Phono preamp hum

Yes, the ground wire is secure.  I have tried grounding it a few different ways with the same result.  Like I said, it sounds fine for about 30 minutes, then the humming gradually becomes louder the longer it plays, or even not playing.

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. 

Did you try changing the tube?

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. 

@lewm  No it isn't.  @Kokomo  Nope, I have given up on it.  I have another unit that works fine.