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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
jea48
4,425 posts
 
jdpawnbroker OP6 posts

12-13-2019
5:33pm

Cheater plug did not make a difference. Hum was still there.

Did you put a piece of tape on the bottom of the cheater plug ground tab, or over the trim screw that hold on the electrical outlet cover plate, so the ground tab does not make electrical contact with the metal screw?
The trim screw is electrically connected to the equipment ground.
jdpawnbroker OP
10 posts
 
I did not use tape but the ground tab was not in contact with the screw.
atmasphere's avatar
atmasphere
11,803 posts
 
This sounds to me like a power supply problem such as a failing filter capacitor in the power supply. Such a part can indeed cause more hum in one channel than the other but the problem would be in both. Sometimes you might be able to get it to stop humming for a little bit by simply turning the amp off and on again, but only sometimes. My recommendation is to seek service on the amplifier.
jdpawnbroker OP
10 posts
 
Thanks for your responses and advice.  Seller asked me to ship it back to them so they can take a look.
jdpawnbroker OP
10 posts
 
Follow up:  I sent the amplifier back to the seller, which sent the following:

"I was able to get the amp on the bench today and test everything out. Everything is working great. The hum that you were hearing is remedied with a slight tweak of the bias to get the pairs of tubes biased correctly. The bias light system can get you very close but very slight adjustment of the bias cleans up all of the remaining noise and gets everything perfect."

The "bias light system" mentioned above is the VAC amp's bias system whereby a light comes off or on while you turn a screw to determine whether bias is correct.  Does it make sense that a slightly incorrect bias could create a hum audible from 8 feet away?

FYI, I bias my current amp all the time with a multimeter and without use of a light system and have had no such issues.
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