I recently acquired a Dynaco PAS3 preamp. It has a new pc board replacing the selenium rectifier and associated filter capacitors. There is a slight hiss or hum at low volume. It doesn’t sound like power supply hum. It doesn’t increase when I turn up the volume. In fact, with no input signal the noise goes away with the volume turned up beyond a certain level. Any clues on the possible cause would be appreciated. Could it be a noisy 12AX7 on the PC-5 board? They’re all new JJ 12AX7s. Also the power source has a bucking transformer to lower the mains voltage about 7 volts, a vestige of when I was running Eico 20 amps. Could this be causing the problem? I’m using it with a Dynaco ST-70 which appears to be free of noise with no input from the PAS3.
Strange.
As for noice/hum on line level, as the volume pot is first, noice produced at zero volume comes from the lineboard. If hum
changes when volume is increased is due to cancellation or addition ( depending on phase)
In your case lacking instrumentation i would suggest a different troubleshooting,
1/ solder a short wire between eyelat (2) to (1) and (9) to (1) . This shorts the input of the amp and
eliminates ant mysterious problems with the volume pot (which is cheap and sometimes break in
mysterious ways)
If hum persists, problem is in the board or in the B+ or in the filament feed. Possibly
miswiring of the bass controls could affect hum also.
( if you mentioned hiss i would suspect plate resistors 470k and 100k )
If hum disappears your problem is in the wiring selector volume or balance controls.
if hum remains a second wire could be installed across the 1.2M resistors ( one end of this is
connected to pin7 (grid) on second triode. Thus further reduces the problem area.
