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    Bad 12AU7 - Cause or Effect?

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    cgtayloriii

    Posts : 1
    Join date : 2018-08-20

    Bad 12AU7 - Cause or Effect?

    Post by cgtayloriii on Sat Oct 27, 2018 8:00 am

    After operating perfectly for over a year, my VTA ST-120 started repeatedly blowing a fuse within a minute or two of being turned on. I removed the cover and found a blown 12AU7 (white top, cracked glass) in the V1 socket on the driver board. I've ordered a new tube which should arrive in a few days. My concern, however, is whether the bad tube caused the fuse to blow, or whether there is another problem that caused both the fuse to blow and the 12AU7 to fail. I don't want to stick in a new 12AU7 if something else is going on that I need to fix first. Any help would be appreciated.

    Charles

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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Bad 12AU7 - Cause or Effect?

    Post by Peter W. on Sat Oct 27, 2018 9:11 am

    It is quite unusual, but not entirely unknown, for a miniature dual-triode tube to self-destruct. That being written, if the blown tube is of Chinese origin, then that it failed in such a way is no surprise, but still not typical.

    If you have no additional means of testing the amp, and if you have done a basic check of the wiring and so forth, when the new tube comes in, watch it like a hawk for the first few hours. If nothing untoward happens, and if there are no strange sounds or visuals from the amp, it was probably as simple as a defective tube. Otherwise, let the diagnostics begin!
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    peterh

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    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Bad 12AU7 - Cause or Effect?

    Post by peterh on Sat Oct 27, 2018 9:24 am

    cgtayloriii wrote:After operating perfectly for over a year, my VTA ST-120 started repeatedly blowing a fuse within a minute or two of being turned on.  I removed the cover and found a blown 12AU7 (white top, cracked glass) in the V1 socket on the driver board.  I've ordered a new tube which should arrive in a few days.  My concern, however, is whether the bad tube caused the fuse to blow, or whether there is another problem that caused both the fuse to blow and the 12AU7 to fail.  I don't want to stick in a new 12AU7 if something else is going on that I need to fix first.  Any help would be appreciated.

    Charles

    The V1 tube does not in itself have the possibility to blow the fuse.
    It might, on the other hand, inject noise to the power tubes in an amount to blow the fuse, but it seems unlikely.

    You could follow the instructions from the build manual, removing tubes, power on etc
    and at the end of the instructions have all tubes, including the new 12au7 in place. That would be
    a safe road.

    Even if you simply replace the broken 12au7 you should as a first action check the bias on
    the power tubes.
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    sKiZo

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    Join date : 2013-04-01
    Location : Michigan USA

    Re: Bad 12AU7 - Cause or Effect?

    Post by sKiZo on Sat Oct 27, 2018 6:56 pm

    There's a difference between a tube failing and exploding, right? I can see a potential for an internal short spiking the circuit hard enough to blow the fuse.

    So how about picking up a couple cheap "sacrificial" tubes for testing? Some RCA clear tops maybe, quite nice sound to them, and cheap enough so no great loss if one goes up in smoke.
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Bad 12AU7 - Cause or Effect?

    Post by peterh on Sun Oct 28, 2018 1:36 am

    sKiZo wrote:There's a difference between a tube failing and exploding, right? I can see a potential for an internal short spiking the circuit hard enough to blow the fuse.

    So how about picking up a couple cheap "sacrificial" tubes for testing? Some RCA clear tops maybe, quite nice sound to them, and cheap enough so no great loss if one goes up in smoke.
    Peeking at the schematics; the 12au7 is surrounded by 30k and a LM334, whatever happens
    inside the 12au7 it won't load the B+ hard enough to blow the fuse.
    Only possibility is that the filament develops a hard short, and a new 12au7 will cure that!
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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Bad 12AU7 - Cause or Effect?

    Post by Peter W. on Mon Oct 29, 2018 8:33 am

    peterh wrote:

    Peeking at the schematics; the 12au7 is surrounded by 30k and a LM334, whatever happens
    inside the 12au7 it won't load the B+ hard enough to blow the fuse.
    Only possibility is that the filament develops a hard short, and a new 12au7 will cure that!

    I expect that is the case. The "bad" tube lost its vacuum, melted down, shorted the filament winding which - *LUCKILY* was enough to blow the fuse. Typically, it is the cheap fuse that is protected by the expensive transformer.

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