Ngd: 69 Plexi - Modifications, Originality?

Discussion in 'Let's Talk Vintage' started by LPman, Oct 24, 2016.

  1. LPman

    LPman New Member

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    Hey Guys! I’ve recently acquired this 1969 Marshall metal face 50w Plexi amp (serial: 3205A) for a really good price, but it had some modifications through its life. It was modded in order to be the perfect gigging amp for clubs and I was told that all of these mods are 100% reversible. Please take a look at the internals and let me know what you think!

    There are two basic modifications. The amp received a foot-switchable volume boost circuit for solo boosting with an independent volume pot (you can see on the faceplate pic). The foot switch plugs into the lower ’volume II’ input and the pot was put into the place of the higher ’volume II’ input – it looks tidy and reversible.

    The second mod was a 2204-style master volume: the stock ’volume I’ pot became the ’pre-amp gain’ and the stock ’volume II’ became the ’master volume’, just like on a 2204. I guess the amp’s circuit was changed internally to be exactly like a 2204.

    There were some hardware modifications too. I was told that the laydown mains transformer had been changed to a period correct stand-up one. It’s a Drake 1202-164 transformer. I suspect that the output transformer was replaced too. It’s the correct Drake 784-139 tranny but it seems to be a later period one, likely from a 1973-74 Plexi. Anyone can confirm this?

    Because I’m no expert, my questions are the following for you guys.
    - Do the possibly ’73-’74 trannies change the characteristic ’69 tone in any ways, despite being the exact same Drake models that were used in ’69?

    -Also, can you confirm the mentioned mods in the circuit and their reversibility? Do you see any other modifications in the amp?

    I don't want to reverse these mods, I'm just really curious about the state of the amp and everything about it. I love how this thing sounds! Thank you in advance for your help!


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    khingpynn likes this.
  2. Ron W

    Ron W Member Sponsor

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    Congrats on your JMP50 metal panel!

    I recently acquired JMP50 S/A 1972A, and had Greg Germino do a sympathetic restoration to put it back as close to original as possible. Mine has a 1202-164 PT from a '74 Artiste; it looks very similar to yours including the part # stamp font & size. Per Greg, some of these PT's from the early-to-mid 70's have slightly lower voltage than the earlier ones. Mine has a plate voltage of 395 volts when biased to about 60%. He indicated that the only change tonally would be that it breaks up a little earlier than one running at the typical 425 volts. FWIW, my chassis is for a standup PT, ie. no hole punched for the laydown style PT.

    I'm sure some of the sages here will chime in soon...in the meantime, crank it and enjoy!
     
  3. 66SuperTremolo

    66SuperTremolo Active Member

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    Greg's the man for correct/sympathetic resto's !!!

    New end covers on both Trannies, both have been re-wound retaining the original laminates ??
     
  4. LPman

    LPman New Member

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    Thanks for the responses so far. Please, if you see anything unusual about these trannies or the internals of the amp, let me know! There are no Marshall expert techs in my area so you guys are my only chance to know more about the amp.

    Is there any way to know the exact date of these transormers? Or any source I can ask? I can't find a website for the company (Drake) to contact them regarding ID'ing.

    Do you see anything unusual about the internals of the amp besides the mentioned mods (master vol & solo boost)? Are these mods really reversible in case I have to sell the amp in the future for any reason? Thank you!
     
  5. boola1

    boola1 New Member

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    The OT and PT are almost certainly not original. The choke also doesn't look original, looks more like a Fender than a Marshall. I've not seen that font printed on the OT and PT before but I would guess they date from the 80s. Several caps (3 bright yellow) and resistors (long green) replaced on the board. Looks like it has probably also lost the .68uf cap from under that add-on board. It wouldn't be hard to reverse the mods.


    EDIT: the choke is original. It's more commonly seen on '68 amps so I guess yours dates from fairly early in '69
     
    Last edited: Oct 26, 2016
  6. m1989jmp

    m1989jmp Active Member

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    +1 for the choke, my '69 50 watter has the same one.
     
  7. neikeel

    neikeel Well-Known Member

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    Should be a nice players amp.
    Started off life as a shared cathode bass/organ or PA model features all correct for late 69-early 71.
    A list of changes for info (as per original request):
    Replaced PT and OT.
    Replaced diodes (originally diode block and snubber caps on the board)
    Replaced output tube sockets with ceramics (these are on top - usually fitted flange inside).
    Replaced dropping 10k dropping resistors (may have been cc)
    Replaced PI plate resisitor
    Replaced PI caps
    Added 250pF cap in tone stack (paralleled with the original)
    New Xicon 33k slope resistor
    Unusual presence arrangement for the year, suggests a 25k pot was used as a replacement and a resistor added (like post 73 amps)
    Added screen resistors
    Replaced bias caps and filter caps.
    I wonder if the rear panel is a repro or recovered because the screw holes seem in an odd place, hard in the corners but might just be how that one was put together.
    Obviously some one has added red marker dye to the solder joints, probably to make it all blend in rather than fake originality.
    Can't see what is under the added board it looks neat enough.
    What does the added pot do? Set the level of volume boost for the footswitch?
    All the mods are the result of making the amp usable in todays environment and presumably some fault rectification, but will still be pretty loud without an attenuator.
    Everything is reversible (except the hole next to the bulgin) but if it sounds good I would go with it:shred:
     
  8. LPman

    LPman New Member

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    Wow Neikeel! I really appreciate your detailed response, it's a huge help! I knew that this forum will be the way to go!

    I have a question to you (and all members who might know the answer) regarding the transformers:
    - What do you think about the OT and PT? The codes seems to be correct for the era but the font types are not. I have seen this type of font in some 1973-74 JMP amps, but they are not typical. How old are these transformers in your opinion and how do they compare to the original '69 trannies? Can I still get an authentic 69 Plexi tone out of this amp in this state with these possibly later trannies?

    - And how could I make sure if the amp was originally an 1987 or 1986 model? Any indication about this anywhere on the amp?

    - Should I reverse the smaller mods - cap, resistor and diode changes - in order to get a more authentic '69 Plexi tone or should I leave them alone. I really like the tone of the amp but I'm curious about every possibilities.

    You are right about the added pot, it sets the level of the footswitch-able volume boost.
     

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