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Old 15th Apr 2021, 2:19 am   #9
nzoomed
Tetrode
 
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 99
Default Re: 807 tube amp, any ideas?

Quote:
Originally Posted by Diabolical Artificer View Post
I refined the 120w amp over several years, put a hell of a lot of work into it and hours of research, I'd advise against using that schematic,it was about Mk5, the finished amps were about MK79 : ) 807's do have character, thing is if you want to run 6 you'll need to go through about 100 valves to get six matched.

I've bench tested numerous 807 valve amp circuits from extended class A, Class AB2, Class AB2 ultra linear which killed several 807's and several other mad ideas, in the end they're best run straight in PP with a regulated G2. The extended class A article is worth reading though, it worked quite well, but you need the right OPT. The problem running 807's in UL is that Vg2 max, it means you have to keep your HT within 400v, 400v HT is not quite enough to get the best out of a pair or more 807's. My thinking is your already chucking about 10w per valve away on the heaters,you've gone to a lot of trouble and time to build a chassis, prototype etc,why not run the valves at their optimum configuration?

I reluctantly ditched the idea of running 807's with Vg2 over 300v, it just doesn't make sense, for a start no new 807's are being made, they are a finite resource, running them at full whack is a bit cavalier.

I hope I don't sound too discouraging, and am glad some of my original work inspired you to build something. I'd advise you to download the STC 807 application report, find a set of parameters that's about right, then find a decent pair of OPT's, then build the OP stage and PSU and crack on from there. Designing an amp from scratch is good fun, educational and immensely frustrating at times,bit like bringing up kids or keeping chickens.

All the best, Andy.
I only plan on running a pair on each channel, so matching wont be much of an issue. Believe it or not, you may be surprised to learn that 807s are still made in Russia and China, but dont know what their quality is like, but ive got a lifetime supply of the things anyway.

From what Radio Wrangler is saying, it sounds like running 807's in UL configuration is the same as a triode so the screen voltages dont matter., or am i missing something?

I guess I could try at a higher voltage if thats the case, nonetheless, my guitar amp has them running at 400V fixed bias and not UL configuration.

I have all the gear to wind transformers, and all the bobbins, laminations, etc, but dont know where to start on winding one yet.
I know some people who can wind a decent OT if needed.
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