My 6384 SE Experiments

In 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.

The Breadboard Test Amp

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. 

First Test Circuit

Below is the resulting distortion spectrum at 1W into 8 Ohms. 

0.70% Distortion @ 1W

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.

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.

Second Test Circuit

Below is the resulting distortion spectrum at 1W into 8 Ohms. 

0.25% Distortion @ 1W

Amplifier Zout was measured at 1.1 Ohms. Things definitely got better.

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. 

I decided to try feedback from the output tube plate to driver cathode. Below is the circuit:

Third and Fourth Test Circuits

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. 

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:

0.27% Distortion @ 1W

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.

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:

0.14% Distortion @ 1W

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.

Fifth Test Circuit

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.

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:

0.046% Distortion @ 1W

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. 

I then substituted the 826 output tube and results can be seen here. 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. 

Anyway, I hope that explains the progression I went through to arrive at this approach.

An Ultra-Low Distortion 826 SE Amp

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.

It's currently a breadboard amp with a very sophisticated forced air cooling system. ;)

The Amp

Here's a simplified schematic:

Simplified Schematic

I'm currently running B+ at 520V. Output transformer is an Edcor 5k:8. Nothing exotic. 

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. 

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. 

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. 

Sensitivity of the overall amplifier is set to ~0.7V. This can be easily changed by altering the 520R/220k resistor ratio. 

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.

See below for distortion measurements at different power levels:

0.014% @ 100mW

0.017% @ 500mW

0.027% @ 1W

0.040% @ 2W

0.063% @ 5W

0.063% @ 10W

0.30% @ 19W

I measured Zout at 0.7 Ohms, so damping factor is over 10, which is probably pretty unusual for a SET amp. 

Here's a 10 kHz square wave:

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.

Here's a screenshot I took of the input tube plate waveform at 19W output:

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.

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.

Idea for Ultra-Low Distortion Unity-Coupled Amp

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)

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:

1. 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. 
2. 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.

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.

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. 

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.

Some day I'll work something like this into my Unity-Coupled amplifier.