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1969/70 JMP Head

Discussion in 'Let's Talk Vintage' started by Alpesitehen, Jul 17, 2021.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Even the back panel stock plates are in great shape, not even chipped in the corners. NICE.

    Aside from that one added hole, it's about as pristine as you're likely to find in years of searching.
     
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  2. AndyD

    AndyD Well-Known Member

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    Agreed with the above. Feet?!!!!
     
  3. Pete Farrington

    Pete Farrington Well-Known Member

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    Marshall’s feat of sheer engineering excellence there must have sent innumerable EL34 to an early grave too :mad:
    But rather than being short lived, it seemed to have continued through much of the 70s, eg here’s that non cascaded 2204 with it, schematic dated Nov 1976 https://www.thetubestore.com/lib/th...arshall-JMP-Mastervol-50W-2204U-Schematic.pdf
     
  4. Ned B

    Ned B Well-Known Member

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    It looks like this one has the ball feet, so I was wrong.
     
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  5. Alpesitehen

    Alpesitehen New Member

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    Yes, it has ball feet. I was able to try out the amp, sounds as good as it have to. Ridiculously good condition.I think, I will hunt it down.
     
    Last edited: Jul 20, 2021
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  6. Spanngitter

    Spanngitter New Member

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    Looks like a nearly untouched gem one.
    However, now it comes into euqation if you want a player's or a collector's item.
    For a collector's amp i would just dust it off and leave it as it is, if you want a player then there is a longer list of things recommended like:
    - Adding a Cap Job (those Dale do last very long but after +50 yrs they might be dead) and have those replaced with some F&Ts
    - Add Screen Resistors (no cement blocks, use 1k / 3-5W Enamel Resistors like TAD's Z-RGE1K) by moving wire from Pin4 to Pin6 and add Resistor between Pin4 and 6
    - Let a Tech check those WIMAs Durolit for any leakage (those tend to crack the casing and collect humidity which makes them leaky)
    - Consider to have that Bulgin cable/connector checked if good or have Bulgin receptacle replaced by an IEC Version (like TAD's MBBREP ) and your PE Contact moved to an separate connection (not on common ground bus).
    - Check Fuses for correct type and rating and have Fuse Holders measured for any resistance
    - Consider to check that Bias Pot for any possible contact issues
    - Replace the Snubber Caps (one already missing on the BR)
    - Bridge Standby with an 220k/2W R so Caps do get preloaded slowly and there is no huge inlet current rushing thru Switch when powered on
     
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  7. Alpesitehen

    Alpesitehen New Member

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    Some of you said this is a bass amp. 1987 means its not a bass amp. Am I Wrong?
     
  8. Alpesitehen

    Alpesitehen New Member

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    I would like to use the amp too, not in concerts just for myself at home, but on high volume ofc. If I have to do these things are they reversible? If yes, these tech work effect the value of the amp?

    Thx
     
  9. junk notes

    junk notes Well-Known Member

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    1959/1987 100w/50w Lead
    1992/1986 100w/50w Bass
     
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  10. neikeel

    neikeel Well-Known Member

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    :agreed:
     
  11. stickyfinger

    stickyfinger Well-Known Member

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    You probably wont have to do any of those things but filter and bias caps maybe and would be a good idea non the less.

    The Wima caps are not Duolit. They are TFF and usually just as solid Mustards. I personally always leave my standby switches in the on position and turn down the guitar vol for short breaks. Screen resistors may need to be added but if you have Mullard XF2 you wont need them.

    Amp is all so bass spec.. that right there would be a deal breaker for me. For JMP Id want lead spec. Still a great amp but the JTMs sound better for this voicing.
     
  12. Spanngitter

    Spanngitter New Member

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    If you insist on playing high volume I would at least add Screen Resistors.
    Nobody who understands the requirement would complain if you add those, just stick with vintage correct parts.

    For the remaining it really depends on what you are out for, as I stated above: if you want a player or a collector's item.
    Me personally I have no problem to accept an old amp with a cap job if good parts are used (and F&Ts are good) and defective components replaced with either good NOS or actual parts (I know it hurts your eye if you see an Vishay MKT1831 in an old Marshall but they are as good as any Mustard was) but it avoids that you play that thing and it explodes after a while due to components failing. And all that on the Turret is replaceable with not much effort but if you fry your OT or PT due to a failing component then you are in trouble.

    I also have customers which attempt to keep these amps as original as possible but you need to make the decision: just look at it and enjoy the ownership or have the look and joy of actually playing one of these gems.
     
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  13. Alpesitehen

    Alpesitehen New Member

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    This amp should have a bright cap?
     
  14. TAZIN

    TAZIN Well-Known Member

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    If you mean a Volume pot bypass cap then the answer is no.
     
  15. TAZIN

    TAZIN Well-Known Member

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    You go by what the circuit is and not necessarily by the Stock Number badge found on the rear panel of the cab or the stamp found on the paper tag adhered to the chassis. The folks at Marshall make mistakes.
     
  16. Jack Lickson

    Jack Lickson New Member

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    I'm not the OP, but I appreciate everyone's willingness to assist. I just joined MF and will be posting some photos of my '70 JMP as soon as I'm able. Need some expert insight.
     
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  17. neikeel

    neikeel Well-Known Member

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    I'm sure people will help as they can. The more hi res pics and info the better (serial, tags, circuit boards, transformer code stamps, even the feet of the head cab give detail!)

    Suggest you create a new thread for it.
     
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  18. Jack Lickson

    Jack Lickson New Member

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    Thank you. Will create new thread for sure.
     

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