Ringing/hum sound from Ampeg Gemini I

[ubiquity55]

Recently bought an old Ampeg Gemini I. It has the original tubes (7591 power tubes) and sounds great. However, when I turn the amp on and leave it on for at least 5-10 minutes, a ringing/humming sound comes from it. This happens when I'm not playing through it. When I start playing my guitar, the sound seems to stop, but picks up again shortly thereafter. While it's humming, if I knock on the top of the amp a couple of times, I can get it to stop momentarily, but it usually starts up again. I've had tubes weaken on other amps but these sound pretty strong to me, so I don't think it's them, but I'm not much of an expert. Wondering why the knocking on and/or jostling of the amp has an effect. Any help would be appreciated as I want to record with this baby and the hum is a bummer.
Thanks,
ubiquity55

[Enzo]

Is it actual hum? The same note as when you touch a guitar cord tip? Or is it just a long ringing note coming from the amp?

Isolate the problem. Does turning down the reverb make it stop? Does turning down ANY of the controls make it stop? REverb pans can feed back.

Look in the back while it happens. Reach in there and grasp each tube firmly - use a rag or glove if they are too hot for you. Does it make a difference grasping any tube? Mucrophonic tubes can feed back.

Take a pencil or pen or just use your fingernail, and tap on each tube. Does sound come out the speaker? If you find a sensitive tube, it might be microphonic.

Wiggle each tube in its sicket. Does the sound through the amp change at all?

[jrfrond]

I'm thinking that it might be a combo of motorboating and microphonic tubes, since you are describing a "hum" (low-frequency oscillation?) and "ring" (microphonic tubes excited by the motorboating). Of course, I am just speculating.