Did you try rolling in a fresh pair of driver tubes? I think those Tucker boards used the 6N1P right? Those aren't exactly the longest lived tube in the world, and when they get flat, that plate voltage will start drifting up.
Otherwise, you can just order the flat rate repair service for two amps, and that will take care of what you need done. It may take a little while for the boxes to go out and come back to me, so toss a note in one of the boxes that's just a print out of this thread and I'll remember what to do.
-PB