So something's wrong with the power supply or the bias circuit. The tubes are still biased cold and I'd need to replace my 3.3k bias range resistor to get more adjustment. If I adjust the bias pot all the way down (minimum resistance) I get ~52 mA but the plate voltage drops down to 315V, way below where it ought to be. I see 348V on the plates (should be closer to 415V) and my bias probe shows ~40 mA of current, resulting in 13-14 watts plate dissipation. I wanted to solve the noise issue first, but maybe these are related. Apologies for not mentioning this sooner. Regarding amp operation: the thing that's puzzling me is that the amp doesn't seem to want to bias correctly. The junction of the two 270k and 4.7M resistors seems the most sensitive (it's very loud) but pin 2 and the wire joining those two points all produce a lot of noise when touched. I've also tried swapping V2 for a new 12AX7. Tapping V2 does not induce much noise tapping pin 2 directly does. Regarding sensitivity: the tube does not appear to be microphonic. Have you tried tapping on the tubes to see if they are just as sensitive (or more so) than that junction?
The loud thump could be a microphonic preamp tube. It's amplifying a (roughly) 100mV signal up to over 40W!!! Even vintage amps could be considered very high gain by other types of amplifier standards and that can make some very high sensitivity. Even with a chopstick sometimes, as recently noted by one of our other posters.Īssuming your cathode voltage is correct (which I believe it is) what problem are you having with the amp in operation? I suppose with that V2 lead being so sensitive I might use a shielded cable for it. Is the sensitivity on this V2 lead causing some instability in the amp in operation? Hum when touching a lead is normal. Notice that the following stage has a higher supply voltage, the same plate resistor value and the same cathode resistor value but only shows 1.7V for the cathode voltage. Typo on the 2.2V spec for the cathode in the schematic.
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