1966 Bandmaster and 60Hz hum....?

[acorkos]

Not sure where the hum's coming from, but it disappears when I pull the phase inverter tube. Tried pulling the other preamp tubes, but the hum remains. Different power tubes, same hum (power tubes are matched within 0.2mA). replaced all the filter caps....same hum. Replaced the bias board components...same hum. If I lower the bias current to 5mA (8% of max plate dissipation), which is the pot turned all the way CCW, the hum is mostly gone (but the amp sounds like crap). I moved wires around, but no change in the hum. Any clues as to where the problem lies? thank!

[Enzo]

COnsider that when you pulled the PI tube, the hum went away. The power tubes were still in it and running, biased by that same bias supply, and with B+ filtered by the same filter caps as when the PI tube was in. So clearly the power tubes, bias supply, and filter caps are not the source of your hum.

Since pulling the preamp tubes has no effect, the source of your hum is after them.

So the first thing I'd do would be try a new PI tube.