5qSlsvc_C-EEXMYMv72138nN4IVp-pcSvTs9HBTUlI/s400/50W60mA.png" width="400" /></a></div>
<div>
<b>50W</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDYgFFxPLFfqYM18aso6ycRSwx6E4pjVkevK2ImKbGw94nAOSQw_HqXWPeFv-3s7dvrFBsv5qNVJ4sAbucKwEkbfOEPLxN5IO0v861VIo-2LPZT1VmKZQjiohyphenhyphensrwrh0dZwWkKAnQlmsA/s1600/20Hzsquare.BMP" imageanchor="1"><img border="0" height="195" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDYgFFxPLFfqYM18aso6ycRSwx6E4pjVkevK2ImKbGw94nAOSQw_HqXWPeFv-3s7dvrFBsv5qNVJ4sAbucKwEkbfOEPLxN5IO0v861VIo-2LPZT1VmKZQjiohyphenhyphensrwrh0dZwWkKAnQlmsA/s400/20Hzsquare.BMP" width="400" /></a><br />
<b>20Hz square wave</b><br />
<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNDBMTc3-Zj7-BdjFj4jz7CPAS3gSRX2cG26yuAFyg-XQ8GbvVrNFChftkIBsYUR-W5-WEk5vCL8lz8aBk05A2hl297z1HrD2NJQjHn_gheTaX5b6xnn_hoileOU7ximriH7HQ3_PnAoM/s1600/1kHzsquare.BMP" imageanchor="1"><img border="0" height="195" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNDBMTc3-Zj7-BdjFj4jz7CPAS3gSRX2cG26yuAFyg-XQ8GbvVrNFChftkIBsYUR-W5-WEk5vCL8lz8aBk05A2hl297z1HrD2NJQjHn_gheTaX5b6xnn_hoileOU7ximriH7HQ3_PnAoM/s400/1kHzsquare.BMP" width="400" /></a><br />
<b>1kHz square wave</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6x-l1ZqDHqSm4MlxBe6keo3KEoqjtfjVNkhPehCW533t5Biwi89TMMppIiFUefTu5tMmvuUh4XODn7cXWcxFUHt4hJl6B7EKEIsVbRCat_MpEdfz6FtRpNKH386tHgkigRUcfGu3UAWY/s1600/10kHzsquare.BMP" imageanchor="1"><img border="0" height="195" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6x-l1ZqDHqSm4MlxBe6keo3KEoqjtfjVNkhPehCW533t5Biwi89TMMppIiFUefTu5tMmvuUh4XODn7cXWcxFUHt4hJl6B7EKEIsVbRCat_MpEdfz6FtRpNKH386tHgkigRUcfGu3UAWY/s400/10kHzsquare.BMP" width="400" /></a><br />
<b>10kHz square wave</b><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiXoXVO3hb5mZuH7dpdOyyshuzYD6lWKtyvFF1Xy5ATEzoVJCtagnBd1iEPGjzc9O3v47w20onQCnyjg2xlxCTgKauFVDFywfWdO7U4yENBLE-dRXCyv3YTd7k0AKyqEY3EGuEo5Z7I-jk/s1600/IMG_0589.JPG" imageanchor="1"><img border="0" height="266" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiXoXVO3hb5mZuH7dpdOyyshuzYD6lWKtyvFF1Xy5ATEzoVJCtagnBd1iEPGjzc9O3v47w20onQCnyjg2xlxCTgKauFVDFywfWdO7U4yENBLE-dRXCyv3YTd7k0AKyqEY3EGuEo5Z7I-jk/s400/IMG_0589.JPG" width="400" /></a><br />
<b>The finished product.</b></div>
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Overall, I think this is a great result on this amp. I think it's a pretty sound approach.</div>
Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-72156822356641228842017-01-27T09:40:00.001-08:002017-01-27T09:40:30.640-08:00A Test Amplifier for Tube CircuitsHave you ever just needed to test a driver-type tube stage but your sound card output can't put out enough voltage drive to get it where it needs to go? Well I have, and here is how I solved my problems.<br />
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I searched around and looked at high voltage op amps and found a few candidates. The one with the lowest distortion and noise specs that I could find was the ADA4700. I built a simple non-inverting stage with a gain of 20 and a 100k feedback resistor, powered by a couple of medical isolated 48V wall-wart power supplies (Meanwell GSM25U48-P1J) and got some pretty good performance. See below:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi_GhhYBQMh9pGzXQlJRugvOR9-EeTNpDWJ0_vcHL_H8uCNC3W33Oc9w802DXmAqJZD2111NQD8QvMfsZEHBYNv62UzPSiA5NPFOMIMZxpbgU6tnXLYcl_JQrwVfLqXMng8ffp2GA077kY/s1600/SeriesADA4700_10V_FullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="152" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi_GhhYBQMh9pGzXQlJRugvOR9-EeTNpDWJ0_vcHL_H8uCNC3W33Oc9w802DXmAqJZD2111NQD8QvMfsZEHBYNv62UzPSiA5NPFOMIMZxpbgU6tnXLYcl_JQrwVfLqXMng8ffp2GA077kY/s320/SeriesADA4700_10V_FullBW.png" width="320" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqCo1OnTO-xcFdRIL_aYG9vgPAnJn54BS5r24tiJHkFEbtrg921EjnWh7egfidYikjJ84vlSg2rD5T8dgitQaMs_kr8m1YUWCvzav851LCSkgbnu48CTwzHRo6ExmGSA58ozeYzRqFIrE/s1600/SeriesADA4700_20VFullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="154" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqCo1OnTO-xcFdRIL_aYG9vgPAnJn54BS5r24tiJHkFEbtrg921EjnWh7egfidYikjJ84vlSg2rD5T8dgitQaMs_kr8m1YUWCvzav851LCSkgbnu48CTwzHRo6ExmGSA58ozeYzRqFIrE/s320/SeriesADA4700_20VFullBW.png" width="320" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVwhLU1ogzdqy5S0joC6mhPMN1zj9jIZLtTIVd_s1hyAUizGsrb6liYr3BGCSQ5tcLRk7kVwwEu0YC-1wQllI1Dnf80xiBDPah-ZAF_KQXXOrzMbHm58W9tAnmz08Jqz691fh19tfdVdk/s1600/SeriesADA4700_30VFullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="152" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVwhLU1ogzdqy5S0joC6mhPMN1zj9jIZLtTIVd_s1hyAUizGsrb6liYr3BGCSQ5tcLRk7kVwwEu0YC-1wQllI1Dnf80xiBDPah-ZAF_KQXXOrzMbHm58W9tAnmz08Jqz691fh19tfdVdk/s320/SeriesADA4700_30VFullBW.png" width="320" /></a></div>
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The above results are for 10Vrms, 20Vrms, and 30Vrms respectively. I was pretty happy with that performance, but I was getting more distortion than I expected from reading the ADA4700 datasheet.<br />
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Later, I was reading a small signal audio design book by Douglas Self and he brought up the point that some (most?) op amps suffer from much higher distortion when used in a series-feedback configuration than if used in a shunt-feedback configuration. He notes that the LM4562 is a notable exception.<br />
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I wanted to have the high input impedance of a series feedback configuration but the low distortion of shunt-feedback so I decided to try a two-stage version with an LM4562 inverting amplifier as the input stage and the ADA4700 inverting amp to get the voltage output up to 30Vrms. Particularly, I wanted to minimize higher-order harmonics so that I could recognize them in the tube stages that I wanted to test.<br />
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Here is the schematic of what I came up with:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3GFWAp2Gj57aTn8TYhvH-Q3dRjXOL9SNEvr7-j_bHdr2BWMvewL6nxJKaa-7teJ_PKwZMM0txj1jM3dkgRqXSV8cjy93GcQC8E3YXquAb3SBtU9SzbN2mbxFY6YI2qNbY94dPOE1WfBg/s1600/IMG_20170126_200714125.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="180" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3GFWAp2Gj57aTn8TYhvH-Q3dRjXOL9SNEvr7-j_bHdr2BWMvewL6nxJKaa-7teJ_PKwZMM0txj1jM3dkgRqXSV8cjy93GcQC8E3YXquAb3SBtU9SzbN2mbxFY6YI2qNbY94dPOE1WfBg/s320/IMG_20170126_200714125.jpg" width="320" /></a></div>
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I had the idea that I could put filter capacitors across the feedback resistors and that might have some positive effect on suppressing higher order harmonics. I added a bit of gain to make up for the loss and set the -3dB point at 1kHz for the filtering of each stage. Here is a picture of the circuit as built:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh34E2qYGrPmKuca5dRK1ljJwKkrIV0gfPdjLAF31mzgvqpMwykCSNEl2P_d9IAiCjXSwPoSdI8VCf8IfvBk0i1bM8Rl0FNwtMyVgeJDQWaNvKbL5on9uFMnNg2OxFljfjQoyNscdpaUKA/s1600/IMG_20170126_201954862.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="180" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh34E2qYGrPmKuca5dRK1ljJwKkrIV0gfPdjLAF31mzgvqpMwykCSNEl2P_d9IAiCjXSwPoSdI8VCf8IfvBk0i1bM8Rl0FNwtMyVgeJDQWaNvKbL5on9uFMnNg2OxFljfjQoyNscdpaUKA/s320/IMG_20170126_201954862.jpg" width="320" /></a></div>
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The ADA4700 only comes in a surface-mount package with a thermal pad on the bottom so I bought a prototype adapter, bent the leads, and flipped it on its back. I make a makeshift heatsink out of a couple of pieces of solid-core wire. I don't think this is really needed at the loads I am driving but it was easy to do, so I did it. I stuck with a surface-mount LM4562 because it fit well right next to the ADA4700 on the prototype board. The filter capacitors are placed in parallel to the feedback resistors and easily soldered/unsoldered on one side to take them in or out of the circuit. How does it perform?<br />
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First, here is what the distortion of my test setup looks like (including an M-Audio M-trak II plus and a high-impedance input version of Pete Millett's soundcard interface):<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi5vx8QIKwNUlDTnO4UMx5PpQmnRLeu9AVDv-PA4SXkGyjvui3Phb8I7wEooDeR6BsO8WPBk66oEQwZMhEsVRYWUyY6CnrJ_nbaJaEBWMIhASHIOuK0an50JfFl1stKUezQnBW-X6XE39o/s1600/milletloopback2V.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="152" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi5vx8QIKwNUlDTnO4UMx5PpQmnRLeu9AVDv-PA4SXkGyjvui3Phb8I7wEooDeR6BsO8WPBk66oEQwZMhEsVRYWUyY6CnrJ_nbaJaEBWMIhASHIOuK0an50JfFl1stKUezQnBW-X6XE39o/s320/milletloopback2V.png" width="320" /></a></div>
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Here is distortion of the LM4562/ADA4700 circuit at 10Vrms, 20Vrms, and 30Vrms output, without the filter capacitors in circuit:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwphSzd0pl6PTJyDvaAF6CrLcl8atiOv7EVr8cPn9OoaRqNx4XDTsGsPOKfpT2tPswPYRM2KFZ7wWvfqkbqz5poeQYVoSHvNI1XJkGrYCROKoMR_rHKO4hMrv8xloV4s02Xatmz7SKHiI/s1600/ParADA4700_10VFullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="154" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwphSzd0pl6PTJyDvaAF6CrLcl8atiOv7EVr8cPn9OoaRqNx4XDTsGsPOKfpT2tPswPYRM2KFZ7wWvfqkbqz5poeQYVoSHvNI1XJkGrYCROKoMR_rHKO4hMrv8xloV4s02Xatmz7SKHiI/s320/ParADA4700_10VFullBW.png" width="320" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjPnLEecNQ57Vv4GqZQAoKI5n5GKpRi1m7cGRxee-ZT1bdG4DYNu_OoD58LZezvDtSL_rPHrvGPc4OwqjOUjQdeImaic8F9AAWVe-R1u-Vi8ge7dTJJdCbpv1puE9x213a0reVX4JvOToY/s1600/ParADA4700_20VFullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="152" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjPnLEecNQ57Vv4GqZQAoKI5n5GKpRi1m7cGRxee-ZT1bdG4DYNu_OoD58LZezvDtSL_rPHrvGPc4OwqjOUjQdeImaic8F9AAWVe-R1u-Vi8ge7dTJJdCbpv1puE9x213a0reVX4JvOToY/s320/ParADA4700_20VFullBW.png" width="320" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLPA4kNad9UOOmrYYlhFfwjlzGUKfe9bGg0HVbXlxUST-SmHIaQ7rlyKj40Kjd9RaucqbV0LsOyK0-pqomeuf_zX1qx2gMNjpm51Ej7fobT6D7mw0KLdHhBZEaxpaBCS2wSemkBw4zGoI/s1600/ParADA4700_30VFullBW.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="152" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhLPA4kNad9UOOmrYYlhFfwjlzGUKfe9bGg0HVbXlxUST-SmHIaQ7rlyKj40Kjd9RaucqbV0LsOyK0-pqomeuf_zX1qx2gMNjpm51Ej7fobT6D7mw0KLdHhBZEaxpaBCS2wSemkBw4zGoI/s320/ParADA4700_30VFullBW.png" width="320" /></a></div>
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I was happy with the reduction in distortion of the new circuit. Let's see what happens with the filter capacitors in:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCTOmNll2hMlMc5K4D_R6Eeq68xqCsbnjpMcq91S6KGW4RPumD_4lRhzFnOH2kGyIOAKPSVdN9GXMTSyIRhvrN5f-RqpUwHbMDf7idSYoeOLVD1yM5iwMoiFFO3gIVzkdRAsB4xDFkZ_M/s1600/ParADA4700_10VFilter.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="153" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCTOmNll2hMlMc5K4D_R6Eeq68xqCsbnjpMcq91S6KGW4RPumD_4lRhzFnOH2kGyIOAKPSVdN9GXMTSyIRhvrN5f-RqpUwHbMDf7idSYoeOLVD1yM5iwMoiFFO3gIVzkdRAsB4xDFkZ_M/s320/ParADA4700_10VFilter.png" width="320" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_C-oVDo6cgpVTfAuA5X0h1BuMogZvhhbV4TgMpUYDN846fE0C7nRQtCIZvZYoSZbtL76pDhxaqIaWpp1qhVJ20XbkKQGEp9O8jgHtGyQZ9jTiFZfGlLYViAccPLug-C0XBawq32cvbHk/s1600/ParADA4700_20VFilter.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="153" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh_C-oVDo6cgpVTfAuA5X0h1BuMogZvhhbV4TgMpUYDN846fE0C7nRQtCIZvZYoSZbtL76pDhxaqIaWpp1qhVJ20XbkKQGEp9O8jgHtGyQZ9jTiFZfGlLYViAccPLug-C0XBawq32cvbHk/s320/ParADA4700_20VFilter.png" width="320" /></a></div>
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The first thing I noticed is that the noise floor is significantly lower at frequencies above mid-band. This could be helpful in detecting small levels of higher order harmonics in testing. As you can see, THD is also slightly lower. The main reduction comes from suppression of 2nd harmonic. Third and higher harmonics seem to come up a little bit. Noise floor seems unaffected in the 30Vrms plot but that is because I have to switch ranges on my Pete Millett sound card interface to a setting with a higher noise floor. The sound card interface becomes the dominant noise source in that plot.<br />
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I think I will stick with the unfiltered configuration mostly but I can see situations arising where I might want to put the capacitors back into the circuit.Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com0tag:blogger.com,1999:blog-2914493245344265727.post-24760028205665609512015-05-09T21:36:00.000-07:002015-05-09T21:40:39.185-07:00A Push-Pull Amplifier with Simple Plate-Grid Voltage Feedback Driven by a P-Channel FETFor this post, I want to go back to some stuff I did in December 2013. Some time before that I read O. H. Schade's paper <i>Beam Power Tubes</i> and was fascinated by the characteristics shown in that paper of what plate-to-grid voltage feedback was able to do to the characteristics of a 6L6. In fact, those characteristics looked more linear than a 300B. A 6L6 could never replace a 300B, because it doesn't have a high enough plate dissipation rating, but the linearity was impressive nonetheless.<br />
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I started to wonder what KT88 (which does have a comparable plate dissipation rating) characteristics would look like with plate-to-grid voltage feedback. I followed the procedure in Schade's paper to plot curves in Excel, which are shown below:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjgflKcGRSA5UQLrCAGZ84kLBLu2NZlUTymw9xNtEbXMi6T8WYNVFL5raBxsFFcG-KILe-W9zEoV4esEk-RHrKusEWnG9mCGag3o9ot-4-xmtBb8ozVRdjwYIVTuDxNjpG2ni8oQsg4868/s1600/curves.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="286" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjgflKcGRSA5UQLrCAGZ84kLBLu2NZlUTymw9xNtEbXMi6T8WYNVFL5raBxsFFcG-KILe-W9zEoV4esEk-RHrKusEWnG9mCGag3o9ot-4-xmtBb8ozVRdjwYIVTuDxNjpG2ni8oQsg4868/s400/curves.jpg" width="400" /></a></div>
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I decided that in order to do a fair comparison between a 300B and the KT88 with feedback, I needed to plot on the same area. In the plot above, the pinkish purple plots are KT88 with feedback with the feedback resistors set for a mu of 5. The Yellow curves are 300B which has a mu of about 3.8. The Cyan curves are KT88 triode connected which has a mu of about 8.<br />
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As you can see, the 300B is more linear and has a lower rp than a triode-connected KT88 but has a lower mu. But the thing that is really neat is that the KT88 with plate-grid feedback has higher mu, lower rp, and better linearity than the 300B. We have the KT88's high transconductance to thank for that. It also has the benefit of being able to reach saturation without driving the grid positive, so we can drive the plate to a much lower voltage without needing a driver to supply grid current. All of the pinkish purple curves represent negative grid voltages. Needless to say, this got the wheels spinning in my head. I was considering building a 300B amp some day but they are kind of expensive tubes. KT88s are cheap by comparison.<br />
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Now I had to figure out the best way to apply the feedback. Most of the practical designs involved capacitive coupling of some sort. I really like to keep anything that goes to the grid of a power tube direct-coupled if possible so that if the amp is over-driven, there will be no cap to charge and cause blocking distortion. There are ways to minimize this but I saw an idea in a post one day on the tube amplifier forum at diyaudio.com that showed a totally simple idea of how to apply plate-grid feedback, using a p-channel FET just like this:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkO1-YxzRqp9EL0oytUH__FuQ92V0yY1Tc9HD47DmsTpttCDzUMjEgjoKyqltV38jrgcwiziptTGCwNlwrgorXr2L5uti3NhnYxZtiMPLa8bgs4iXHvUmnHrhG7qooHUB_xl7jMJPtAXI/s1600/Feedback.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkO1-YxzRqp9EL0oytUH__FuQ92V0yY1Tc9HD47DmsTpttCDzUMjEgjoKyqltV38jrgcwiziptTGCwNlwrgorXr2L5uti3NhnYxZtiMPLa8bgs4iXHvUmnHrhG7qooHUB_xl7jMJPtAXI/s400/Feedback.png" width="387" /></a></div>
It was simple and elegant. I was in the process of building a push-pull KT88 amplifier for my brother so I altered the design a bit and came up with this:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjulqvKe2El-J0-xDTMXjoZhi8m8k3zXdJJY-bopTHuO4UEif4n4Nhq8YR6ormgV5iNLWtmv5TVSnqqYlakXqmi2eMmCzh8a-P0em9pk6iDLyT4mTRYAWv6rNuv6jOMbuYnanvv3TX8i0E/s1600/AudioClark.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="256" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjulqvKe2El-J0-xDTMXjoZhi8m8k3zXdJJY-bopTHuO4UEif4n4Nhq8YR6ormgV5iNLWtmv5TVSnqqYlakXqmi2eMmCzh8a-P0em9pk6iDLyT4mTRYAWv6rNuv6jOMbuYnanvv3TX8i0E/s400/AudioClark.png" width="400" /></a></div>
Pictures of the construction/testing:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiFE64lbiZpuD3OR2B8dhVyYuDiNW5EZdPscK8eLvOQiPRAG28uXxqB3PfC13atD8B2FBasECv8zgpQ1GdtWcK8UlMTnfWSCssGVVJCrD0DjaC8ZEUdjQ4wUI2MieAFUmd4izgW2eeV44c/s1600/996064_10201856174958461_1242312272_n.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="225" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiFE64lbiZpuD3OR2B8dhVyYuDiNW5EZdPscK8eLvOQiPRAG28uXxqB3PfC13atD8B2FBasECv8zgpQ1GdtWcK8UlMTnfWSCssGVVJCrD0DjaC8ZEUdjQ4wUI2MieAFUmd4izgW2eeV44c/s400/996064_10201856174958461_1242312272_n.jpg" width="400" /></a></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiI9d2UXCy5Geg4cliHtibR6FWQopuYFEgs757M_QjCZTHZoYs1mqO-XhwqWIonl_peb9jgwQAhUIfyX9NvtOK2bT-Jj85X_RLBynXy4NpN10utCcLh6Md7BRkvsiCPM9fBvUXjGIButnM/s1600/1426296_10201856175518475_291678444_n.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="225" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiI9d2UXCy5Geg4cliHtibR6FWQopuYFEgs757M_QjCZTHZoYs1mqO-XhwqWIonl_peb9jgwQAhUIfyX9NvtOK2bT-Jj85X_RLBynXy4NpN10utCcLh6Md7BRkvsiCPM9fBvUXjGIButnM/s400/1426296_10201856175518475_291678444_n.jpg" width="400" /></a></div>
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I was desperately trying to finish this amp in time to deliver it to my brother as a Christmas present so one of the things I regret is that I didn't take many measurements. I remember it put out almost 50W at clipping but I only saved screen captures of 1W and 10W distortion measurements. This was also the first time using my measurement setup based on Pete Millett's buffer box. I built a high-impedance input version so that I could take measurements with a X10 or X100 scope probe directly on tube plates. It works well but is much more susceptible to noise pickup. I later spent some time calming the noise down in the measurement setup, so please excuse the noise peaks below the fundamental. I also later discovered that my diodes that I was using to protect my sound card input were generating high-order products at higher amplitudes, so some of the high order products in the second plot may not in fact exist.<br />
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Here is distortion at 1W:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3_so9R2rPwHPGneRKppGbLG4isEcSJL0DFhyphenhyphenpJohbc9-jNSZDaq1EwVlW2NzqE_wq33A34X97yDip025OPJWQTZg6MOQZbrcH2v8xMuGhJOfaKiikiG0W7ODD-G0ZZdtmEK-XcIH-S0A/s1600/ClarkDist1WST.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="225" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj3_so9R2rPwHPGneRKppGbLG4isEcSJL0DFhyphenhyphenpJohbc9-jNSZDaq1EwVlW2NzqE_wq33A34X97yDip025OPJWQTZg6MOQZbrcH2v8xMuGhJOfaKiikiG0W7ODD-G0ZZdtmEK-XcIH-S0A/s400/ClarkDist1WST.png" width="400" /></a></div>
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Distortion at 10W:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh6j5aEz6Jkla0-97WywOjLXs8uirJPf7T7SglhanpTgaozGvv0J2xno7K8uL1XBY-QGXpcQK4Zz3TOZLlBUCanxfrd0-C0BLsiClFGTybnssGQ5xWKIG9GRZbJ3eS8hwnvVRdrHGyzKf0/s1600/ClarkDist10WST.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="225" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh6j5aEz6Jkla0-97WywOjLXs8uirJPf7T7SglhanpTgaozGvv0J2xno7K8uL1XBY-QGXpcQK4Zz3TOZLlBUCanxfrd0-C0BLsiClFGTybnssGQ5xWKIG9GRZbJ3eS8hwnvVRdrHGyzKf0/s400/ClarkDist10WST.png" width="400" /></a></div>
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Curiously, even though I put that AC balance pot in the input stage to null out gain mismatches between the two sides of the 6BL7, I never was able to get the second harmonic on the output of the amplifier to be lower than the third harmonic. I wasn't expecting that.<br />
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Anyway, it sounds awesome, my brother couldn't be happier, and I don't think I'll ever build a 300B amp.Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com3tag:blogger.com,1999:blog-2914493245344265727.post-20234368434603989312015-01-12T18:20:00.001-08:002015-03-14T22:19:44.310-07:00A Unity Coupled KT88 Amp with Plitron Transformers<div class="separator" style="clear: both; text-align: left;">
I was always fascinated by the Unity Coupled output stage in old McIntosh tube amps. In many ways it is an incredibly elegant solution to many design problems in output transformers. Ever since I first studied the circuit I thought it would be really neat to build one myself, my way. </div>
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Obviously, the most difficult design challenge to overcome is the large voltage drive requirement of the output stage. I got to wondering if it would be possible to build a Unity Coupled amp without a bootstrapped driver and if it would be possible to do that well enough that a global feedback loop wouldn't be necessary. Not that I have anything against a feedback loop, but negative feedback tends to linearize things at the cost of making overload behavior worse. If I can make an amp good enough without feedback I will do without it. So a few years ago I challenged myself to see if I could build such an amp without feedback, but I left my options open in case it was needed. Here was what I built:</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhEdUBvj-vbkxS91fSm4rzi4RWhnegwGAzdoDwN1Otarq9Y67N1G79ti2UNjUXAhyL93zpodRXtZSYM17ddV0zCLvJhSH5E1DnMDABJZ4BtnxlR_qZvYAXMorpLDiO6vrkxrUc61RKNncc/s1600/UC.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhEdUBvj-vbkxS91fSm4rzi4RWhnegwGAzdoDwN1Otarq9Y67N1G79ti2UNjUXAhyL93zpodRXtZSYM17ddV0zCLvJhSH5E1DnMDABJZ4BtnxlR_qZvYAXMorpLDiO6vrkxrUc61RKNncc/s1600/UC.png" height="296" width="400" /></a></div>
I got the output transformers from Plitron and they cost a pretty penny but are really outstanding transformers. The impedance ratio is 4k:5 Ohms or 6.4k:8 Ohms. I was a little concerned about this at first. I don't like driving pentodes into saturation since that is really hard on the screen grid. I thought the impedance ratio was a bit high and actually corresponded a bit with the transformer designer and told him so. He thanked me for my input but stated that he would not be doing another design at a lower impedance ratio.<br />
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I eventually settled on making a floating screen regulator from a MOSFET source follower to reduce the screen voltage and make it so that the load line for the KT88s hits the knee of the pentode curves. I think that this is just a really healthy choice for the KT88s and will probably make them last a lot longer without any real sacrifice in output power. As far as I know I'm the first to use this solution on an amp like this. The simplified schematic doesn't show it but I added a current limit circuit on the source follower that will make the screen voltage sag if the screen draws too much current.<br />
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I have a source follower driver for the KT88s, not so much because they need it but because I wanted to present a very high input impedance to the 841s that develop the high voltage drive.<br />
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For the driver, I had some very unique requirements. I needed about 200Vrms (560Vpk-pk) of drive to take the output stage to clipping. That means I had to have a driver capable of dealing with an 800V or so B+ and be able to idle at 400V if I wanted some headroom. Most small tubes can't idle at 400V since it takes special cathode materials to withstand the ion bombardment that happens at these higher voltages. Looking at oxide coated cathode tubes only power tubes and a few selected types like 6SN7, 6BL7, could handle high idle voltages. I also discovered that thoriated tungsten filament tubes typically have higher idle voltage ratings since thoriated tungsten filaments are fairly resistant to ion bombardment.<br />
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At some point I came across the 841 which is a high impedance triode with mu = 30 that is meant for zero bias operation and needs to be operated in A2 to swing significant voltage. It has absurdly linear curves over a very wide voltage range, though:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjX3-DTY3TWpdOYC1-bt5Y6tN99EBf9ZMGEKOQdBEzlnmPcLb__M9dMF35BVzL4BmX8xJR0wMdIciYFYh_PB4WuGhYGA4lNoTtQIUAyOimUWQUUy0i4djAFtIsvTPzQ0v7dWgZ7XRmL1KU/s1600/841.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjX3-DTY3TWpdOYC1-bt5Y6tN99EBf9ZMGEKOQdBEzlnmPcLb__M9dMF35BVzL4BmX8xJR0wMdIciYFYh_PB4WuGhYGA4lNoTtQIUAyOimUWQUUy0i4djAFtIsvTPzQ0v7dWgZ7XRmL1KU/s1600/841.png" height="321" width="400" /></a></div>
I thought if it were driven with a source follower and loaded with a CCS it would do very well. I was not disappointed. This is the distortion performance at 230Vrms output running at 10mA and 400V idle:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3pARXNn_F8AnY55WFqtjyswfqoH1N_TYpVoKnDaNffg9IO4aWptkjQFmEI3EZ10XErbf9yCo8A4wcwCGD-4eIdax6hNNCwdj3oYbvqcbjLb_1zB5Y2a7VJsd7kvfta80vJdoxIJXlu2A/s1600/Driver230Vrmsout.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3pARXNn_F8AnY55WFqtjyswfqoH1N_TYpVoKnDaNffg9IO4aWptkjQFmEI3EZ10XErbf9yCo8A4wcwCGD-4eIdax6hNNCwdj3oYbvqcbjLb_1zB5Y2a7VJsd7kvfta80vJdoxIJXlu2A/s1600/Driver230Vrmsout.png" height="225" width="400" /></a></div>
Now you will notice a hump in the noise floor. I suspect that might be caused by the white hot filament in a high impedance tube but I really don't know that for sure.<br />
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Last of all is the input stage, which is pretty unremarkable. It is just a 6SN7 LTP with CCS tail operated in a very linear range. You may be wondering what the deal is with the pot. I knew that I would have to count on balance between 841s, which are made of pure unobtanium (I was able to acquire three sets for this amplifier with great effort). I added a pot so that I could attempt to introduce some imbalance in this stage to counteract any imbalance in the gains of the 841s.<br />
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All high voltage supplies are implemented as Maida-style regulators.<br />
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Anyway, it is time for pictures of the completed amp:<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaPZxyehoCtAwjrip2NCwRpj0tE3ANUarouiEcjmrSUdQTFRwJz3sLaOTv4OaEFCHJ6f3ucbk2QMT5kfvaIZy9SgWkCdQ4WgrpmoVIELlPRldLC4p54Y2baMYRQkK58DyYuW61u8yOdFM/s1600/20150110_111600.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhaPZxyehoCtAwjrip2NCwRpj0tE3ANUarouiEcjmrSUdQTFRwJz3sLaOTv4OaEFCHJ6f3ucbk2QMT5kfvaIZy9SgWkCdQ4WgrpmoVIELlPRldLC4p54Y2baMYRQkK58DyYuW61u8yOdFM/s1600/20150110_111600.jpg" height="225" width="400" /></a></div>
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Kinda ugly inside. Oh well. I really love the glow of those 841s. They kind of steal the show from the KT88s.<br />
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<b>Measurements:</b><br />
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I measured output impedance at ~1 Ohm. I'll look at doing some distortion measurements this weekend and update this page (hopefully).<br />
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As far as listening tests go, the amp just does everything effortlessly. It sounds great. The bass is strong. I can crank it to ear bleeding levels and it doesn't break a sweat. I'm pretty happy with the sound.<br />
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If I put my ear right in the tweeter I can hear a slight hiss, maybe something to do with that noise hump in the output of the 841s. I was hoping it would be dead quiet but apparently not. There is no hum or buzz though.<br />
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Oh, and I should thank Tom Christiansen for his excellent filament regulator boards and Guido Tent for the automatic bias board. I love that my unobtanium 841s have a soft start on the filaments and that I always have perfect balance on my KT88s. Both boards work well and are highly recommended.<br />
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<b>Addendum 3/1/15:</b><br />
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<b>More Measurements:</b><br />
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When I initially got this amp together, I flipped it right-side-up a bit prematurely. I was excited to have it finished and thought I would test distortion with it right-side-up.<br />
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I realized later that I had not adjusted the pots in the input stage plate loads to null out the gain imbalance in the driver tubes. The problem was that the amp was so heavy that every time I thought about flipping it over to make the adjustments, I just felt this overwhelming lazy feeling and flipped on the music instead. Besides, it sounded quite good despite making almost 1% distortion at 10W.<br />
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Anyway, this weekend I finally got back to it and I'm really excited with the distortion measurements with everything tuned up.<br />
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I measured:<br />
0.025% @ 1W<br />
0.15% @ 10W<br />
0.22% @ 20W<br />
0.24% @ 30W<br />
clipping occurs at ~ 40W.<br />
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All were 2nd harmonic dominant, despite this being a push-pull amp. I could really get that 2nd really low by introducing imbalance in the phase splitter, but I never could get it under the 3rd.<br />
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All in all, I think these are pretty good figures for an open-loop amp. </div>
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Then I tested bandwidth. These output transformers are supposed to have ~500 kHz of total bandwidth per the manufacturer.</div>
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The amp was down 3dB at 120kHz. I'm guessing that the limiting factor in the amp is the 841 driver stage. I did some back-of-the-envelope calculations and it looks like since the 841 has such a high plate resistance, it only takes a 22pF load to put it 3dB down at 120kHz. An 801A would probably perform much better here in terms of bandwidth but they are too expensive and I don't really need to make an amp with half a megahertz of bandwidth. <br />
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I knew that would be the price of using a high-impedance triode as the driver, but the 841's curves sure are pretty. The other thing about the 841s is that I think they are the dominant noise source in the amplifier. I think those white-hot filaments combined with the high plate resistance aren't particularly good for low-noise design. This amp has a tiny bit of background hiss (with ear close to the tweeter), whereas other amps I have built have had none, and I used the same input stage I always use.<br />
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That said, I spent a bit of time listening after making the adjustments, which mostly just took out a bunch of 2nd harmonic distortion, and I think that modern pop or heavily electronic music definitely sounds better now, but simpler songs sound slightly less alive. I would say that songs with lots of electronic sounds and loud bass lines are now subjectively clearer. <br />
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My conclusion: The 2nd harmonic distortion can add a pleasant effect to sounds, but any non-linear transfer function, even if it only generates even harmonics, will generate IM products all over the place when you start mixing a lot of sounds together. Those IM products don't sound good and will muddy the sounds if you start mixing enough together.Heath Workmanhttp://www.blogger.com/profile/05876235572567271859noreply@blogger.com4