The amp had a loud 60 hertz hum with nothing plugged in, so my first thought is the filter caps are the problem area. I pulled the phase inverter tube and the hum was still there, so it's being introduced early in the power supply.
The first filter cap on this amp is a discrete one and the other 3 in the power supply are in a mulit-cap can. Since the hum is generated early in the power supply, I subbed in a good cap for the discrete unit, and the hum is still there. So I proceeded to sub in 3 more caps for the multi-cap can, and…the hum is still there. The only remaining cap is the fixed bias supply filter cap, which is where the problem turned out to be.
I got to the problem, but realized that if I had paid attention to the frequency of the hum, I would have started with the bias filter cap and saved some time. The only way you get 60 hertz hum is from external sources, or somewhere before the rectifier tube. The rectifier tube is a full wave device which takes the incoming 60 hertz AC voltage and flips the bottom half of the wave up to produce 120 hertz pulsating DC (see the diagrams below). If the problem had been the filter caps the hum would have been up an octave, close to the open B string on a guitar. As is was, the hum was closer to the B at the 2nd fret of the A string on a guitar.
|60 hertz AC|
|120 hertz pulsating DC|
|60 hertz pulsating DC|