Phono preamp hum

No quiz.  It has been back in the box for a couple years.  I keep looking at it wondering what's wrong.  Thought maybe I could get some help as I am no tube genius.  Maybe I will give it away and let it drive someone else crazy.

I’m sure it’s an easy fix. But the fixer needs physical access to the unit.

I’ve had a bad tube behave like that in the past…I replaced the tube, and never heard it again.

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.