The Amp Garage

That's my schematic, and that's way the amp was that I had on the bench. I though it weird too, but I just captured what was there.

I've always found T&M/CE shipping prices to be on the high side. Now I guess they're even higher!

Try strapping an appropriately sized cap across the HT line. I've seen (in rare cases) voltage-sensitive motorboating. I'll bet the HT doesn't drop as much with two tubes.

The label says 200VA so I'd get a 300VA step-up just to be safe. You'll want a transformer type as opposed to anything electronic.

22nF and 1M has a -3db point of < 1hz, so it's pretty much just for DC.

Here in the States, the "120v" nominal might be 127v coming out of the wall, which makes the B+ 6% higher as well, e.g., 400v -> 423v or 450v -> 476v. This is particularly a problem with older amps using a 115v primary. Certainly the old original RCA 6V6s Fender used were robust, but I don't know if...

It's kinda constant, fuzzily. Not really constant. An inconstant. :) I went back and measured a bunch with my meter diode check function (probably very low current) and the Vf readings were all over the place. The UF400Xs were really low, ~0.41 Vf. So much for leftover schooldays assumptions... :ro...

All my life a standard silicon forward-biased junction has dropped .6 to .7 volts when conducting. The Vishay 1N400x data sheet (https://www.vishay.com/docs/88503/1n4001.pdf) says Vf varies between ~.6v (at low current) and 1.1v at one amp. We have a batch of 1N4007s that are forward-dropping 0.45v ...

You'd likely need bigger (than 25w) resistors for the Robinette solution, and/or they'd need to have access to plenty of cooling air. Note that I've done something similar (have resistors soak up 75% of the output power), and it's not really that much quieter. I agree with the 99%, I put a triode sw...

Well, the "Fender 22921" listed on the schematic is a standard reverb driver transformer, available lots of places. Why not just use that?

Depending how "pure" you want to be, you could also use a MOSFET source follower on the output, so you could save the triode for gain instead.

Nice work, Martin. Thanks for the hard data.

The suppressor is there to improve the curves by reducing the tetrode kink. Evidently it works a little better if its potential is below, as opposed to the same as, cathode voltage. That is true. I didn't mean to imply it had no effect, but rather that 'curve improvement' was probably not the prima...

> Tone Lizard's Marshall Myths
Aha, you're probably right.

I could have sworn that I read somewhere (but cant find it right now) that making the suppressor grid negative helps limit max plate current in extreme situations, which makes a little sense intuitively (and is supported by Andy's anecdotal evidence). Since the grid's primary purpose is to direct el...