Hello, I have a Marshall DSL15 that is humming a 120 Hz tone, even with all controls down. This tells me the trouble is in the power section. I checked the bias voltage of both 6V6 tubes, and they are there. However, when I checked the voltage across the plate current sensing resistors, one is around 24 mV, and the other is virtually zero. I swapped the tubes, and the voltage values on the resistors went the other way round. It looks like I have a bad 6V6 tube, but I'dlike to confirm it before I go buying a new one. Any thoughts? Thanks!