what to check when pwr tube blows?

[allsavy]

I traded a pair of GT6L6ge #3 with an amp tech for a pulled set of #6 to get more headroom and he told me my tubes (almost new) looked like they were way over voltaged or something due to the paint and logo discoloration. I checked all voltages at idle with the tubes in and nothing seems unusual. The pulled tubes lasted about one year of light duty but started crapping out once I began using this amp for rehershals recently. I put in a new pair of GT6l6 #4, re-biased and within the first 1/2 hour of rehersing one tube began to arc like crazy. Could be a bad tube but what might I check in the circuit to be sure the issue doesn't lie eleswhere?
The amp is a fender princeton reverb ll (a/b push pull fixed bias.)

[Enzo]

If a tube is arcing inside, I think bad tube.