Troubleshooting Hum in a Tube Guitar Amp

There are many different ways that you can get hum in an amplifier, and its hard to troubleshoot without the amp at hand. But generally; hum can enter in three ways: Directly, inductively or capacitively coupled.

Looking at your case; did you try to short the reverb return input at the amp when disconnecting the reverb. If the hum is still present, the problem is in the V4A stage. (shorted cap etc...). If not, you could have an open R28, a bad cable or a broken tank. The tank would be most likely of the three. (Yes, the tank can be faulty and still give sound.) Did you try to connect the send to the return directly? What did you get? Even if the hum is still there with the tank disconnected you still have an open input that would pick up hum capacitively. This would be the same as a corresponding fault in the tank or the cable. Good luck!

It turned out that the loud 60Hz hum was a result of using a footswitch whose cable wasn't shielded (not the original FS). This allowed the long cable to pick up 60Hz hum inductively from nearby power cables and couple them directly into the rest of the circuit. The high impedance of the V4A plate and surrounding resistors made it particularly susceptible to hum.

Rewiring the FS with shielded mic cable almost completely eliminated the hum to the point where you can hardly hear it unless you turn up the reverb all the way (and even then it's very faint).

For good measure, I replaced all the capacitors in the power supply as well to try to eliminate any remaining hum. Now the amp is nice and quiet (until I play of course). I also went ahead and cleaned all the pots which got rid of crackle.

Thank you to Lars for guiding me through this troubleshooting!

Here are some clips so you can hear for yourself:

Before: https://youtu.be/W0_c5Ka2TyA

After: https://youtu.be/2zCLHwAeT20