I restored this 500C a few years ago. http://audiokarma.org/forums/index.php?threads/not-messin-around-anymore-500c-rebuild.545832/. It was pretty happy with a set of matched EH7591s, but I wore those out after 3 solid years of listening. Decided to try a set of the new production Tung Sol 7591As, the 500C didn't really like them from the start. I could only get -16.6V negative bias on them, for a draw of 40mA per tube, best I could do. I've been running them for probably a year that way. I noticed recently that one of the tubes (V10) was looking a bit heat stressed on the plates. So I took the Fisher out of the rack for some testing. My line voltage is 125VAC, the receiver is seeing 110 after the pair of CL70s I installed during the restoration. Pin 3 on the outputs is a nice 435V, pin 8 is 403V, pin 6 is -16.6V (that's as good as the neg bias gets on my 10K pot). The tubes are all over the place for current draw, V8 is 20.4mA, V9 is 43.8, V10 is 25.6, and V11 is 38.3. Looks like I have a problem with balance between the tubes, since the even numbered ones are trashed. Would an IBBA fix this with a new set of tubes? Is there anything else to check out to see why this 500C is killing tubes? Phase inverter adjustment, perhaps?