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
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I had the same problem with my Tube/SS Hybred and TT I just yesterday recieved a Granit Audio Ground Zero it solved the ground loop hum I had which is most likly what you have. Good luck
Do you still have the hum when the preamp is disconnected entirely (with no cable attached to the amp inputs)? Are the pre and power-amps connected to the same outlet ? Just ruling out possible sources of ground loops.

also how old is the amp ? If it's old it could be failing power supply capacitors ? Just a wild guess.

The low frequency hum is almost certainly mains frequency (60Hz), so it's either coming through the amp's power supply, or via poor grounding somewhere.
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.
Thanks to all three of you for your responses. I have plugged the amp into a grounded wall socket with nothing else plugged into the outlet, and no preamp or other input to the amplifier, and the hum remains. It does sound about 60 hz or so, so I'm sure the "ground" hum verdict is correct. I have had the amp resting on brass cone points, as well as on its own stiff rubber feet, on my Target stand, and isolating it from ground vibrations doesn't seem to have any bearing on the problem. Neither does placing weights on top of it. The consensus opinion seems to be that repairs are called for. Any idea what such a repair might run, and/or where I should take/ship it? I hate to pay top dollar to a hi-end specialist if it's the kind of problem that a reasonably competent tv/audio shop repair guy could fix. I live in Charlotte, NC. Thanks, Ben
did you try a ground cheater yet? it will cost you $1 and should be tried before you take in the amp for repair
I agree with Elevick. I had to go throght the same process of elimination until I found the culprit. The "cheater" plug did the trick. Very frustrating, however necessary. Good luck
Another culprit of hum is incoming cable if running your Tv outputs to your stereo. If this is the case you can buy an isolator from Radio shack. I also had a Grant tube head amp and unfortunately could Not rid myself of 60 hz.Good luck
I'll give the "cheater" plug a try. Thanks to all! Ben
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.
Same problem after I added my external Adcom 5802. Low lever hum audible from speakers when no source sound playing. Disconnected other power leads. When I unplugged the TV power, the hum increased a few dB. Big clue. I then disconnected the CATV coax from my VCR and magically the hum got disappeared. Goodness. Looked on the web and found instructions for hooking 2 radio shack 75-300 ohm transformers in series as an AC noise filter. $7 in parts, and works like a champ. No deterioration of cable video quality discernable, and speakers now absolutely quiet. No need to spring for a $50+ isolator. Wishing all other 'hummers' similar success.
Ben, If you have nothing else plugged into the amp, and it still hums, It's not a ground loop problem. Ground loop hum occurs when one grounded component has a different electrical potential in the ground than another component. By using cheaters, you are eliminating the ground from all but one component, so there is no ground loop. I had to put cheaters on everything but my pre-amp, including one that de-grounds coax cable. You may have a different problem- the tubes picking up 60 cycle noise (a harmonic of 120 cycle alternating current). I can't say for sure, but I would exhaust the cheater idea before you re-tube.
Your ,"Hummer" is driving you nuts.Oh,I thought you were dating my ex.