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PPIMV installed now it’s squealing

Discussion in 'The Workbench' started by Rotorcraft230, May 17, 2021.

  1. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    6853934B-01B1-4E29-83E4-44D39BDBA9EF.jpeg 6E3240C6-F47B-4058-8C5D-3781D1329E45.jpeg 0A997B83-471F-4BE5-9387-891415EA7804.jpeg 6601A5B3-833F-46E6-8A7F-4CD7C768624A.jpeg 4CED6EAF-E9FA-4A54-9DF8-F541B290BB8A.jpeg 6FA7F4EA-4FB9-40E1-BA2D-98387C0B84C9.jpeg 01D68E3E-BB3B-48E2-AB97-0B6F194A8BA3.jpeg So I have a Metroamp JTM 45 that I built and installed his PPIMV. Now the controls squeal. The wire he sent is 2 wire with a ground wire. I hooked it up and it works but I think the wires are too close to the preamp circuit, I used the lower input on channel 2 for the pot. Do I need to move the pot or can I use different wire with shielding and ground the shields.
     
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  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Remove the jack entirely. Both for safety reasons and because that jack is basically an antenna. That may stop the squealing, and you can always reinstall it later if you want.
     
  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Or mount the PPIMV pot in a speaker jack hole and see if that makes a difference.
     
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  4. junk notes

    junk notes Well-Known Member

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    Outer shield grounded?
     
  5. pedecamp

    pedecamp Well-Known Member

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    If you followed his instructions I wonder if theres something wrong with the pot? I'd ask Metro...
     
  6. neikeel

    neikeel Well-Known Member

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    Don’t put ppimv there unless you use shielded wire
    You are taking output signal right across your preamp and back again
    Move the pot either to the polarity switch position or back of the amp.
    Simple.
     
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  7. Pete Farrington

    Pete Farrington Active Member

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    Master volume control in close proximity to an input? That’s what’s known as a bass ackwards layout ;)
    Maybe if you keep hammering you’ll get that square peg rammed into the round hole?
    Placing circuit nodes with perhaps 100dB of voltage gain between them so close together seems like asking for trouble.
    I acknowledge that some folks do it with apparent success, but that doesn’t make good / sensible practice.
     
    Last edited: May 17, 2021
  8. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Am i dreaming or does it seem these PPIMV's cause more problems than they are worth? Just an observation that's all.
     
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  9. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    No it isn’t.
    He has 2 412 cabs he uses.
    Didn’t think about putting it there until I herd the sequeling. Thanks for your input
    thanks Pete for the vote of confidence. Yea I’m a dumb ass, I really don’t want to drill a hole is the chassis that spot was clearly not the correct spot

    Gotta rethink this.
     
  10. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    I was just moving it out of the way for the pot to see how the PPIMV acted there well now I know. Gotta find a different place
     
  11. JP2036

    JP2036 Well-Known Member

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    Thats why most MV pots on NMV amps are mounted on the back.
     
  12. neikeel

    neikeel Well-Known Member

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    For ultimate low noise you can do it like George at Metro does.
    He installs the pot on the back of the amp near the PI, he flips the pi output caps and wires them PTP on the pot.
    Don't forget that the grid stoppers go right on the output tube sockets too (if that is what you have hidden in the shrink wrap?;)
     
  13. Travis398

    Travis398 Well-Known Member

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    You could wire one of the cabinets with an extra 1/4" jack so one plug from amp and connect cabs.
     
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  14. Pete Farrington

    Pete Farrington Active Member

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    Not at all, we’ve all been there, the 1st time is an opportunity for a learning experience. It might apply however if someone went and did the exact same thing over again.
    Sorry, perhaps I might have hammered the point home a bit too forcefully, I was hoping to dissuade you from continuing to persist in trying to make a flawed plan work.

    Might the screen of the cabling be connected to the bias supply output? If so, that’s fine, it will provide a screening function, as it’s a low impedance path to AC 0V common.
     
    Last edited: May 17, 2021
  15. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    That’s a good idea but they are JCM800 from 1980-1983 don’t want to cut any holes in them.
    I can check into that and as far as the heat shrink on the wires was they were too short so I made them a bit longer.
    I did use the bare wire in one of the bundles to connect both pots and then to the junction of the 220k resistors
     
  16. william vogel

    william vogel Well-Known Member

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    The part I’m having trouble understanding is the guy plays through a full stack but needs a master volume control on a jtm45.
     
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  17. Matthews Guitars

    Matthews Guitars Well-Known Member

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    It can be problematic any time you have a strong signal running close to the sensitive front end of the amp. Which is what happens when you put a PPIMV in the input jack cluster.

    You're creating a situation that is primed and ready to start up a positive feedback loop.

    For the same reason you don't want to put the microphone right in front of the speakers, a master volume should be kept away from the inputs unless great care is taken to keep it from getting into a feedback/oscillation scenario.

    If you want to put the master volume in the input jack cluster, use high quality shielded cable both on the PPIMV input and output leads AND upgrade the input jack wiring with shielded cable as well.

    And, where the input and PPIMV cables must cross, dress them to cross each other at right angles. This will keep capacitive coupling down to an absolute minimum.
     
  18. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    72C0E1BD-AF8B-499B-801E-05EC3D887D1A.jpeg 0EADDF09-7D0B-4009-87D1-8676548019ED.jpeg
    I’m gonna give the input side another go. I’m a helicopter mechanic but new to amp building. We have very high quality twisted pair shielded wire used in navigation systems for the aircraft. It’s 20awg and it’s Teflon. I’ll rewire it this weekend and report back.
     
  19. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The twisted pair isn't going to help much. Pick ONE conductor in that cable and use it for the audio signal. Pick the lead with the blue tracer. That's good wire, though, and should do a fine job.
     
  20. Pete Farrington

    Pete Farrington Active Member

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    If I had to do it, I’d use some thin metal plate and form a screened enclosure around the pot.
    By that, a square peg might indeed be persuaded to form an interference fit in the round hole :)
    Because even with screened cable, without a screened enclosure, the pot terminals will couple back to the input jack terminals and form a positive feedback loop. Given suitable control settings, input conditions and valve gain, the potential for oscillation can’t otherwise be eliminated, due to the close proximity of the jack and pot terminals.
     
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