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    Blown KT120

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    vegaseddoc

    Posts : 15
    Join date : 2009-11-21

    Blown KT120

    Post by vegaseddoc on Sun Jan 01, 2012 9:54 pm

    I'm running an ST120 with Tung-Sol KT120"s. The tubes have about 300 hours on them and one arced during a very bass heavy song. Not sure what portion of the tube failed but I have no bias voltage on the tube. I had them biased at .550 V so they should not have been running any where near the reported specs. No damage to the amp. Placed a back-up set of power tubes and everything seems to be fine. Anyone else have trouble with early failure of KT120's in their amps?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Blown KT120

    Post by Bob Latino on Sun Jan 01, 2012 10:15 pm

    Hi,

    On the contrary those Tung-Sol KT120's tend to be much more reliable than KT88's when used in KT88 designed amps like the VTA ST-120 - BUT - as you have found out ... Anything can happen. If you lose a tube within the first 90 days most tube vendors will replace the tube. Keep your sales receipt at least for the first 90 days ..

    Bob

    vegaseddoc

    Posts : 15
    Join date : 2009-11-21

    Re: Blown KT120

    Post by vegaseddoc on Mon Jan 02, 2012 11:11 am

    My curiosity got the better of me so I took the tube apart to see what failed. Turns out that the fine wire that connects pin 8 to the cathode completely melted. I guess that is why the amp didn't blow the fuse because the circuit opened rather than shorted. My suspicion is that the wire had a defected such as a kink or a gouge that caused an area with increased resistance and therefore heat. When I loaded the tube hard it failed. Interesting though that my Sovtek KT88's with approximately 2000 hours on them are still kicking strong.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Blown KT120

    Post by anbitet66 on Mon Jan 02, 2012 11:39 am

    Doc,

    I had a similar experience 15 years ago. I had been given a (formerly) matched pair of GE 7581's. One of them was totally dead. It lit up, but otherwise did nothing. I used my tube tester to confirm it had no output at all. If I cranked up the filament voltage to 12 volts I could get grid emission, but not much else. I eventually found the wire to pin 3 (plate) where it attached directly at the plate was welded poorly. The connection broke, and the tube died a very unfortunate death. It came from a monoblock amp and the owner said he replaced both tubes since it didn't have the output it once had.

    Like any mechanical device, there is quality control, and the lack there-of. I'm sure GE, like any other manufacturer has their share of bloopers that pass all quality checks, but ends up failing in the field. Keep in mind the tube was 25 years old or more when I got it, not quite the same as one much newer.

    At least you know it most likely wasn't a fault of your amplifier Very Happy .

    Tony

    tubes4hifi
    Admin

    Posts : 1282
    Join date : 2008-11-30

    Re: Blown KT120

    Post by tubes4hifi on Mon Jan 02, 2012 3:26 pm

    sounds about right, back in the "good-old days" this would be a one in a million failure.
    With new tubes, maybe more like one in a thousand failure.

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