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Tube amp volume shuts down after 20 minutes

Discussion in 'Amp Input - Normal or Bright' started by axis69, Aug 11, 2019.

  1. axis69

    axis69 Senior Stratmaster

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    its a jcm900 12" speaker combo and after 20 mins the volume starts to sputter here and there and then shut off completely.Put it on standby for 5mins and it works fine again.
    What do you all think?Tubes,solder joints?
     
  2. thomquietwolf

    thomquietwolf Dr. Stratster Strat-Talk Supporter

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    Oh good
    My Carvin 16/5 does the same
    But
    No touch
    No play
    2-5 minutes
    Back again..
     
  3. Believer7713

    Believer7713 Most Honored Senior Member Silver Member

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    Sounds like the problem I had with my JCM2000 DSL401. The bridge rectifier was overheating. I had to lift it off of the circuit board about 1/2 inch so it could get air flow under it. Never had another issue in the last 5 years.

    Sent from my SM-G975U using Tapatalk
     
    rocknrollrich, fezz parka and axis69 like this.
  4. axis69

    axis69 Senior Stratmaster

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    yeah,the back of this amp gets super hot..even the metal plate on the rear gets really hot...idk how the speaker.copes!
     
  5. circles

    circles Resident Pinball Enthusiast Strat-Talk Supporter

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    Yikes.
     
  6. gjohnson441496

    gjohnson441496 Strat-O-Master

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    I had that problem once and a tube change solved it.
    btw only one was the culprit.
     
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  7. vid1900

    vid1900 Most Honored Senior Member

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    Sounds like the Bias Supply has kicked it.

    There is a "type x" safety Cap and a resistor that will probably both need replacing.

    Stop using the amp because you are killing your power tubes.

    If you are a repair kind of guy, I can tell you what to replace, if not you are going to need to take it to your amp tech.
     
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  8. vid1900

    vid1900 Most Honored Senior Member

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    jcm
     

    Attached Files:

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  9. axis69

    axis69 Senior Stratmaster

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    it smells also- not burning but...i have a fan on it now and it hasnt shut down ....but i wont run it anymore til its fixed...thx
     
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  10. rocknrollrich

    rocknrollrich Senior Stratmaster

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    It's been a while since I looked under the hood of mine, but I believe they used individual diodes for the rectifier.
    Not a square block package (jcm900 that is).
    They typically don't get hot as they are well specd for this model.
     
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  11. rocknrollrich

    rocknrollrich Senior Stratmaster

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    I would take it to a competent tech.
    Honestly, these amps are not as difficult to work on as the dsl/tsl models that came after them.

    If it's getting really hot on the back panel, this is not normal. I have one and it doesn't require a fan.
    I suspect what is getting hot may be a transformer, probably the power xformer. Something is making it very unhappy.

    Are your tubes red plated ( do the grey peices inside glow red)?
     
  12. axis69

    axis69 Senior Stratmaster

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    the tubes glow orange...
     
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  13. rocknrollrich

    rocknrollrich Senior Stratmaster

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    There is a filament that runs up through the middle of the tube.
    That is a heater filament. That should glow (orange).

    If you look at the grey plates of the tube, they should not be glowing at all.
     
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  14. Lester H

    Lester H Strat-Talker

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    "Oh good
    My Carvin 16/5 does the same
    But
    No touch
    No play
    2-5 minutes
    Back again.."


    My Carvin Belair used to do that at start up but you never knew when it was going to do it. It would sit there for minutes then suddenly be ready to go and I could play a 4 hour gig and it would never acted up. Then start up fine for 3 or 4 days then do it again. I changed tubes and cleaned the jacks with no luck.

    I opened it up and tried to get it to recreate the problem with a chop stick pushing on things. Finally pressing on the pre amp tube PC board I could get it to do it. Tightened all the preamp tube socket pins and re-flowed the solder on the sockets and it hasn't done it in months now.
     
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