Q: My tube guitar amp makes a low-level crackling noise all the time, sort of like bacon frying. It doesn't get louder when the volume is turned up, it is pretty much always at the same volume level — just loud enough to be noticed and annoying. What can I do to stop this?

A: Since the level of the noise doesn't change when you turn up, the problem is probably not in the preamp section of the amp or the preamp tubes. More than likely you have a bad power tube or possibly a bad phase-inverter tube. Replace the power tubes one by one with a known good tube. If the noise persists, replace the phase inverter tube. If the noise still persists, a trip to your favorite amp tech is probably in order.