Tube Failure

Who makes the amp? Do you have to bias your tubes or self bias?

Power tubes in a Mac and VTL 2-5000 hours on power tubes.

Signal valves depends on the duty and what it’s being used for, sometimes only half the valve is being used, but you still need good valves to begin with. Same 2-5000 hours.

I’ve lost power and signal tubes..Sometimes problems with one can adversely effect the other.. One can take out the other..

You say 6550? Not KT88s, they are a little different.. US vs UK ay?
I like TungSol 6550s most others are a shot in the dark..EXPENSIVE
300-1000 a set, for well matched quads.

Regards

Seems like it was a minor issue. Agree it is strange that the tube would blow upon turn off and not turn on...but you did state that it had an issue on prior turn on.
Wonder how often a driver tube blows before a power tube? Has anyone actually had a driver ( small signal) tube blow on them?

This happens quite frequently with some Chinese tubes and other cheaply made ones.I went to top SS amp as i could not deal with this any more i had ARC, CJ etc.Good luck though.However there are some very dependable tube amps out there but there will always be badly bad tubes as well.Good luck.