Not Urgent --Anymore--

I had a MM amp given to me for Free. I turned the amp up to 11 and the thing cut out twice and then that side of the output transformer would not work any more?? So I ordered new tubes and used the otherside. When I got the tubes JJ's 6l6's I wanted to check the bias before I installed them to make sure it wasent somthing crazy (I work on electronics and have seen some strange stuff). Only one of the resistors was showing a reading?? I wandered if this have some thing to do with the one half of the output transformer burning out but thought it was not fesable for it to work this way since the amp is not sterio. I crossed voltage on one of the ICs when I was trying to track the circet and the voltage came back. My best guess is the Ic overloaded and the dead short forced a reset??
Anyway I left the amp at 28 on one 31 on the other it sounds great I don't care much about tube life. with the other set of tubes in the bias voltages changed with volume now the voltages are stable can anyone explain this I work with all kinds of stuff everyday but I am new to Valves.
I'm going to have to build a pedal for this amp and want to include a tube preamp with 3 ax-7's any good schematics on this kind of pre-amp out there?? After awhile I'll have to include pictures of this amp inside and out it's in great shape.
The board says June of 1980
Chassis#2275.75
Tweed 210-75 (Not a HD)