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Amplifier Hum - Only Amp Connected

I have read many threads on here about various amplifier hum.

I recently bought a used tube amplifier.  When I connected the amp and turned it on, I immediately heard a hum from the mid-range driver of both speakers. The hum is audible even if the pre-amplifier is turned off/muted. In fact, I unplugged all interconnects from the amplifier and still heard a hum when the amp was on. I then unplugged the the interconnects and unplugged everything else in the room from the outlets (turntable, pre-amplifier, TV, and cable box), so that the only thing in the room connected to the wall was the amp. I still hear the hum from the speakers when it is only wall>power cord>amp>speaker cable>speaker.

This has to mean that it is a problem internal to the amp, correct?
jdpawnbroker
atmasphere's avatar
atmasphere
11,803 posts
 
It is normal for an amplifier to pick up a little bit of buzz with no input or with the preamp turned off. Does it have the buzz with the preamp turned on?

If no you're off the hook. If yes, this type of buzz can be caused by a ground loop. For testing purposes only, place a ground cheater adapter on the power cord of the amp. You can get these at almost any hardware store. Run the system with the preamp ON. Did that fix it? If yes, you have a ground loop and some exploration will be needed to sort out a solution. If no, other exploration will be needed.

Try these and report back- we'll see what the next step (if any) is needed.
twoleftears's avatar
twoleftears
5,318 posts
 

From PS Audio.

https://www.psaudio.com/ps_how/how-to-find-and-fix-hum/

jdpawnbroker OP
10 posts
 
There absolutely is a hum when the preamp is turned on.  The hum does not change in any way when the preamp is off or on, including when the interconnects between the amp and preamp are removed or connected.

I will try the cheater plug this evening.  Does it make sense that a ground loop would be worse in one channel?
kalali's avatar
kalali
1,977 posts
 
If the buzz is 120Hz, given the amp's age, the power supply filter caps are probably not suspect. Based on what I've read, a 120Hz buzz is typically caused by something faulty in the rectification stage depending on whether the signal is half wave or full wave rectified. If the VAC amp is tube rectified, that might be an area for investigation. That might also explain the uneven amplitude of the buzz. I'm sure others with more hands-on experience can correct me if I'm mistaken.    
paulcreed
559 posts
 
Been through this one myself. I agree with cheater plug to start off or break ground pin off a cheap stock power cord. Have you swapped power tubes left to right since you said it's louder in one speaker, or maybe a rectifier tube. Electrolytic cap, transformer in power supply would cause hum but that would be last stop after trying all the free solutions listed.
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