Vintage JTM-50 - S/11295 - 1968 - what it it worth? How to rennovate (if at all)

Discussion in 'Let's Talk Vintage' started by redfish, Apr 6, 2021.

  1. neikeel

    neikeel Well-Known Member

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    I had one exactly the same until w couple of years ago.
    I’ll look at the external pics agsin
     
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  2. redfish

    redfish Member

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    It has the same yellow 0.68 250v 'cube' caps as S 11295
     
  3. redfish

    redfish Member

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    It has the same yellow 0.68 250v 'cube' caps as S 11295
     
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  4. neikeel

    neikeel Well-Known Member

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    As both SuC and myself have said the caps are original, same for the bright cap, the little grey ceramic cap on pi etc.
    The fuse holders are original.
     
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  5. Marshall50w

    Marshall50w Well-Known Member

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    I took one of my Marshall's to Bletchley HQ 10+ years ago and they didn't change the Bulgin plug on it. I would suggest that you take it to Marshall (being so close) to obtain an appraisal of the amp of what they think needs doing and the cost. You will have a better idea then for your next step. New reissues of the amp are around £1300 so I guess the amp is maybe £3k depending on its originality/ work required and of course what someone would pay for it.
     
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  6. neikeel

    neikeel Well-Known Member

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    ;)
     
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  7. Matthews Guitars

    Matthews Guitars Well-Known Member

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    3K? Probably it's worth twice that.

    Neil will know exactly what to do to make this amp live its best life. My guess....put proper knobs on it. Leave the Bulgin socket alone, just put a new cord on the proper Bulgin plug. Replace the incorrect fuse holders with a proper pair of Belling-Lees. (I think that'd be right for '67-68, right?) Replace any incorrect components with correct ones. Not that that's very many. Test the filter and bias caps for proper performance. Don't change any original part that doesn't NEED changing.

    And hopefully those original British Mullards sre still strong.

    I've never seen those yellow cube caps before.
     
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  8. TAZIN

    TAZIN Well-Known Member

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    With the exception of the 56K resistor in parallel with the original 47K NFB resistor the turret/eyelet board is all original.
    Both switches appear to be replacements as is the Presence potentiometer.
    Someone reworked the rectifier assembly and added the two capacitors along with that large RS resistor at the H.T. Fuse.
    The Treble pot is a period correct type but still maybe a replacement due to the disturbed solder joints.
    Obviously the output transformer is a replacement.
     
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  9. junk notes

    junk notes Well-Known Member

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    :yesway:
     
  10. junk notes

    junk notes Well-Known Member

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    Both V1A and V1B 100k also changed. Not cap as I mentioned prior.
     
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  11. Ivan H

    Ivan H Active Member

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    Nice amp. My JMP50, serial No. 11225 is pretty much identical, same yellow Philips 0.68uf/250 "cube" type cathode bypass caps with red lacquer intact on solder joins, (C296 mustards elsewhere), same fuse holders with red lacquer on solder joins intact. Also has the grey tubular ceramic PI fizz cap. Originally had the Torotor (small black bat) toggles (standby has been replaced with non stock item).
    Was advertised as a '68, but I have since been told that it "may" be from late '67 due to the paxolin type voltage & impedance selectors & the inter-node wiring being done on the underside of the board rather than on top.

    While I think of it, the "mains" fuse holder on mine has been damaged (though is still operable).
    I do have the Belling Lee type fuse holders, but would prefer not to disturb the red lacquer on both fuse holders if possible.
    If anyone at all knows where I can source one of this type fuse holders seen in the OP's amp I would be very appreciative. Cheers
     
    Last edited: May 12, 2021
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  12. neikeel

    neikeel Well-Known Member

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    Those fuse holders only seem to be around 68-69, coincide with the brown paxolin selectors. I have never seen any for sale elsewhere or on any other amps. It was one of those I had to repair on my 68 JMP 50 as there was corrosion and intermittent contact on the HT and hard to track crackle.
    The rectifier mods are pretty obvious (these had the flying diodes, post tube rectifier era and pre rectifier block era).
    Toro tots are almost impossible to find too although I have some that look the same externally (short round black bat type). Regarding v1 plate resistors you do see those now and again (and on v2 socket). They are RS branded 10% and that plus a bit of drift to 120k or so can actually give v1 a bit more perceived gain. Other places you see that type of resistor is tone stack 56k on bass amps and 470k mixers.
    Wonder when the OT was replaced. Looks a bit like a Partridge or the EV brand and appears to have been there for a while.
     
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  13. redfish

    redfish Member

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    The OT must have been installed before mid-late 70's .... as that was when I bought it - and no work done on it since then
     
  14. StuC

    StuC Active Member

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    Both my '67 JTM50 and my '69 JMP 50 have these fuse holders. Never gave them a second thought before. :hmm:

    OT is definitely a Partridge. What are the numbers on the top?
     
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  15. redfish

    redfish Member

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  16. neikeel

    neikeel Well-Known Member

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    Where did you see those pics!!!
    Yes def Partridge

    looks like TG9624?
    Do you know where they were used Stu? Not one I recognise.
     
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  17. Matthews Guitars

    Matthews Guitars Well-Known Member

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    So those fuse holders are correct for this amp? Making notes and saving photos for reference....

    If anybody needs belling-lee type fuse holders, there are always a few on the UK ebay site. I've got about 15 of them saved up for worthy Marshalls that may need them in the years to come.
     
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  18. redfish

    redfish Member

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  19. Ned B

    Ned B Well-Known Member

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    Does anyone know the date code system for Partridge transformers?
     
  20. StuC

    StuC Active Member

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    I posted my thoughts on the old Plexi Palace forum. This is only what I came up with after looking at dozens of amps of different makes, so it's by no means definitive, but it seems to fit.

    Usually, there's a part number and a batch number. It seems to me that the part number follows the pattern T(X)(1234) where T presumably stands for transformer, X is a letter denoting the decade when the part was designed and 1234 is a four digit number.

    The second number takes the form (X)(1)(234) where again X is the decade, but this time the decade in which the transformer was made. The first digit appears to be the year in that decade and the last three digits a batch or job number.

    The relevant decade letters are G for the '60s and H for the '70s. Prior to 1967, it's rare to see any codes on the transformers. '67 and '68 codes are usually hand written with a paint marker, later ones are stamped.

    So for example, TG5479 H1422 would point to a '60s designed part produced in 1971.
     
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