I have a inexpensive Bellari phonostage that sounds pretty good after a 15 minute warmup, but develops a terrific hum after about 30 minutes. It has one tube. Anyone have an idea?
It's equally possible that by removing and replacing the tube, in your scenario, you improved contact at one or another tube pins, thereby curing hum that had nothing to do with the tube itself.
It could conceivably be a tube socket problem but unlikely to be the tube itself, although strange things do happen. I would try wiggling the tube gently in its socket or even just tapping it with the eraser end of a pencil to see if that changes the character of the hum. Good idea.
It’s impossible to solve this sort of problem “long distance”, once you’ve eliminated the obvious. You did not answer the other obvious question; is your phono stage wall wart plugged into the same outlet as your linestage and amplifier? If not, try that. If yes, then maybe consult Bellari in Colorado.
Have you checked the grounding? Make sure the ground wire is tightly secured. Then try removing the ground wire. Is your linestage plugged into the same outlet as the Bellari? If not, make it so.
Does that unit have an AC filament supply? If so, I could imagine that there is a leak of AC into the signal that takes time to occur when you start from cold, because something needs to heat up before it leaks. It's important to know that this is a real "hum", meaning 60Hz or 120Hz pure tone. If not, we will start over.
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