bias feed resistors? Master Volume mod...

[martin manning]

Assuming you don't have a bias meter, I'd check the current on each half of the OT, using 1Ω cathode resistors, or the measured DC resistance of each half of the OT, and the voltage drop when powered up. You can measure the heater voltage to ground on each side of the heater string and se if it's balanced. If the master doesn't affect it, then that means it's beyond that point in the circuit and not likely to be heater hum. Another possibility is failing power supply capacitors. Is this your DSL401 you're talking about?