Advice please? Amp keeps blowing reverb tube [12ax7]

Discussion in 'Amps and Cabs' started by snow and steel, May 29, 2015.

  1. snow and steel

    snow and steel Supporting Member

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    I've got a amp that keeps blowing a tube. The amp is less than 2 months old. I'm keeping the manufacturer's name quiet at the moment because I am in the process of dealing with them about this issue and giving them the chance to make this right.

    The tube it keeps blowing is a 12ax7 that controls the reverb. I can put in a new tube, and the reverb works for 5-15 minutes, and then suddenly the reverb goes out. The tone of the amp does not change in any other way - the reverb just goes out. So far it has gone through three tubes in one day.

    When I contacted the manufacturer they suggested replacing the tube, and it blew. When I got back to them on this they said try it again because if the reverb comes back it must be the tube. Sure enough, a new replacement and it blows again.

    I'm curious if anyone might have an idea of what might cause this?
     
  2. snow and steel

    snow and steel Supporting Member

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    no one has any theories?
     
  3. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    If it is the reverb circuit driver, it probably requires a 12AT7, not 12AX7. That 12AT7 can handle greater current than a 12AX7.

    - T
     
  4. snow and steel

    snow and steel Supporting Member

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    the tube sheet that comes with the amp specifically lists a 12ax7. I'm not honestly sure if its considered a driver or what, but it is the tube that the reverb runs through. tone does not change on the amp in any way other than reverb no longer works.
     
  5. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Hmm... I still wonder if that's a typo. Perhaps the reverb is solid-state powered, and that 12AX7 amplifies the reverb return. I'm just guessing here without seeing the circuit.
     
  6. snow and steel

    snow and steel Supporting Member

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    I can send you the info privately, if you'd like and are willing - like I said I'm trying to give the manufacturer the benefit/time to make this right.
     
  7. zenas

    zenas Member

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    Is this amp made by one of the big companies that used to make good amps?
     
  8. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    So, from looking at the manual, it seems you're talking about "V6" - yes? That V6 is listed as a 12AX7, and it functions as both the reverb send and return. It is more common for the reverb send to be a tube that can carry higher current (12AT7, 6K6GT, 6V6GT, etc.). I can't advise any changes from a 12AX7, after viewing the manual, but your dealer may recommend the make of 12AX7 they've found to be robust in that position. I'd hold tight until you hear back from their rep.

    Best of luck!

    - T
     
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  9. kbgear

    kbgear Member

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    My first thought was a bad/shorted/missing coupling cap or plate resistor, but it would take a tech to confirm.
     
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  10. snow and steel

    snow and steel Supporting Member

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    Yup, that's the one.
     
  11. J M Fahey

    J M Fahey Member

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    I suspect a shorted cathode cap.
    The plate load is "shorted" for DC because it's a transformer primaray.
     
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  12. J M Fahey

    J M Fahey Member

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    Ok, just extrapolated published 12AX7 datasheets (which deal with normal use) into abuse , in this case getting 300VDC on plates without a , say, 100k plate resistor but instead the MUCH lower resistance primary of a reverb driver transformer, AND getting 0V bias (shorted cathode cap or floating grid which lost its ground reference or even worse: lossy coupling cap turning grid *positive* (ugh!!!) .
    In any case I get around 6.5mA plate current, which multiplied by 300V gives me 2W plate dissipation.
    That's for one tiny triode plate and 200% the maximum allowed power.
    No surprise they die in minutes.
    [​IMG]
     
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  13. snow and steel

    snow and steel Supporting Member

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    I'll certainly mention that to them in passing, thank you very much!
     
  14. guitarjazz

    guitarjazz Member

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    Take it to Click and Clack.
     
  15. snow and steel

    snow and steel Supporting Member

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    wow... that is.... helpful?
     
  16. zenas

    zenas Member

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    Yeah one of the Tappit brothers past away. The shows been in reruns for I don't know how long.
     
  17. snow and steel

    snow and steel Supporting Member

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    Amp went back to the manufacturer. They at first said "bad tube", and when the new tubes burned out they said I might have damaged the new tube in install. When it did the same thing and burned out the second tube [third including the factory on] in one day they decided it might be a transistor that controls the reverb that burned out. Should be there Monday - we'll see what they say.
     
  18. snow and steel

    snow and steel Supporting Member

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    Heard back from the Manufacturer today. They are mailing the amp back to me. They said they have fixed it. Hopefully that fixes everything.
     

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