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My "Hummer" is driving me nuts

I am new to tube equipment, so know very little about it. I purchased a British-made Grant power amplifier, 50 watts? per channel, four large tubes, four small ones. Sounds terrific, however, when the volume is turned all the way down, I can hear a low-frequency hum from both speakers. Not loud, but definitely discernable. Increasing volume does not affect the hum. I have ruled out the preamp as cause, having swapped in a second pre, also ruled out cable interference, as I've set up the system in two different rooms with different configurations and different cable and hum remains constant. My questions are, what might be the source of the hum? Will swapping out tubes eliminate it? Any suggestions on what steps to take next? Thanks! Ben
balthus

Showing 2 responses by elevick

elevick
2,638 posts
 
Ground Loop Hum most likely.
Unplug everything but the amp, power it up and listen for hum. If no hum, great! Continue by plugging in and turning on each component 1 at a time until the hum appears. Once you find the culpret, you can either deal with that units ground or the amp. This means that there is a ground loop issue which is easy to fix either with a "cheater" plug for 3 prongs to 2 prongs and then reverse the way it is plugged in.
If the amp hums when turned on, hooked up to the speakers buy no signal into it, then you have to deal with repairs most likely. Sometimes it is just vibrations that can be dampened by putting something under the feet of the amp. Weights on top may solve the issue but amps need to breath. Otherwise, in for repairs. It is very unlikely a tube problem.
elevick
2,638 posts
 
Forgot one other item: Do you have any other household items on that same circuit? A tv upstairs, a fishtank downstairs, a lamp in the living room...? This stuff can cause a hum. Try shutting off the breaker and see what else goes out.
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