Pro Jr volume loss....

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bigroy, Mar 14, 2006.

  1. bigroy

    bigroy Member

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    Just bought a used Pro Jr to use as a practice amp. I took it to an open mic last night and the thing was singing, and then all of a sudden, the volume just dropped to almost nothing. I turned it off, back on again, and the volume was fine for the first few notes, then quickly and smoothly dropped back to a level that was barely heard over the drum kit. We weren't playing that loud either. Any time the volume would drop, if I just stopped playing for 10 seconds or so, it would come back strong, but then gradually drop again.

    I am suspecting the first preamp tube, and was going to retube it with some JJ's anyhow, but I'd like to troubleshoot it over the next few days while I wait for the new tubes to arrive.

    Do you think I'm on the right track with the preamp tubes?
     
  2. Dave C

    Dave C Gold Supporting Member

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    I'd suspect power tubes first , but it could be a preamp tube or a crummy connection somewhere in the amp as well.
    Dave C
     
  3. bigroy

    bigroy Member

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    I'm replacing the whole tube set anyway, hope that takes care of it. We'll see.
     
  4. Poppa Stoppa

    Poppa Stoppa Member

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    I'm having EXACTLY this problem at the moment with my PJ. I put in a complete replacement set of tubes, and it didn't cure the problem.

    I took the back off and waited til it happened again - and found I could cure it by pushing the grey ribbon to the second preamp valve (with a chopstick to avoid shocks). I figured it was a dry joint on that ribbon. So I took it to my local amp repair guy and...now the problem won't recur. Poke it, prod it, it won't happen again. I've got to go over there this weekend and play a guitar through it until it dies again.

    So let me know if you find the cause, and I'll keep you posted.
     
  5. bigroy

    bigroy Member

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    I was hoping the tube swap would do it. I have to work till noon today, then off to Chicago to see Donald Fagen tonight. I was going to install the new tubes and speaker tomorrow afternoon. If that doesn't cure it for me, I have a great amp tech in Indianapolis that can usually track down anything.

    I'll keep you posted on my results as well.
     
  6. mbratch

    mbratch Member

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    Sounds like a bad solder joint for sure. If you are handy with a soldering iron and understand good soldering technique, you only need to reflow all the connections on that cable that go to the tubes.
     
  7. bigroy

    bigroy Member

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    Full retube, and mine still has the volume sag. Off to the shop tomorrow.
     

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