Tube Question

I've said all I can say. Good luck.

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.

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 guess you've chosen not to respond to my question about the source of the hum. If you don't know what's broken it will be difficult for a repair shop to fix it. If the problem is due to AC grounding in your system it may not exist when the unit is in a repair shop, for example.
Anyway: Bill Thalmann at Music Technology in Springfield VA is the best on this side of the Mississippi 

Weve established that if the tubes have never been replaced now is the time.

First, please clarify whether you are talking about a mechanical hum coming from the unit itself or a hum that is audible on your speakers.  If it's a mechanical hum, I agree with the above two responders.  You could try tightening down on screws that hold the transformer laminations together, if the transformer is not potted. Or you could isolate the transformer from the chassis on rubber grommets.  Neither is guaranteed to work but might work.

If it's a hum on the audio signal, then the issue is mostly likely due to equipment grounding, not a tube, although it's also possible you have a dirty tube socket.  All that said, if you've never changed the tubes since the 90s, it's probably a good idea to do that too.