Re: Update on Williamson Amplifier
Posted: May 12th, 2020, 1:29 pm
David McGown wrote:Grover,
One thing I noticed about the Kiebert article is that he used the 5687 at the same current at the 6SN7 it replaced. Plate resistors are 50K. That is really a poor operating point for that tube, it really need more current for linearity. Which is why I currently have 20K plate resistors (for the original planned ECC99). What did you use, if you remember?
David
I built it as he designed it because I wanted to hear it. I've never been a huge fan of the 5687. I think it actually sounds better at lower currents but that's just me. At high current it sounds harsh to my ears. What I like about the the 6SN7 is the utter naturalness of the sound.
DavidB, I agree that fiddling with high feedback and phase-lead/lag caps can be a fool's errand, but you are a magician with tubes and do things that would be difficult for the average hobbyist to duplicate. I've got your 807 schematic which you generously shared a while back, but I don't know how well it would work without the audio-bias board and some of the other details I would not be skilled enough to implement. I've a pair of Acro TO-290s, 12K p-p at 20 watts, and the big Peerless, of course. I'm game to give it a try but I'd have to simplify it a bit.
David McGown, I'd be interested in your reactions to the stock Haffler circuit, and also interested in how well it holds up to some rigorous stability testing. With bandwidth out to 200kHz I wonder, but I'd imagine it's good enough, probably better than the early Heathkit amps, which need quite a bit of tuning to get them stable. Maybe I'll snag a pair of repro A-431's and put them on the breadboard so we can compare notes.