The previous owner of this amp told the current owner that he had to do "a lot to get rid of the hum." Almost all the components in the amp had been replaced, the negative feedback loop (only on the instrument channel) had be removed, and a huge low pass filter had been installed across the speaker output.
This was not necessary as the noise was caused when someone replaced the output tube "grid leak" resistors with much smaller values. In this amp, that resistors also forms a voltage divider for the 2nd half of the (paraphase) phase inverter, so that stage was fed a signal about 100 times larger than it should have been, causing the amp to oscillate. I fixed the issue, cleaned up the wiring a bit, and reinstalled the negative feedback loop.
