• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Have you ever seen this done to a MkIII?

I definitely did check your work on that one, ha ha. Only because about halfway through reading it, the original .7 made sense. Duh. But yes now I have a firm grip on what you are saying. And in the instance I stick with that coupling, will be able to take some measurements so I have exact ratios. Thank you very much for that explanation, I think that'll help more than me.

Yes Peter I do believe I'll stick with the GZ34. I have a certain fondness for the damper diodes in my creations, but probably to play it safe I'd need 2 anyways, especially if I still wanted to drop some appreciable voltage with them. But I'm still curious about the voltage regulation. I could wrap the amps in some pretty wood and gain 2-3" down under with ease. The hunt is on for some boards but like I said not much yet that is safe up to 575vdc, which is what I'd want.. at least...

And yeah I saw when I looked last night Chris, 480. I've ran quite a few different PP amps up around there, with cooler biases. Since I'll be making it individual, accurate, and easy individual bias (maybe multi turn pots), I might want to 500 just to dial in warmer bias occasionally. Or, maybe I have some luck with the VR idea, and I can fashion a limited range adjustment of the voltage with that?

I ordered some things today. Tubes 4hifi octal driver boards (help with lowering voltage right there), The dual tube bias meters. Probably have them switched, but I like them. The 3/4" spacing binding posts pair (so I'll have to switch 4,8,16, I'm ok with that for normal spacing). Thanks again for the cap shout out Chris. No way I was paying $80 for 4, small, caps. Besides, I have so many already...I'd have to think but surely there's a source for some thin can that I could solder tabs to, and stuff myself. Hmmm...
 
Hello again everyone. Well, actually this one is most for the mods...I started the rebuild process last night. Is it ok (preferable?) to start a new thread, just for this? I've seen quite a few MKiii modification threads around the internet, and here. But I'm definitely planning a few things I don't see in other builds, and I'd like to have it all in one, well documented and pictured, thread about the rebuild.

Thanks 🙏

Loren
 
Audio Research also made/modified Dynaco ST-70's which used the secondary for cathode feedback. They also did output tube plate feedback to the cathodes of the driver tubes. At least this was in the C3 version. There was an article in either Audio Amateur or Glass Audio years ago about ST70 mods which included secondary cathode feedback.
 
Audio Research did that for all of their amplifiers in the 1970s at least. With a single pair of output valves, the feedback is only a coupla dB (varies with output valve Gm at idle) but increases with paralleled output valves in the bigger amps (3dB per doubling number of valves).

Part of the reason it's so universally effective is just that it doesn't try to do too much. Drive voltage requirements for the output stage only increase a few dB, Bode plot lines hardly move at all, doesn't require much matching of OPT secondary halves, very little drama.

All good fortune,
Chris