'67 And '71 Jmp Hum Issues

Discussion in 'Let's Talk Vintage' started by soundboy57, Oct 15, 2016.

  1. soundboy57

    soundboy57 Well-Known Member

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    I have two different JMP's that have fairly loud hum, depending on where the volume knob is set.
    Seems it is on both "volume II" channels.

    At zero, there is hum, but turn it up to about 6-7 and the hum goes away.
    On one, it hums except for a narrow window at about 6.

    Filter cans are all replaced a few years ago, and the amps sound great with all NOS Mullards.

    Bad coupling caps? Seems odd that both amps are affected by volume two.
    I am pretty good with a soldering iron, and have done a fair amount of wiring/parts replacement over many decades, and can easily read a schematic, so I wanted to know what to measure, and where to start or look for the issue.

    Perhaps this should be in the Workbench, but I figured there was more of a chance of any of you having the same experience with these old amps.

    Thank you in advance.
     
    Last edited: Oct 15, 2016
  2. stratburst

    stratburst Active Member

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    Two amps with the same problem could indicate the source of the noise isn't with the amps but with power or the environment. Floressent lights and appliances are usual suspects. Have you tried firing up the amps somewhere else, a friends house or studio to see if the noise persists?
     
  3. pleximaster

    pleximaster Well-Known Member

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    Also check for ground issues. Start using a mild electric spay cleaning on star ground and other ground post. Then tight the bolts in the chassis holding the lugs. Clean input jacks and potentiometers. (Spray jacks and put a plug in and out like a horny rabbit for a minute. ) spray pots and turn them up and down for a couple of times.)

    It is however strange that you have two amps doing the same thing... I also recommend trying them in an other environment and with another guitar and guitar cable, (the combination of could make the setup work as an antenna and pick up noise it certain settings.

    Plexi

    If it doesn't disappear just give the amps to me an buy a Marshall digital Code instead!
     
  4. pleximaster

    pleximaster Well-Known Member

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    Ps

    Also try to lift the green wires going to the preamp tubes coming from the board. Optimal is 90 degrees compared to the blue and yellow ones that you keep flat against the chassis floor.

    Plexi
     
  5. soundboy57

    soundboy57 Well-Known Member

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    Thanks so much for the responses. I have 2 other vintage JMPs that are silent, no hum at all on the same outlets.

    I will try cleaning everything mentioned, checking the grounds and checking the wires.
     
  6. pleximaster

    pleximaster Well-Known Member

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    Do that these are the easy fix... and never harm anything. It could be a lot of other things... Neikeel will probably chime in and give some other good advice to what it could be. ;)

    best plexi
     
  7. soundboy57

    soundboy57 Well-Known Member

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    I won't have a chance until next weekend, so will update. Thank you so much. Pretty sure it is a minor fix, they otherwise sound great.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    There is 3-4 main issues which cause HUM.

    1. Tubes, don't assume that NOS tubes are noise free. Swap them w/ new ones and compare.

    2. Filament HUM (actually it's buzzing). This is the most common cause.
    Remove V2 preamp tube and the noise goes away....this helps confirm that it's filament noise.
    FIXING: you need to re-arrange the location of the filament wiring. This can eliminate the buzzing. The Marshall filament wire layout is about as noisy as it can get.
    Move filament wires in the preamp (connected to pins 4/5 and 9) away from grids (pins 2 and 7).
    CLUE: what helps is filament wires vertical, and grid wires horizontal, routed away from the sockets. Separate wires and don't let them touch each other.
    BUZZ from filament is amplified at V1 and V2 preamp tubes. This is where the filament wiring needs to be moved / modified.

    3. Filter Caps: you changed your caps w/ fresh ones? Including the bias filter caps?

    4. Oscillation: high frequency oscillation will cause the power amp to pass power supply ripple (HUM). It's better to have a scope to confirm this cause.

    5. Coupling Caps: not usually the cause of hum. Don't run out and change them all, cause it's usually a big waste.

    "Seems odd that both amps are affected by volume two."

    No it's not odd. What's happening is that volume 2 side of preamp tube is picking up filament AC and amplifying it.
    Also, volume 2 has bigger bypass cap for cathode of preamp, and this allows more low frequency to be amplified.
    Not unusual at all, in a Marshall it happens all the time.

    But as I mentioned above: moving the wires...it's possible to eliminate the noise.
    I can get 99% of the noise out (or more) just by moving the filament wires.
     
    JCarno likes this.
  9. JLBIII

    JLBIII Active Member

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    Are you running a grounded AC cord or the original 2 prong one?
     
  10. soundboy57

    soundboy57 Well-Known Member

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    Both 3 prong.
    Won't have time til later in the week due to work. Thanks ampmadscientist, very much for the help.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Remove V2 preamp tube and the noise goes away....this helps confirm that it's filament noise.
     
  12. soundboy57

    soundboy57 Well-Known Member

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    Thank you so much for the help. I haven't had time to tear in to it yet due to other obligations (work)...I will keep you posted :)
     
  13. soundboy57

    soundboy57 Well-Known Member

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    Thank you. I did all of these things, it was mainly a couple tubes...NOS Mullards, go figure...cleaned all the jacks, pots, moved some wires...
    done.
    Sounds great, minimal noise....you da man. Thanks again :):):)
     
    Kris Ford and JCarno like this.

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