May is a very busy month for me personally so I have only a short update this week. My last post was about the Quality Amplifier, a forerunner to the well-known Williamson amp. I proposed a modernized 6V6 A2 version based on the same topology that could probably crack double digit power.
In the conceptual outline above we have a MOSFET rather than a tube as a concertina splitter to save heater power. The unity gain MOSFET splitter feeds a tube differential pair to generate enough voltage gain to drive the outputs. MOSFET grid drivers with a CCS load help the differential stage cope with the drastic impedance changes at the output tube grids in A2 operation. Outputs are wired as triodes, of course.
The A2 drivers will go on a PCB. I also have one designed for the MOSFET splitter, though that’s simple enough to wire any way you might want. We’d need just three tubes per channel: a pair of 6V6s and a dual triode driver. My driver pick at the moment is probably the 5965 (or a pin-compatible 12AT7 if it needs to be new production).
I’m also happy to report that progress is being made on the preamp project. Hopefully I’ll have pictures to start sharing in the very near future!
wauwatosa tube factory 
Hi if your using 6v6 as your output tube I would not wire them in triode mode. I would wire them as a pentode set-up. A point with your fet be careful of oscillation
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Thanks for your comment, Ian!
I’ll test this in UL as well as triode. Part of the appeal to me here is squeezing out power from a triode with an A2 driver and keeping it open loop like the original Quality Amp.
I imagine UL would get close to the same Zout and power output without much distortion penalty and lower driver requirements. There’s a Baby Huey 6V6 version floating around that uses this approach with feedback.
Another related design is the Musical Machine: pp 6V6 triode driven by a LTP. Though this doesn’t do A2.
I have gate stoppers on my FET boards and will watch for oscillation!
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In my search for a DIY push pull amp I’ve heard all about the Williamson amp & that it can be a very finicky amp to dial in. I never heard of the quality amp but will research it. I do have a 807 matched quad that I’d like to build into a stereo amp
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