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Re: Single 45 Amp question and pics

Posted by Perseo on March 1, 2007 at 05:08:29:

The spec sheet shows a 440 v CT transformer rated at 50 mA, or an Antique Elec Supply PT 142 which is 325-0-325 at 50mA. Those seem like pipsqueaks for a "power" amp, and going to 2A3s makes matters worse.
As mentioned below, 2A3s draw 2.5V at 2.5 amps, vs 2.5 v at 1.5 amps. Do you have adequate power to the filaments for a 2A3?
6SN7 can be a good tube at 128v. 32k ohms is a relatively high resistance load on 6SN7, but your gain might be only 15 - 17 at a guess, which is not enough to fully drive 45 unless your source is giving you more than 3 volts peak in. With 280v B+ on the 45, I bet it would need 45 v peak to drive it to full power, and I believe a driver stage ought to have more headroom (Lynn Olson suggests 50% extra, he also suggests some ability to deliver current to the grid, which makes a switch to say a 6SL7 problematic).
You could try a current source or choke for the driver stage. I would suggest the fabulously linear and undiscovered 6C8G (octal, grid cap would look good on your beautiful amp) with the little Hammond 90H chokes, running at the same 5 mA you have now. Your gain would be more like 32. May need to fiddle with the cathode resistor.
I would also look for a honkin' power trnasformer, if yours is currenlty only a 50ma one.