Kendall, my Rog, a broadcast engineer who's played with these kinds of things all of his life, says it wouldn't be the plug-in vibrator (oh, my!) as you wouldn't get anything if it were bad.

If there is a lot of hum, it could be the filter capacitors which can be easily replaced. He says you just take them out and replace them; get them from any electronics parts store. And, if there is nothing else coming out a vacuum tube is probably bad.

If you want to pick his brain, send me a PM and I'll give you his number.:-)