Some year JMPs are reputed to be higher gain than others. Why?

Discussion in 'Let's Talk Vintage' started by Matthews Guitars, Oct 21, 2021.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I've heard this more than once. Some years of the 1987s and 1959s have more natural gain and crunch to them than others. The 1972 50 watter is one of them that is specifically mentioned in the Doyle book as having a volume control contour that goes from zero to max output in very little volume pot movement, and beyond that it just adds more and more gain.

    So, what are the circuit differences that cause this?
     
  2. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Pot taper would be my first guess.
     
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  3. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Bright cap value increase and negative feedback reduction.
     
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  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Got any knowledge of the specific value changes of those parts?
     
  5. jmp45

    jmp45 Well-Known Member

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    I had a 73 1959 stack IIRC about 3.5 - 4 was all you needed so maybe so.. IDK.
     
  6. Dblgun

    Dblgun Well-Known Member

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    Depending on the year there were several different values, NFB was increased from 47k to 100k and 4 ohm tap, cathode resistor from 820 to 2.7k with the .68, coupling cap from .022 to .0022, different caps on presence pot for just a few. Plate voltages and filtering varied as well.
     
    Last edited: Oct 22, 2021
  7. Seanxk

    Seanxk Well-Known Member

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    Didn't they have a 5K bright cap from 69
     
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  8. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I think so, or in 68 but the NFB decreased a couple times later.
     
  9. stratburst

    stratburst Active Member

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    My 72has linear taper Treble and Mid pots, 100k NFB tapped at the 4 ohm winding of OT. Split cathode in 2.7K on Ch 1 side not 820 ohm like a true plexi circuit. The .005uf brite cap also makes channel 1 more crunchy.
     
  10. AlvisX

    AlvisX Garage Guerrilla Gold Supporting Member

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    Some have a .68uf on V2a Cathode ,some don't .....try switchin that in and out n see what happens
     
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  11. Seanxk

    Seanxk Well-Known Member

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    .68 on the Presence pot?, not sure when that came in but I thought it was later than 72.
     
  12. Matthews Guitars

    Matthews Guitars Well-Known Member

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    What I'm after is to make my '73 SL a bit less really clean. Even dimed, it only offers a taste of distortion. Rather than "mod" it, I'd like to start with just revising the circuit to match that of the crunchiest version of the circuit that ever came out of the Marshall factory.

    I'm one of those guys who likes a rich overdrive right out of the amp. No pedals necessary.
     
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  13. stickyfinger

    stickyfinger Well-Known Member

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    I've built a switchable JMP 50 lead that did 69, 72 and MK2 that switches circuit with one toggle flip. Three different negative feedback circuits and V2 resistor/cap combinations. The modes all had approximately the same max gain at 10 but a slightly different voicing.

    Some amps are reportedly having a unusual lower voltage power for a particular year. So get a 71 50 watt that is a transitional circuit that has the V2 .68 bypass cap and the 100k/4ohm feedback and this amp will be extremely hairy.
     
  14. neikeel

    neikeel Well-Known Member

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    There are quite a lot of mixed specs here.

    The early (68/69) Superleads had 820/250uF and 820/0.68 on cathodes. That gives a smidge more gain.
    The 73 should be a fire breather. 2k7/0.68 and 820/250 v1
    4n7 or 5n on vol 1 pot.
    1k cathode v2a. 100k/4ohm NFB (ie less NFB than the 47k/8ohm) and 25k/4k7/0.68 on the Presence pot. The voltage in these is often in 500v+ region too.
     
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  15. neikeel

    neikeel Well-Known Member

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    Oh and given the changes at different times most likely the 72 50w had yellow chicklets, both v1b and v2a 0.68s and 5nF on vol1.
    As repeatedly stated the 5nF is not a frickin bright cap. Look at the frequencies it allows through! At 2 on the volume is almost full signal.
     
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  16. Pete Farrington

    Pete Farrington Well-Known Member

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    What’s the preferred terminology for it then :hmm:
    The volume bypass cap? :D
     
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  17. neikeel

    neikeel Well-Known Member

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    Well it is a much more accurate description, nest pas?
     
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  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Here's my '73 as it is now. (Except that the added resistor on V2, yellow wire, has been deleted since this photo was taken. Never understood why anyone would have added that anyway but it was there when I got the amp.)

    It had a few monds in it (some weird coupling caps) and I reversed those mods but I may have just gotten the wrong coupling caps for it. This should be clear enough to read their values. It was my first time doing significant rework on a Superlead.

    DSC_6711_crop.jpg
     
  19. neikeel

    neikeel Well-Known Member

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    So you have 0.022uF coupler on V1b (should be 0.0022uF) and I cannot see the vol 1 bypass cap, other than the V2 plate resistor you mentions looks pretty stock for a 73.
     
  20. Scumback Speakers

    Scumback Speakers Well-Known Member

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    I'm not sure a gutshot of my 71 JMP 50w helps, but here you go.
    [​IMG]
     

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