i have a 1988 fender the twin (red knob). i recently retubed with matched quad JJ 6L6GC's and i biased and blanced per manual, via the external test points and my multimeter. no problems. after borrowing two of the 6L6's to use for a couple of hours in another, new amp, i replaced them into the twin and thought i'd recheck bias. so, when powered up and taken out of standby, i immediately saw one tube (not one of the "borrowed" ones) begin flourescing wiht all sorts of fireworks inside the tube and there was a loud hum out of the amp. i immediately went back into standby and powered off. next, i swapped the affected tube from one socked to another and tried again. the same tube flouresced some more and it seemed there was a bluish glow in the power tubes. i ran the bias back by eye rather than multimeter and the glow went away. so i powered off. i got a replacement, matched JJ 6L6Gc to swap for the affected tube and powered up. now, there was no hum, no fireworks and all the power tubes are looking to be glowing normally. the problem is that the amp will no longer bias or balance to manual specs of bias = 0.040 and balance = 0 volts. what i am reading on the mulimeter is a bias that won't go below about 1.25 and balance that won't go below about 10 volts. any ideas on whatmight be causing these readings? i'm afraid to play the amp until this is resolved. i have no problem taking it to an amp tech but would like to have some idea, if possible, as to what is going on. any advice appreciated.