Tube Question

Tubes can 100% absolutely positively hum! I used to use Sovtek tubes and they were horrid, many hummed like crazy. New Electro Harmonix are much better. NOS US made are best and quietest. I definitely recommend getting new tubes to try.

What made/model is the amp?  As already said, there's a strong chance there's nothing to repair.  If you send it anywhere, have a good long talk with the repair person before you send it.  The problem may be inherent in the AC coming into your house, in the wiring of your house, or in the model or unit itself.  Lots of variables...

2leftears, Most likely, it would be the ground scheme of the audio system itself and what is plugged into what receptacle and/or how any one piece of gear is grounded internally (some mix chassis ground with signal ground; some isolate one from the other), but you're basically right.

Invictus, Please tell me in what way your anecdotal report proves that tubes per se can be a prime cause of "hum"?  Let's be clear also, that when one uses the term "hum", it typically refers to 60 Hz or 120 Hz noise.  Tubes certainly can be noisy, especially aging tubes, but when they are noisy it's usually a mixture of many frequencies, which sounds quite different from "hum".  So, I guess a grossly defective tube that is shorted out could play a role in causing horrendous hum, but in that case you'd get no music at all.

Anyway, maybe the OP will come back and explain his problem more clearly.

I thought of one way in which a tube could cause classic hum as I defined it above: If you have an AC filament supply and especially if you are powering Directly Heated Triodes (DHT), the AC on the filament, which is also the cathode in a DHT, can be amplified and produce audible hum.  However, the cure is not to replace the tube but to fiddle with the filament supply or change over to DC filaments.

@jcbach , does your amp use a 3-prong power cable; the type with an earth ground? If so, try using a cheater-plug into the wall receptacle. This will eliminate the possibility of a ground loop.