Fuse blown, replaced rect tube as advised and

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AbstractLunatic, Jun 16, 2006.


  1. AbstractLunatic

    AbstractLunatic Member

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    still blows the fuse.

    Amp has EL34 and GZ34 tubes. I was playing with the volume cranked and I hit the amp with a fuzz. Instant quiet... 3A sloblo fuse is fried. I call the maker and a new rectifier tube is on the way because that what they think it is. So I bought a new tube today because I can't wait. I got a 5AR4 because that's all I could find today. I got a couple of fuses, put one in, power up the amp and nothing. Standby was still engaged. I pull the fuse and it's blown.

    What should I do now?
     
  2. John Phillips

    John Phillips Member

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    A 5AR4 is equivalent to a GZ34, that's nothing to worry about.

    By 'standby engaged', do you mean the amp was set to mute or play? Sorry, this terminology is confusing. If (as I'm guessing) you mean with it set to mute, you can probably rule out a power tube short.

    If you have at least one more fuse, try again with both the rectifier tube AND the power tubes pulled (leave the preamps in). If it blows now there is a major problem (you could finally eliminate all tube problems by pulling all the tubes, but preamp tube faults almost never cause fuses to blow).

    If it doesn't blow, try it with the power tubes back but the rectifier out - if it now doesn't blow that will eliminate a power tube filament short (different from a normal power tube short, and rare but not impossible).

    If the only thing that causes the fuse to blow is the rectifier tube being in, either you have another bad tube (not impossible) or there's a more serious problem.
     
  3. donnyjaguar

    donnyjaguar Member

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    What type of amp is it?
     
  4. pfrischmann

    pfrischmann Member

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    OR, You could make Gerrie Weber's Current draw tester....essentially a 100w lightbulb in series with the amp. It takes about $10.00 worth of parts and can really help with trouble shooting a problem like this.

    Saves a lot of time changing fuses and money in fuses.
     
  5. AbstractLunatic

    AbstractLunatic Member

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    Pulled all the tubes. Fuse still blows. :worried I guess I have to send it back to the maker. It'll take a week each way just for shipping. :( Very bummed...
     
  6. Blue Strat

    Blue Strat Member

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    Wow! A shorted PT in a brand new amp?
     
  7. danieldroukas

    danieldroukas Member

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    Blue Strat...
    If I understand what he's saying correctly, there's the possibility that the standby switch could be after the first filter section and, if this/these caps are shorted, cause the fuse to blow, right? If the rect. is out of the amp and it still blows then yeah PT is likely bad.
     
  8. Blue Strat

    Blue Strat Member

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    He said the fuse blows with NO tubes plugged in. This would seem to limit contenders to the PT or possibly something shorted on the heater circuits.
     
  9. John Phillips

    John Phillips Member

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    If it blows fuses with no tubes (including the rectifier) in at all, a blown PT is unfortunately the most likely cause.

    The only other possibilities are so unusual they're very unlikely... unless the amp is like a Mesa Dual Rectifier, which has both tube and solid-state rectifiers. Mine had a short in the solid-state rectifier, which is why I know about this one :).
     
  10. ekp

    ekp Member

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    It could be bad wiring too... That is not unheard of...
     
  11. donnyjaguar

    donnyjaguar Member

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    If there is a blown MOV in there it could be the cause too. Much less expensive than a power tranny.
     
  12. Texasamp

    Texasamp Member

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    Check you rectifier socket to make sure you are not arching AC to ground.
     

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