My newly refurbed and 6550-converted 1973 Ampeg V4B is doing this weird thing that has happened three times now. It works great, sounds great for an entire gig. I leave it on standby during breaks. Turn it off at the end of the night. Next time I turn it on, it hums. So I re-tension the tube socket "springs" or what ever they're called. The amp works fine. For the entire next gig. Until I shut it off, it sits until I turn it on and the hum is back again. So obviously heat from the output tubes is causing the tube socket springs to open up. What can be done?