I bought a second (or third or whatever) hand valve amp yesterday.
Previous owner had it for ten years, without changing valves once.

It worked perfectly fine until just now.

Clean stills sounds as it should.
Crunch channel is humming louden than it should, and there is a background noise like someone's writing really fast on an old typewriter.
Ultra channel is humming even louder.

The humming and typewriter noise increases and decreases with the amount of gain.

What might be wrong with it? (I know the question sounds dumb. I know it's probably the valves. Just want someone else's opinion.)
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Professor Plum in the Studio with a new Amp

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You should just re-tube the amp. Chances are quite high that is the issue. Even if it is something else, on top of fixing whatever it may be you should also re-tube it. Long overdue for that.