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RE: Tube amplifier hum

It is possible you have a couple of things going on. Some background, electrical noise can be conducted or radiated. Meaning it can come through electrical connections or through the air. Some designs are more susceptible than others to one or the other or both. You should contact the manufacturer who may be aware of the problem and have a fix that you can implement.

When you tried the experiment of moving to another room, no interconnects. To really do this test, use shorting plugs on the inputs. Non loaded inputs can act as antennas. This is a common problem with higher gain mic and phono inputs, but still worth trying with shorting input plugs on your line level inputs.

In the days before polarized AC plugs, when you had a hum problem, the first thing to try was turning the AC plug around. You can't do that with today's polarized plugs, but sometimes home wall sockets can be incorrectly wired (hot and neutral swapped). Hardware stores sell test adapters that will tell you if a socket is incorrectly wired or you can check it with a voltmeter. You can also have resistance neutral to ground, which a voltmeter will also tell you. Plug the amp in and measure the other socket neutral to ground, AC volts should be very low, in the millivolts.

So what can you do? Redo your experiment with shorting input plugs and verify the rooms/circuits you are trying are correctly wired and low resistance neutral to ground. You should be able to determine if the problem is conducted or radiated. If your problem is radiated and as someone mentioned grounding the chassis doesn't help, this link will show you the thickness required of different materials to shield for 60/120Hz.


The graph you want to look at is mid way through on the right.

Good luck.

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  • RE: Tube amplifier hum - fredtr 08:33:37 05/21/17 (0)


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