Tube Amps with a Twist: P-Channel Shunt Feedback KT88 Amp Revisited

This is Part 2 of a series on an amp I designed and built. Click here for Part 1.

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.

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:

The first measurement I made was output impedance at a few different output tube idle currents (450V B+). Here are the results:

3.7 Ohms @ 50mA
3.3 Ohms @ 60mA
3.1 Ohms @ 70mA

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.

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:

After changing the feedback resistors, the Zout were measured as:

2.9 Ohms @ 50mA
2.4 Ohms @ 60mA
2.2 Ohms @ 70mA

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: