Yep, exactly what I was thinking. I figured when I had her cranked and vibrating good, one of the tubes must have backed out.

What I can't figure about it though is that both pre-amp tubes felt tight and they both still had the spring cap style keepers on them, shouldn't have loosened off.

What keeps bothering me in the back of my mind is what is going on with the socket or something in that area?

On the plus side everything is still humming like a top and working great. And if for some reason it stops again, it will give me the excuse to drop the chassis and have a look around. No schematics on the net that I can find but how hard could it be :D

Power of positive thinking!