Marshall JMP 50 watt lead '72, now this thing rocks!

Discussion in 'Let's Talk Vintage' started by Jucciz, Mar 5, 2012.

  1. Jucciz

    Jucciz New Member

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    Got a new baby to my collection, a 50-watt large box Lead 1987 from '72. I also found a relatively rare Marshall 2x12" cabinet model 2045 from '74(ish). I bought it empty and loaded it with a Celestion G12-65 and a Weber C1265 so now it can handle a total of 130 watts - should be enough even for a JCM800 2203. This mini-stack just screams.

    Here's a picture of them together with my '60 relic Stratocaster (Cunetto neck and Sheptone pickups):
    [​IMG]

    There's a little story about the amp on my web page, which I guote here:
    A metal panel point-to-point Marshall JMP 50W Lead (model 1987) from 1972. This amp came to me with the most suspicious power tube configuration I've ever seen: a near-dead RCA 6L6G and a very strong Tungsram EL34. Also the bias resistor had drifted from 68k to 89k so the tubes couldn't be biased warm enough. Luckily I found a nice 56k carbon composition resistor as a replacement and now the bias trimmer has a usable range again. A nice pair of Telefunken EL34s really does justice to this baby.
    The power transformer is an upright one. In 1972 Marshall used both laydown and upright power transformers and even though the laydown power transformers were more common in the early years of Marshall amps, some of the later '72 models have laydown transformers as well. Wonder why they put upright ones on some of the early '72 ones. My guess is that they put in whatever they had in stock at any given time.
    There are six rare Wima "chicklet" capacitors on the board as opposed to the all-mustard set you'd expect from most amps of the era. Marshall only used these capacitors in '72 and only in a handful of amps. Some say these capacitors sound a bit more aggressive than the "mustard" capacitors. I'm definitely not going to swap them out, to me they sound great! On top of all this: the grid wires are orange as opposed to the usual green, but the solder joints seem untouched, so I think they are original. All those facts combined it's no wonder that even though most old Marshalls sound great, they all sound a bit different!

    Here's a link to more detailed pictures:
    https://www.dropbox.com/gallery/4302688/3/Music gear/Amps & Cabs/Marshall JMP 50W 1972?h=674fa3

    I'm going to get this thing recapped as it has a slight buzz - nothing dramatic though. I think I'll get the fuse holders replaced as well and install a pair of screen grid resistors just to be sure. Strange that my '71 already has those and this baby doesn't. I'm using Telefunken EL34s that should be able to handle all the voltages this amp has to offer. Did I mention the amp came with three original Mullard BVA ECC83 preamp tubes and the weakest of them tested at 100% - the rest were stronger. :)

    Now a couple of questions:
    1) I read on the other forum that the white bias voltage wire should be moved from the rectifier lug to the standby switch, because otherwise there's a risk of damaging the PT or the rectifier. Do you agree? On my '71 the wire is already connected to one of the standby switch lugs.
    2) Have you seen a lot of Marshalls with orange grid wires instead of green?
    3) Anybody else here with a '72 Marshall loaded partially with those yellow chicklet capacitors?

    If I had to describe this amp with one word, it would be "Van Halen". Not that it would be exactly the legendary brown sound, but I'm very happy to play the early VH stuff with this baby - no need for pedals or anything, there's exactly the right amount of gain and punch. I love it.

    Feel free to comment!
     
  2. Rockbert

    Rockbert Active Member

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    Yes. The bias supply should be wired up to the hot side of the standby switch.
     
  3. Jucciz

    Jucciz New Member

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    Yeah, I'll get that sorted out immediately. Wonder why they put it on the rectifier in the first place. Any advantages or just disadvantages?

    The snubber caps have also been removed - which isn't that uncommon I guess. Do they actually even serve a purpose nowadays? I mean, I've heard some techs even say that they're more dangerous than useful and they simply should be gotten rid of. I've understood that they can eliminate popping and other noise while switching from/to standby - but I can't hear a single pop even without them so I'm not particularly missing them.
     
  4. Marshallhead

    Marshallhead Well-Known Member

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    Re: good work

    Another strange newb post. What's with these things?
     
  5. SoloDallas

    SoloDallas Well-Known Member

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    Re: good work

    Newb in what sense? The OP seems to have way more knowledge of what counts and whatnot.

    Additionally, he got himself an older amp - and a good one too, have the same but with both 1986/1987 circuits inside - which is not a common newb thing.
     
  6. Marshallhead

    Marshallhead Well-Known Member

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    Re: good work

    I'm referring to Nightwishh's bizarre response I quoted in my post, not the OP.

    There some kind of spammery going on - similar replies have appeared on other threads.
     
  7. TradAmpGuy

    TradAmpGuy New Member

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    They dont are about spam here... just member count

    Nice amp btw
     
  8. Söulcaster

    Söulcaster Well-Known Member VIP Member

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    shit man you sound like someones forcin you to be here, or you just tryin to drum up some work?:wave: whinge a lot?
     
  9. Adwex

    Adwex New Member

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    I'm not sure.
    1- You're wrong, any spam that comes to my attention is deleted immediately. I guess you missed the countless times that it happened?

    2- I don't see any spam in the newb's post. Strange, yes, but no spam. What's even more strange is that you felt the need to comment. Irony, perhaps?

    3- Thanks for the attitude. So you stopped contributing here but you come back to complain?

    4- I also find it ironic that you claim we only care about member count, when you and a bunch of your cohorts left because you didn't like the way things were run around here.

    Sorry to hijack the thread.
     
  10. Jucciz

    Jucciz New Member

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    Thanks for the bump. :)

    Now that we're at it, I have some news to share. The amp has now been serviced. It was recapped and screen grid resistors were installed. Also the fuse holders were replaced - and I'm glad we did that: one of the "fuses" was actually just a piece of aluminum foil crammed inside the holder. Talk about electrical safety! :D

    In addition, the white bias wire was moved from the rectifier to the standby switch as advised. Now the bias voltage is there immediately after flicking the standby switch - this way the amp is not so hard on the power tubes. Also the bias circuit series resistor is now a 47k. It may not be a Piher but still old stock (probably Vishay or similar) and it looks very good and period-correct-ish. I also installed new captive nuts to the chassis since the original ones had been long gone.

    The V1A had a 320uF cap that measured 560uF (!) and it had been paired with a non-stock 2,2k resistor. The 2,2k got replaced with a correct 820ohm resistor (the same type as the 47k series resistor in the bias circuit) and the drifted 320uF cap got replaced with a 220uF Philips.

    To summarize, the new electrolytics are:
    TAD Gold Cap 50+50uF filter caps
    Sprague Atom 8uF/150V bias supply caps
    Philips 220uF/63V V1A bypass cap

    Here are the latest pictures after the service:
    https://www.dropbox.com/gallery/4302688/4/Music gear/Amps & Cabs/Marshall JMP 50W 1972/#new?h=cdc36f

    I'd be glad to hear your comments!
     
  11. Jucciz

    Jucciz New Member

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    And here she is with a '73 angled cabinet and a '72 straight cabinet:
    [​IMG]

    So... it's ALMOST a '72 fullstack. Not quite, but pretty close. Sounds pretty OK'ish though. ;)
     
    2210Rocker and Blokkadeleider like this.
  12. Blokkadeleider

    Blokkadeleider Well-Known Member

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    Nice!


    Gr,

    Gerrit.
     
  13. Doug_1970

    Doug_1970 New Member

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    Are the speakers sharing the power evenly? I've tried mixing speakers up before, and even with the same impedance one of them always seemed to be dominant.
     
  14. Jucciz

    Jucciz New Member

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    I've hadn't had problems so far. The trick is to match the sensitivity as well. On the other hand: if you have a 8ohm 97dB speaker combined with a 16ohm 100dB speaker, they should theoretically be equally loud hooked up together as the 16ohm speaker gets 1/3 of the power and the 8ohm speaker gets 2/3. The 3db difference is compensated with the double power the 8ohm speaker gets. Of course in this scenario you'll end up with a bit awkward total impedance, but it won't necessarily be a problem.
     
  15. 71Smallbox

    71Smallbox New Member

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    Would my 71 have this same circuit error???
     
  16. Jucciz

    Jucciz New Member

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    Mine didn't. The '72 has been so far the only one that had the wrong kind of wiring.
     
  17. 71Smallbox

    71Smallbox New Member

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  18. 2210Rocker

    2210Rocker New Member

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    SWEET Stack man and glad you were able to bring the JMP 50watt back to life, did you shake any pictures off the walls yet? Looks like you'll be having many years of rockin power now.

    Rock it,
    2210 Rocker
     
  19. Quasar-Kid

    Quasar-Kid Well-Known Member

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    I just want that in my living room - it's like ART..!!!
    put little spot lights on it
    a velvet rope around it
     
  20. Jucciz

    Jucciz New Member

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    An old Marshall IS a work of art.

    By the way: did anyone notice the two cabinets have differend kind of chequeboard front cloth? Marshall has used both larger and smaller figured chequerboard around '72-73 and it seems that these cabinets are from the transition period - from different sides of the fence, so to speak.

    I have the Doyle book but it doesn't seem to discuss this matter much, if at all.
     

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