Had another KT88 failure last night. Fortunately I was sitting in a semi dark room listening to music and watching my ST-120 and suddenly there was a flash in the rear left KT88, blew the fuse etc. (I know that I should get out more but I enjoy listening to the sound of the music form this amp). Have checked the tubes with my tester and all tubes seem to check OK. The tube that flashed, tests OK for leakage and shorts however its transconductance is considerably lower than the other three. If I had not been sitting watching the amp I would be up a tree as to which tube was the offending one. The question is, should all four tubes be replaced with a new set? (The other three are all over a year old with about 1200 hrs. but do test about 500 points above the minimum acceptable value for KT88's). Or can I get away with just replacing the one? I also tested the rectifier and it seems to have come through OK. However I wonder if this type of failure puts enough stress on it and I should put it aside as an emergency backup?
Just out of curiosity has anyone had their power tubes last long enough to actually die from general use? Of course I refer to new issue tubes. All of the tube failures that have occurred with my amp have been sudden shorts internally.
Thanks in advance for your input.