Yes, I had to double check with the boss before really saying a lot.
Back around 2017, I had a local repair client in Woodinville, WA who had brought me a lot of hopeless projects he had amassed over the years, and one day he showed up with a box of parts and paperwork rather than some broken piece of vintage tube gear. Apparently he had paid someone else locally a long time ago to develop an amplifier design around the 811A tube, but nothing had really worked out for him and he wanted me to look over what he had. The design was quite odd and had the 811A cap coupled to the previous stage and zero biased, the output transformer was only 5K, and there wasn't any feedback around the amp. Having pointed out these initial obvious defects, he asked me to continue the project, and that set me off on a lengthy tangent of designing and building a lot of class A2 amplifiers and learning as much as I could from the few competent designs posted here and there.
Eventually I did manage to resolve the challenges of working with the 811. Primarily the massively Rp of the 811A (around 30K) needed a lot of feedback to work into any kind of reasonable output transformer, but global feedback around a single ended amplifier ends up creating an oscillator. Ultimately the solution was to apply the same methodology that Paul Joppa used in our Seductor EL84 amplifier to deal with this issue, as this avoids global feedback and creates a far more stable amplifier that works into a relatively standard output transformer. Still, nailing down this circuit was a lot more of a guess and check process compared to a rigorous design, so it took two years of fiddling to finally get the circuit into its final form.
Having delivered 18W 811A SET amps to my local customer, he immediately asked me for something bigger, and I stretched things out with about 60% more B+ and a lot more plate current, and I managed to (just barely) make a pair of 50W 810 SET amps. With a nice payout from that job, I began to wonder what I could make that would pass muster with Doc B. I knew he wouldn't go for the plate caps on an 811A, and also the voltages needed to be kept more in the realm of a 300B amp. Going for power well beyond what a 300B could deliver meant 25+ watts and the list of tubes I could find to accomplish this seemed pretty small, but there was the new production 805A triode that seemed to fit the requirements.
With a slightly better grasp of circuit optimization and help from PJ, this amp (first picture below) came together in just a couple of months in 2019. With some custom made Sowter parallel feed iron, these monoblocks were awesome! I really regret selling them, and their current owner is very happy with them! This was an amplifier that I wanted to make more of, and it used our Kaiju power transformer and just a large 3K parallel feed output transformer and a manageable 30H plate choke. Power output was right on at 30W with new production tubes, and slightly more with old stock specimens.
In the five years that followed form this project, I've built a whole bunch more class A2 single ended amps with this same topology, and PJ has offered a lot of support in nailing down better design techniques. PJ also served up a massive quadruple parallel 6550 circuit that belted out 50W and is one of the heavier tube amps I've ever had to move! We have ultimately settled on this 30W target on our standard 300B power transformer, and PJ designed up some proprietary iron for this project that is working quite nicely.
The results of this iteration are shown in the second photo (odd colors chosen help ensure they stay in my listening room). Power output from new production 805A tubes is 15W at 1% THD, 28W at 5% THD, and 30W at 10% THD. Power available at 35Hz is only a watt or two lower than these numbers. Frequency response is -1dB at 16.5Hz and 22.3kHz, and the damping factor is a bit higher than our typical amps at 4.4.
Sound wise, these amps sound large and fast with ample reserve power and a good ability to remain composed through difficult passages. The higher damping factor is very rare from a high power SET amp, and this leads to tighter bass and better synergy with a wider variety of speakers.
