KT88 Failure

General question to the members.

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.


ArlanB

Bob Latino wrote:Hi Arlan,

Another thought, and what I have done before, is to replace that PAIR of output tubes on that channel and use the one good tube of the pair that is in there now as a back-up tube in case one of the other KT88's goes bad on your amp.

Bob

After a while those extra tubes come in handy for matching other pairs as they age.

IMHO, you can never have enough KT88's


Steve

Or 6L6GC's, or 6BQ5's, or 6BM8's, or 12AX7's, or...

Was I going to make a point...hmmm....

Bob, Westy56,

I will do the "replace both tubes" on that channel and save the one as backup. I have 3 Gold Lions, 3 J/J's, 3 Elektroharmonics, and a brand new set of Penta Labs KT88SC as backup. Since the tubes in the amp are Penta Labs I will use a pair of these first. I really like their sound and they did last longer than their 7 month warranty although as Bob indicated they should last longer in relationship to their cost.

Bob and Steve thanks for the input.

ArlanB

If you end up buying a new KT88 or a pair... I'd buy a spare GZ34 if you are running a GZ34.. These are the most common tubes to fail in a ST120 amp. I've lost 2 in a year. ONe a nice fat big bottle Sylvania NOS..