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Peavey Classic 50 212 crackle/pop after checking most things

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  • Peavey Classic 50 212 crackle/pop after checking most things

    Late 90's/Early 2000's "black tweed" FX Loop Classic 50 212

    The amp was purchased in November and played for about 3 hours straight with no problems, then sat in place until 2 months later.
    The amp started crackling, popping and the sound best described as "cartoon electric chair"
    I smacked the top of the amp and it worked well for another hour, then we took it home

    I unplugged the reverb, pulled the preamp tubes(including PI), unplugged the reverb tank, turned all the controls to 0, left the input unplugged (no guitar/cable), jumped the fx loop
    Still does this after 10-15 seconds of warming up, seems to have gotten worse, it can be very loud

    Also if the amp manages not to pop/crackle, one of the tubes will start to red plate, tried new matched tubes, same thing, not always the same socket, doesn't always follow a tube either.
    Bias voltage is -15.6VDC on all 4 EL84 sockets

    Pulled the board, cleaned and tensioned all tube sockets, cleaned all pots, checked the ribbons for any obvious frays/breaks near the board, checked the impedance of the ribbons, checked all the diodes, reflowed most of the solder joints, replaced the filter caps with the same values, cleaned every post with a molex connection, tensioned every molex plug.

    Chopsticked the wirewound resistors, checked their impedance, checked impedance with every resistor on the power board with a fluke 88V, chopsticking seemed to reveal nearly nothing, though sometimes the power tubes are sensitive (old and new)

    Plate and screen voltage with no tubes appears to be appx 417VDC

    I THINK the problem goes away when I pull the two molex connections from the FX/FS board to the main board, but I'm not sure what that means, because I don't know all that I'm interrupting.

    Out of curiosity, the speaker output jacks are one switched and one typical mono 1/4, what would an intermittent speaker connection cause? would this fluctuate the plate voltages since those go through the OT?

    The crackling and popping is from the speakers, happens with extension cab as well (known good cab)

    since the bias voltage is used for switching is it possible that one of the transistors or something is fluctuating the bias supply? I guess that would affect all 4 tubes though...

    Any ideas or tips would be appreciated, I have my meter ready to check anything you need me to
    Last edited by Mesa; 02-02-2020, 10:52 PM.

  • #2
    When you said you did lots of resoldering, that included any board mounted tube sockets?
    Does it still straighten up with a good smack, or was it just that one time?
    One thing that can cause crackling along with red-plating is oscillation, do you have a scope?
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      Originally posted by g1 View Post
      When you said you did lots of resoldering, that included any board mounted tube sockets?
      Does it still straighten up with a good smack, or was it just that one time?
      One thing that can cause crackling along with red-plating is oscillation, do you have a scope?
      I haven't had it out of my apartment and don't want to smack it in the apartment (I'm on the second story)
      I'm trying to get a nice little tektronix scope on Wednesday.
      What causes a power tube to oscillate?

      Thanks so much for your time!

      Comment


      • #4
        Many things can cause oscillation, bad connections, bad components, poor lead dressing of wiring, etc.
        In this case, as it was working ok for awhile, and responded to being jarred physically, I would suspect a bad/intermittent connection.
        In many cases with modern pcb amps, the tube sockets are board mounted and prone to solder fatigue due to heating/cooling cycles and vibration. That's why I asked if you had resoldered them.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          I recently went thru this scenario with several Peavey Classic 30's. Check each of the plate resistors, starting with the one connected to the tube that's redplating. Lift a leg before you measure.

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          • #6
            Originally posted by Perkinsman View Post
            I recently went thru this scenario with several Peavey Classic 30's. Check each of the plate resistors, starting with the one connected to the tube that's redplating. Lift a leg before you measure.
            Not sure what you mean by this? Power tubes generally don't have plate resistors.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #7
              The plate resistors are on the preamp tubes, 100k & 150k resistors, ill bet one is open.

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              • #8
                Schematic?
                - Own Opinions Only -

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                • #9
                  Here you can see it:

                  https://www.thetubestore.com/lib/the...-Schematic.pdf

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                  • #10
                    So did this ever get resolved?

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