Vintage Super Lead 100 Problem, Please Help!!!

Discussion in 'The Workbench' started by Frank Plante, Dec 23, 2018.

  1. Frank Plante

    Frank Plante New Member

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    Hey everyone,

    I have a vintage JMP Super Lead 100 and it started making a sort of loud static-like sound when I play. It doesn’t have any problem if I don’t play, and the problem seems to increase as I increase the treble/presence. I have tried the head with another cab, it has the same problem. I took it to two different techs, they couldn’t fix it. I changed the preamp and power amp tubes, I cleaned the tubes sockets, nothing seems to work and I’m getting pretty desperate because it really is a killer amp and I don’t have any ideas anymore. Can somebody help me?

    By the way, is it possible to send a video or an audio file so that you can see or at least listen to the sound it makes? I'm don't seem to be able to.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    (Bump this to Workbench?)

    Yes we will help you.
    You need to make several posts before you can attach files...but yes you can attach a sound cloud or you tube file.

    We will probably ask you to measure inside the amp with a volt meter, can you do that?

    Can you take pictures of the inside of the amp?
     
  3. Frank Plante

    Frank Plante New Member

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    Thank you so much!
    I can't even post google drive links to the videos... hmm and yes I can take a couple pictures! How would I take this thread to the workbench section? Sorry I'm new here haha
     
  4. neikeel

    neikeel Well-Known Member

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    Pics and voltages and sound clips are all good:agreed:
    One or two things spring to mind:
    1) Failing preamp filter cap (on V2)
    2) Failing plate resistor
    3) Had similar thing with a JMP - turned out to be a corroded HT fuseholder internals caused intermittent connection with the amp being cranked sitting on the cab, weirdly a similar intermittent static sound that was not apparrent with all my other debugging tests................?
     
    Last edited: Dec 23, 2018
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  5. Frank Plante

    Frank Plante New Member

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    Here are a few pictures, tell me if you need more! Although I still can't post any link so I can't send you the soundclips
     

    Attached Files:

  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I've had a problem similar to this that was caused by a cold solder joint. It can't hurt to check the connections with a magnifying glass.
     
  7. Frank Plante

    Frank Plante New Member

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    I already tried that, thanks though. Another thing to note is that I can play. the sound doesn't cut or anything like that. There's juste a weird static noise behind the sound of the amp, if that makes sense.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Have you measured the bias current with a meter?
    Do you have a bias probe?
    What output tubes are installed?

    Cold bias will cause static noise, almost like the sound of a blown speaker.
     
  9. Frank Plante

    Frank Plante New Member

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    I
    I was planning to order a bias probe in like 5 minutes haha, will the Eurotube one do the job?
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Let's get more close up pictures of this area, output tube sockets.
    Looks like a burned screen resistor but can't see really clearly.

    SLP100.png

    SLP100.png
     
  11. anitoli

    anitoli Well-Known Member

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    All you need to bias that amp is 4 1ohm 2w precision resistors placed across the cathode connection and a multimeter.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    What meter do you have?
     
  13. Frank Plante

    Frank Plante New Member

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    Is that better?

    By the way, can I inbox you the links so that you can post them in the thread? I think a sound sample would be really good thing to have here haha
     

    Attached Files:

  14. Frank Plante

    Frank Plante New Member

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    My brother is an electronic tech, he has a bunch of multimeters
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Looks like screen grid resistor is toast.

    slpscreenresistor.png
     
  16. Frank Plante

    Frank Plante New Member

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    Thanks! That would be the green one right? It is hard to see on the resistor itself, is there a way to know what the value of the resistor is?
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn amp off
    Unplug power
    Remove the 2 output tubes.
    Set meter on ohms
    Measure between pin 4 and pin 6 of output tube socket(s).
    Both sockets should read 1000 ohms.

    If you don't get the 1000 ohms, the screen resistor should be replaced.
    1000 ohm, 5 watt, wire wound resistor.
    Usually these are replaced on both sockets so that the 2 resistors match.
     
    Last edited: Dec 23, 2018
  18. Frank Plante

    Frank Plante New Member

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    Thank you very much! I actually have 4 tubes, so I should replace the 4 resistors?
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out all 4 tubes and test all 4 sockets.
    1000 ohms between pin 4 and pin 6 on each socket.
    But I think at least 1 is bad.
    Yes if you want you can make all 4 the same type.

    If you don't know how to solder find somebody to help you.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    It's good your bro is a tech, he can help you replace the 4 sockets and the screen grid resistors.

    SLPSOCKET.png
     
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