Asking For Oppinions From Little Evidence: Jtm Super Pa

Discussion in 'Let's Talk Vintage' started by TrueFifth, Apr 12, 2019.

  1. TrueFifth

    TrueFifth New Member

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    I may be offered a JTM 100 Super PA with the black flag logo but so far have only seen very little pics, I of course asked for interior pics, and so far I got only from the back.

    I know how knowledgeable you guys are here, and if I'm to drop money on this one, I would wish to make decisions from as good background info I can get.

    I see some spots, especially on the lay down transformer, and perhaps other places as well, I hope this is not caused something wet.

    Compared to other amp pics, the choke looks right, but the writing on it, is that a date?

    Can the pics show that the OT is original? Should one be able to see cloth wires coming out of it?

    I will post more if I get more info / pics, but hope I get some comments for now, it will be greatly appreciated!

     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    It doesn't really show enough, but from what I can see (very little) it so far looks original.
    It also looks like somebody spilled a drink into the chassis...
     
  3. TrueFifth

    TrueFifth New Member

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    Thanks, and yes, those spots, I’m very affraid that should turn out to be because of some liquids too!

    The seller claims no technical knowledge, the amp being far from me, so gotta try make further decisions on very little info...
     
  4. CoyotesGator

    CoyotesGator Well-Known Member

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    Have the seller pull the chassis from the cabinet and send you clear/well lit photographs.
     
  5. neikeel

    neikeel Well-Known Member

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    The transformers are the later Dagnall type that you see in 100w amps from the end of '67 into '69 (so yes I would expect cloth covered secondaries on the OT). The yellow covered filter caps and with the preamp can up top would suggest late 68 or early 69 (there is a member here who goes by the name Tazin, he has these details documented properly) the rust is a little concern if the boards etc got wet so more pics would be useful.
    Normally a Black Flag amp would be purely mid '67 but the PAs mixed up parts being used up so these timelines are blurred, but almost certainly at least late '68.
     
    Last edited: Apr 13, 2019
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    One thing you can do to a late '68 is make sure the output sockets screen resistors are installed.
    On some of these amps there is no screen resistors, this will make the new tube draw too much current.
    Mysteriously, the fuse will blow when the volume is cranked up (now you know why).
     
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  7. TrueFifth

    TrueFifth New Member

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    Thanks, I already have a mid '69 Super PA, and that one does exactly that! So will definitely check that one, thanks!
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    The fuse blows when the volume is cranked up?
    You know where the screen resistors go?
    Did you measure the speaker w/ a meter to make sure it matches the amp setting?

    Either one of those, the speaker or the resistors will make the fuse blow.
     
  9. TrueFifth

    TrueFifth New Member

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    The speaker should be good and checked, I have had the fuse blow once when I cranked it, wondered why since, but since it gets inconceivable loud, I haven’t needed to do that again... But that means, when I get a proper attenuator for it, it could happen again...

    Locating screen resistors would take a little research, I’m also not super tech minded, but I try to pick up what I can!
     
  10. neikeel

    neikeel Well-Known Member

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    The 100w Marshalls all came with 1k screen resistors (except first 45/100 with KT66 that had 470R).
    It is the 50w that ran Mullard EL34s that omitted them as standard until about 71, then all 50w had them.
     
  11. TAZIN

    TAZIN Active Member

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    Marshall 100w P.A. amps are the oddballs when it come to changes/transitions. This amp looks like something built around April 1969 based on what I see. I've seen other Super P.A. amps from 1969 with "black flag" logo front panels and the rest of the amps features being in-line with 1969.
     
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  12. TrueFifth

    TrueFifth New Member

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    The serial number stamped on the back plate is also a little peculiar to me, it is written like this "SERIAL NO OP A/020xx"

    The owner has delivered the amp to a tech, probably for service and/or appreciation, I have asked for more pics of the interior, but so far have not received any.
     
  13. RevJamie

    RevJamie Active Member

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    Can't offer any opinion on the amp other than I have a 69 and a 70 Super PA, and I love those damn things!
     
  14. Jethro Rocker

    Jethro Rocker Well-Known Member

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    Is that a date of 1978 on the choke? Just wondered. Plus there is a similar number on one of the yellow caps. Could be nothing...
    The one on choke is 17-8-78 and the can is 29-8-78. Seems kinda weird if it isn't a date of some sort.
     
  15. TrueFifth

    TrueFifth New Member

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    I have been thinking the same ;-) Still no news from the owner.
     
  16. neikeel

    neikeel Well-Known Member

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    Both are service stamps Gert H
     
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