Pushing a JTM45 reissue to vintage specs worthed ?....or clone ?

Discussion in 'The Workbench' started by Kelia, Oct 16, 2020.

  1. Kelia

    Kelia Well-Known Member

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    Hi guys ,.....first I'm not a tech but have great ears and have a few questions.

    I was wondering if I should have a tech mod my JTM45 reissue to vintage specs
    or have him build me a JTM45 or JTM50 clone instead ?

    Also I remember @neikeel saying that the reissue has lower plate voltage ,400 iirc while
    the original had 450 so how does this affect the tone if everything else is done right like filtering ,caps etc... ?
    Will it sound closer to a vintage 45 or should I get some Marstran trannys and build a clone to get closer ?
    As a side note ,..I have all the parts like Allen Bradleys ,.Pio caps ,Gec's , Mullard GZ34 rectifier etc.

    On this site below,...they suggest what should be done to make it sound more accurate ,
    is this correct ?

    1. Change (C3) 330uF at V1 cathode to 250uF. A low voltage (25v) electrolytic is fine.
    2. Change mix resistors (R16, R18) from 470k to 270k.
    3. Change Ch I mix resistor bypass cap (C6) from 220pF to 500--550 pF.
    4. The 100pF bright cap on Ch I volume pot can be kept or it can be removed if a thicker, blusier sound is sought. Earlier Bluesbreaker amps had it, later ones did not.
    5. 1k cathode resistor R15 on V2 was originally 820ohms.
    6. The .1uF cap at the Presence pot originally was connected from the pot wiper (center terminal) to ground.
    http://raw-sewage.net/jtm45ri_mods.html

    Really appreciate your inputs on this !
     
  2. neikeel

    neikeel Well-Known Member

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    The higher voltage thing is to give a little clean headroom and punch not critical.
    The 330uF to 250uF is homeopathic if you want to have an impact lower than 25uF is required.
    The 470k mixers actually give you more grind but the 500 or 560pF bypass cap works as does the 820R cathode resistor.
    Personally I would build a good quality clone using Valvestorm as a basis and get a Merren OT and Marstran PT and choke.
    I use mustard caps and Piher resistors - but that is my problem!!
     
  3. Kelia

    Kelia Well-Known Member

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    Thank you Neikeel , appreciate!
    CHoke should be 3 or 5 Henry's and did the JTM50 had the same choke if I ever want to go that route ?

    What is the difference between the Merren and Marstran Output tone wise ?
     
  4. neikeel

    neikeel Well-Known Member

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    3H 250mA 100DCR choke (drake 344-114 and Dagnall C1999 are the same spec) is late JTM45 and JTM50 and JMP50 upto late 70s.
    I have both Merren and Marstran OTs and there is a gnats gonad between them, both better than the Classictone (IMHO) both indistinguishable from originals. Price and availability are worth considering
     
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  5. trax1139

    trax1139 Active Member

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    E8A88363-A2D8-4F84-82C3-27FD7E9DFE13.jpeg 7983867E-4E9F-46B6-BA08-22D06EE20ED6.jpeg BC119C2B-252F-4BA8-8AC6-F5924547B232.jpeg don’t know anything about microfarads, trannys, chokes or caps. Started out as a kid with an original 1959 Fender Bassman (POP bought it new in Sept. of 59.) Still have it today. Always heard that the Marshall JTM45 was just a Bassman on steroids. How true. In 79 I bought an early 60’s JTM45 and gigged with that amp for years. Sold it (I regret that)
    Bring it forward to just a couple of years ago. Started playing with another guitarist that had a Fender Bassman reissue. What a joke... not even close. I in turn bought a Marshall JTM 45 reissue. What a joke...not even close. Then I bucked up and bought a Germino JTM45. HOME-SWEET-HOME!![/ATTACH]
     
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  6. Kelia

    Kelia Well-Known Member

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    Nice !!........thanks for sharing ,.........love your Hard Rock Cafe in house !
     
  7. Kelia

    Kelia Well-Known Member

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    Thanks Neikeel ,.......really appreciate your help !........I'm gonna go with your
    clone suggestion,..a lot wiser .
    If I go JTM50 route with a 739-128 and EL34's and want to put some KT66's some day , will the
    739-128 work with 66's or it's totally out of the question ?
    Or the other way around ,....having a JTM45 Output and then try some EL34's ?

    Oh and by the way ,..what the heck does gnats gonad mean ?
    tried to Google it but they talk about fungus !
     
    Last edited: Oct 16, 2020
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  8. 2L man

    2L man New Member

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    If you have an 8 ohm cabinet on 8 ohm output of 3,4k or 4k (which I have been used with EL34) output transformer you can change it to 6,8k or 8k ( which are used with KT66) putting 16 ohm cabinet to 8 ohm output. This drops power output about 20%.

    This link has excellent tool to tune loadline for many common tubes for anode and screen voltages, bias current and impedance:

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/

    Esa
     
    Last edited: Oct 16, 2020
  9. Kelia

    Kelia Well-Known Member

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    Thanks for the input 2L Man !.........will check this out
     
  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    A gnat’s gonad is very small, so a very small difference between the two.

    gnat

    upload_2020-10-16_18-49-34.jpeg
     
  11. Kelia

    Kelia Well-Known Member

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    lol !!..............thanks !
     
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  12. neikeel

    neikeel Well-Known Member

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    2L man said it all (I use the same site for load line calculations too!)
    Only Marstran sells the -128 clone (it has a bit more bass than a -139) so that would be a very cost efficient way of being able to run KT66s or EL34s at or near Marshall specs. In fact KT66s don't really have a problem with 3k4 primary it's just Marshall used 8k (like you would for 6v6) and it worked.
     
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  13. Kelia

    Kelia Well-Known Member

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    Thanks !..............might have some other questions on the way coming ,
    for the most accurate head box and tolex ,...is Sourmash ok ?
    and I see that some clones had a nice square black JTM logo back in the Metro days
    on the left side of the front panel but don't see this anymore on the Valavestorm site ?
    Only saw sites like Modulus and maybe Ceriatone that have them
    but not sure if it will fit a Valvestorm chassis.
     
  14. neikeel

    neikeel Well-Known Member

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    Can't comment on Sourmash myself their initial rep was only so-so but lately only hear good things. I had a couple of excellent head cabs from Metro years ago and all the details were correct, specced by George and built by Mojotone. The Marshall levant is widely available with black being the definitive colour, there are black/green varieties, but take care as many are too green when compared with the original. TAD in Germany do quite a close green/black and I think they are pretty much a Mojotone dealer.
    Now I get headcabs made locally to my original patterns.
    Re: the faceplate. Contact Robert at Modulus he will probably be able to help, particularly if you are buying the rest from him.
     
    Last edited: Oct 17, 2020
  15. Kelia

    Kelia Well-Known Member

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    Thanks Neikeel ,...........I will contact him ,...seems like a nice place with great stuff !
    Also will try to find the best place for a headbox.
     
  16. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    I have a Sourmash JTM45 cab and a Valvestorm amp. Want some pics if so anything in particular?
     
  17. Kelia

    Kelia Well-Known Member

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    That would be great RC230 !,............can you post a picture of each if possible ?
    If you ever find the time of course ,........Thanks
     
  18. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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  19. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    3887F7E5-32E2-444D-BCCE-C040B07F449F.jpeg DB605797-7268-4559-A761-0D7380D98834.jpeg The 45 is on top the other amp is a
    RI 1959 from 1988
     
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  20. Kelia

    Kelia Well-Known Member

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    Thank you very much !..............appreciate !
     
    Last edited: Oct 17, 2020

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