JMP 50 watt bass model 1986 from 1974

Discussion in 'Introduce Yourself' started by Charlie Macarone, Aug 27, 2019.

  1. Charlie Macarone

    Charlie Macarone New Member

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    Hello all, new here to the forum. Just plunked down for this 74 bass head. anyone know other than the PCB what if any changes were made to this amp since 1968? I believe this model always had a solid state rectifier.
     

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  2. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    Welcome to the Forum and congrats on the new amp!!

    I have a stock 74 Lead. This amp looks to be in great shape inside and out. Looks like a couple resistors have been changed out (typical maintenance on an amp of this age) and I am not sure what that big orange thing is there.

    You would probably want to get the filter cans changed out.

    Im sure a few other members will chime in with their expertise.
     
  3. john hammond

    john hammond Well-Known Member

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    looks to me as if those last two signal caps have been changed out, the black ones, and they didn't lift the board to solder them in..the old leads look like they've just been clipped and the new ones solder onto the stubs..very dodgy.
    those two caps look cool, very large for .022.. are they paper in oil?
    Also, ..just looking at the fresh solder where the yellow wires and the black go at the end of those two large brown resistors in series..that has been re-heated. choke goes there ( black wire i think)
     
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  4. john hammond

    john hammond Well-Known Member

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    - not enough maintenance.
    those two light blue ( 10uf)?bias caps look original, time bomb alert right there.
    maybe not to some, but i change these out straight away when I see them original.


    you can see where someone has been tonechasing adding bright caps to both channels volume pots and then removing
     
    Last edited: Aug 27, 2019
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  5. john hammond

    john hammond Well-Known Member

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    that large presence cap, the red one looks fishy, rough soldering and my 74 has a mustard right there..that looks like a much later cap.
     
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  6. john hammond

    john hammond Well-Known Member

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    these are interesting cap changes in interesting locations.
    someone may try and force me to eat ass sandwich, but I can hear the difference when changing that red presence cap from one brand to another ( same value)..and those black .022, I bet they sound ' better' than the original foil film mustards in that location.
    you may want to just play this as it is for a while , and when you do change the components back to original, if you do, change the components one section at a time...you may prefer the sound as it is now.
     
    Last edited: Aug 27, 2019
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  7. john hammond

    john hammond Well-Known Member

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    to answer your question, those two black caps ..the .022 cornell...were, in 1968 .1 uf mustards.

    otherwise that front end, the pre-amp.. looks the same as a 1968.
     
  8. neikeel

    neikeel Well-Known Member

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    The amp is really clean and tidy on the outside and panels - does not look like a recover either. Chassis and transformers look clean and original too.

    It has been converted from Bass to Lead spec:

    V1 cathode split (the yellow cap and resistor next to it added
    V1b coupling cap has been shrunk from 0.022uF mustard to the 0.0022uF yellow
    The tone stack has been altered to lead spec, the output couplers changed to 0.022uF.
    Can't see what the big red cap on the presence is - should be 0.68uF.

    The choice of parts suggests that they had whatever was lying around in the right values (eg lots of carbon comps not normally used in Marshalls in any quantity). I agree with John Hammond that if you were doing all that work - why not drop in new bias caps.

    It will probably be a ripper of an amp:dude:
     
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  9. john hammond

    john hammond Well-Known Member

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    the amp was, i think a 6550 amp. you can see the three switches plus the four output jacks. Did the 6550 amps have compensatory components? you'd have to change them to el34 values to get your 68 specs.Also, were the 1974 50 watt plate voltages the same as 1968? you'd have to check your plate voltage if no-one can tell you. you'd need those plate voltages to be in the ballpark of 1968 specs.

    edit- and preamp voltages need to be in the ballpark as well ( you can take a reading from pin 1 or 6 of preamp tube # 1.
     
  10. john hammond

    john hammond Well-Known Member

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  11. neikeel

    neikeel Well-Known Member

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    I’d play it first.
    It is never going to be a collectors item unless you you’ve a box of the correct parts and time to restore.
    Afaik the 50w never had x4 output jacks as stock. May well have been a 6550 amp with cc 220k as dropper with the grid leaks (ie not OEM).
     
  12. john hammond

    john hammond Well-Known Member

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    hes trying to establish the differences between this particular amp, and a 68, prob likes the 68 and wants this to sound like one, read his first ( and only )post.
     
  13. neikeel

    neikeel Well-Known Member

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    To get 68 tone will be a fiddle.
    The 25k(4k7 resistor) presence pot would go. You would replace with a 5k lin pot and 0.1uF cap
    Then replace the 100k NFB resistor with 47k and hook it up to the 8ohm tap on OT.
    Would need to add a O.68uf cap (100-250v rating) over V2a.
    Regarding the plate voltage the 68 50w would be 420v or so 74 is typically lower but will help give a browner sound at lower volumes.
    Still think that unless the OP has a set of prerola T1221s in a basket weave cab he will probably not be able to tell much difference between the two, even side by side.
    The original spec bass amp in 74 would be closer to a 67 plexi.
     
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  14. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum new MF’r
    :cheers:
    Mitch
     
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  15. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    :welcome:to the forum...

    :cheers:
     
  16. South Park

    South Park Well-Known Member

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    All the jacks have Ben replaced . Make sure that they are switch craft. Check the choke size to looks great see how it sounds
     
  17. whitenoise

    whitenoise New Member

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    Does anyone know how much an amp like this goes for?
     
  18. whitenoise

    whitenoise New Member

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    Would an amp with changes similar to this be a fair price at over 1k?
     
  19. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The four jacks on the back panel aren't quite in a straight line with even spacing so I confess I'm a bit puzzled. While the soldered jumper wires on the jacks look like factory work, I don't think that 50 watt chassis came with four speaker jacks punched even in the chassis itself, did they? And if they did, the extra two aren't quite in line and not quite evenly spaced which isn't consistent with Marshall's production workmanship. But their function as extra speaker jacks is clear.

    I have one guess: As I understand it, the US market amps needed to have four jacks to get UL certification. Bizarre but that's what I've heard. Might it be that this is factory work but the extra holes were added by hand rather than being machine punched?
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Two of the speaker jacks are original.
    2 jacks have been added, could have been speaker or effects or footswitch at one time or another.
    IMHO it's an effects loop that was changed now to speaker jacks..
     

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