Dating my JMP50

Discussion in 'Let's Talk Vintage' started by obx351, Mar 22, 2015.

  1. stratburst

    stratburst Member

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    If the pots are original they are usually the best single reference for dating a Plexi era amp as Marshall didn't seem to buy in as large bulk quantities as the other components. You might see caps dated 65 in an amp from 67 but generally the amp would likely been made within a few months of the oldest pot date code. I will say however You really need to consider everything to get the best estimate.

    Serial numbers are all over the place during this period and can be very misleading. I have 11185 and 11187 (edited) and they were likely built months apart. I've seen amps that were within 10 serial numbers that were definitely were built more than a year apart.

    The date codes on the pots are facing down. A dental mirror and a camera works great for getting it. The image will be backwards which really makes it hard to get it right. I find it's easier to take a pic then look at it in the mirror to read it.

    One word of advice given earlier that is really important. Don't power it up if it hasn't seen power in several years. Have the tech pull the tubes and bring it up slowly on a variac. This accomplishes two things. First it will potentially save your OT if the caps are bad. The OT is a critical component of what makes this amp sound so amazing.

    The other thing it serves to accomplish is it Will reform the caps which may save them. I have a bone stock jtm45 from 1965 and a jmp50 from 68 with all original caps that are as strong as the day they were built. The key is that they have to be played. Store them in the attic unplaced for a couple of years is the worst possible thing that can happen to an amp!

    Regardless of when it was made these amps are amazing and incredibly inspiring to play. Enjoy!
     
    Last edited: Dec 28, 2016
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  2. stratburst

    stratburst Member

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    :applause:
    Great info TZ. Thanks for sharing!
     
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  3. TAZIN

    TAZIN Active Member

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    Yep, using the potentiometer date codes helps too as does the filter cap date codes once you get into 1969. Another 'general' indicator is the use of 1/2 watt AB/CTS (sometimes labeled RS too) style pots which means your heading toward 1968 and later.
    The 1/2 watt AB/CTS (sometimes labeled RS too) style pots normally have a date code represented in the form such as; "B8"; where the letter (A thru M, omitting 'i') represents the month and the numerical value indicates the year (8 = 1968).
    The larger RS 1 watt pots usually have just numbers for their date code (i.e. 8-7; August 1967); where the first number represents the month and the second number represents the year.
    Here's an example from an amp with a mid May 1969 Test Date witten on the paper tag adhered to the chassis:
    All potentiometers are 1/2 watt AB/CTS (sometimes labeled RS too) style pots.
    Volume 1: RS 1M
    ΩLG L8 (November 1968)
    Volume 2: RS 1MΩLG K8 (October 1968)
    Treble: AB 250KΩLG J8 (September 1968)
    Middle: AB 25KΩLG H8 (August 1968)
    Bass: RS 1MΩLG H8 (August 1968)
    Presence: AB 5KΩLN J8 (September 1968)

    So, in this example you can see that the potentiometers pre-date the actual mfg date of the amp by a decent amount making them not the best date indicators in this particular case. Now, the filter caps in this particular amp have a March 1969 date code which is fairly close to the date written on the chassis. BTW, all the Mullard caps were D8N or D8W.

    @stratbust You should post some pictures of 11087 and 11089.
     
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  4. sct13

    sct13 Active Member

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    I am going to apply this to dating mine, I am pretty sure is early 69...

    Tazin: you should write a book...there is probably a market for this stuff. No Kidding....:hmm:
     
  5. obx351

    obx351 Member

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    I'll get gut shots soon. Here's another shot of it with my Park.

    [​IMG]
     

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  6. sct13

    sct13 Active Member

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    Careful ...you might damage your hearing.....:wave:
     
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  7. obx351

    obx351 Member

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    What??
     
  8. sct13

    sct13 Active Member

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    huh?


    whad' he say?
     
  9. obx351

    obx351 Member

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    Just a quick update....the straight 1960 cabinet is a Bass Lead (no wonder it's so heavy!!) from '72 and the 8x10 is a '68, quite possibly bought at the same time as the head. The 1960AX is a '74 lead.
     
  10. neikeel

    neikeel Well-Known Member

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    The x suffix denotes a reissue ;)

    Yours is an original I believe.

    So if the straight cab a Lead and Bass will have G12H30 55Hz T1281s :cool:and is not normally called a 1960 but a 1982B.

    The top A cab if a 1960A will have T1221 G12M25 75Hz speakers.

    If you can confirm.

    BTW any sign of the before and after pictures of your 50w, I still have slight OCD anxieties (yes, my problem I know :naughty:)
     
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  11. obx351

    obx351 Member

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    Wow!! Thanks for the info!! My amp guy has it and I'll ask him for a gut shots of the plexi.

    Here's a pic of the bass lead

    [​IMG]
     

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  12. neikeel

    neikeel Well-Known Member

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    Very interesting, does it have two input jacks?
    The later amps with T1281s have 120w rating
    Do you know what speakers are in it? Grey or greenbacks?
    Hint - pictures?
     
  13. obx351

    obx351 Member

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    Just 1 input jack. I am guessing green because I believe it's a '72. I'll check and and take some pics.
     
  14. neikeel

    neikeel Well-Known Member

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    Where is Mr Bygonetones when you need him:hmm:
     
  15. obx351

    obx351 Member

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    Greenbacks....75hz..CRAZY GREAT SOUNDING

    [​IMG]
     

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  16. Joshabr1

    Joshabr1 Well-Known Member

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    Cab looks like mine late 71/72 with that label. Pete or bygone know better. Mine is 06208 And speakers date to end of 71 oct I think it has big brace and ply back. Is yours ply or chip? The serial numbers where actually factory order numbers I believe. They are not sequential but are vaguely a point to refer. Frame codes is the only good way to date a cab given they are original speakers.
     
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  17. obx351

    obx351 Member

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    Plywood. Thanks for the info!
     
  18. TAZIN

    TAZIN Active Member

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    @obx351 Any new developements on your amp? Did your tech go throught it yet?
     
  19. obx351

    obx351 Member

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    Yes he did. It had some old mods but all the caps and trannys were original. Someone added another gain stage I believe with a '70s ampeg cap. He put it back to stock.....& f'n-A it sounds INSANE! It is the best sounding Marshall I've ever heard. My amp guy, who has done this for a long time, said the same thing. I married it up with the above '71/'72 cabinet. It's a great match. Though the 8x10 sounds great too just brighter.
     
  20. TAZIN

    TAZIN Active Member

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    Cool, I'm glad everything worked out for ya. Did you get any before and after pictures?
     
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