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    KT88 Failure

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    ArlanB

    Posts: 40
    Join date: 2011-01-23
    Age: 67
    Location: Santa Cruz, California

    KT88 Failure

    Post by ArlanB on Wed Aug 31, 2011 12:57 pm

    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
    Admin

    Posts: 1996
    Join date: 2008-11-26
    Location: Massachusetts

    Re: KT88 Failure

    Post by Bob Latino on Wed Aug 31, 2011 2:15 pm

    Hi Arlan,

    Since each output tube is biased separately on the ST-70/ST-120, you can probably get away with just replacing that one tube that went bad.

    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.

    I try to sort of keep track of hours on my tubes but sometimes I lose track. Output tubes should last 2000 - 3000 hours but in all honesty the tubes made today in Russia, China and the Slovak Republic are not as "hardy" as those made in the USA, England and Germany in the 1950's and 1960's. Maybe 1500 - 2000 hours is probably a more "realistic" figure for output tube life today ?

    Bob

    Westy56

    Posts: 63
    Join date: 2011-05-09
    Location: Albuquerque

    Re: KT88 Failure

    Post by Westy56 on Wed Aug 31, 2011 2:37 pm

    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 Wink


    Steve

    anbitet66

    Posts: 119
    Join date: 2009-12-23
    Location: Valley Stream, NY

    Re: KT88 Failure

    Post by anbitet66 on Wed Aug 31, 2011 5:01 pm

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

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

    ArlanB

    Posts: 40
    Join date: 2011-01-23
    Age: 67
    Location: Santa Cruz, California

    KT88 Failure

    Post by ArlanB on Wed Aug 31, 2011 5:39 pm

    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

    mantha3

    Posts: 292
    Join date: 2010-11-10

    Re: KT88 Failure

    Post by mantha3 on Wed Aug 31, 2011 5:54 pm

    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..

      Current date/time is Thu Dec 18, 2014 4:07 pm