Is a JTM-45/JMP bass crossover amp impractical?

Discussion in 'Amps and Cabs' started by music321, Jan 26, 2015.

  1. music321

    music321 Member

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    My ideal amp would have the cleans and overdrive of a JTM45, but also be capable of getting the raw overdrive of the jmpbass. The more I look at this, the more it seems like an impossibility. Granted, there will always be some compromises (e.g., only kt66 or el34 tubes could be used).

    It seems that this isn't just a question of a switch to remove/restore sag in the power supply, or some other change. It seems like everything from preamp filtering to OT specs would have to be changed.

    With all this in mind, it seems that my best option might be to get a jtm with an effects loop, and run something like a "Sansamp" into the return to get the JMP sound (though I don't think that they make a JMP bass sim), and use the amp as a whole for the JTM sound.

    Why this way rather than the other way? This way would preserve the JTM cleans, which I love. I wouldn't have any use for the JMP bass cleans anyway.

    So, are mods and switches possible to get the best of both worlds, or is the modeler through the effects return the more practical way to go?

    Thanks.
     
  2. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Change mix resistors to 470'kmand up nfb to 47k on 4 ohm tap.
     
  3. music321

    music321 Member

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    would this make the amp an "average" of the jtm and jmp bass, or would it allow access to the tones of both?
     
  4. ProfRhino

    ProfRhino Member

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    If you're asking about a JTM45 / JMP50 hybrid, not possible with reasonable effort afaik.
    On the other hand, I recently worked with my amp tech on building a hybrid of a JTM50 & "inspired by" PRS HXDA (which is a hybrid 1986 and 1987 clone in itself).
    So basically it can be :
    -- JTM50 (tube rectified)
    -- Duane Allman's 1986 circuit
    -- Eric Johnson's 1987 circuit
    The amp works great and sounds absolutely killer ...
    My tech asked me not to share schematics or gutshots, but here's a rundown of the basic building blocks, all specific circuits have been collected from the usual online sources.
    - the amp started life as a straight JTM50 clone, great set of trannies and chassis, but kinda average remaining components, so my friend rebuilt the board with all Sozos, TAD gold caps and other quality stuff, added a separate filtering section for the diode rectified JMP variants (the only thing that requires standby for switching, heavy duty circuit-breaker style switch in the back), external biasing and internally jumpered channels, this freed up three of the four input holes for mini switches, recessed so the amp looks almost stock.
    Here are the tweak options we finally settled for :
    - tube / diode switch as described, on the back panel
    - 3way rotary switch, also in the back, giving you the 3 classic Marshall NFB variants, this one makes a world of difference !
    - shared/split cathode switch
    - 3way V2 cathode switch
    - 3way tonestack cap switch
    the rest is mostly straight from the Marshall textbook, tubes are XF2 & i61, the amp sounds absolutely killer, I rarely fire up my real '70s JMPs these days ...
    Adding a PPIMV would be easy, I personally do not need it.
    Just to make it absolutely clear, this is a classic single channel '60s Marshall circuit, not a channel switcher - but it can be a JTM, a 1987 or 1986 depending on the mood you're in. Love it ...
    Massive thanks to my good friend and amp wizard, he takes credit for the bulk of the work. My only part was coming up with lots of unethical concepts and ideas and collecting the necessary info, he provided the reality check and built the beast.
    ymmv,
    Rhino

    oh, and ftr, I had been searching high and low to buy a real HXDA, unfortunately they are not available anywhere in Europe afaics, and if they were, the prices would be insane even before VAT.
    PRS would sell a lot more gear over here if they had a better distribution / service model.
     
    Last edited: Jan 27, 2015
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  5. bbaug14

    bbaug14 Supporting Member

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    Check out the Fargen Retro Classic. Does what you're looking for and very convincingly.
     
  6. music321

    music321 Member

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    Great info. What does a three way V2 cathode switch do? change filtering cap values, or something else?
     
  7. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    over thinking IMO.

    V2 cathode switch is the bpass cap over V2A's 820 K resistor.
    Not all marshalls had a cap there. Some need, some don't, but thats' more a lead spec thing. it gives mid boost and gain.

    For a JTM45 and JMP 1986 'shared' amp...

    I'd just change the mix resisors to 470K
    Up the negative feedback resistor to 47K.

    If you're STILL wanting a bit tighter...change preapmp filtering to 33uF.

    It'll split the difference enough IMO
     
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  8. music321

    music321 Member

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    Awesome. Thanks. I think I was starting to over think it.
     
  9. Roe

    Roe Member

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    a jtm45/jmp 50 bass (1986) hybrid is doable. The preamp is virtually the same really. The power supply, output tubes and output transformer is the difficult part.

    A suggestion would be to try the following:
    - 784-139 OT with mismatched load for faking jtm sounds (from 3.4k to 6.8k)
    - switchable rectifiers and filtering (doable)
    - negative feedback gradually adjustable with pot (dirt easy and effective, often called soul control)
    - compromise output tubes, e.g. 5881s, hard (or somewhat 6l6) sounding el34s, kt66s, kt77s, 6ca7s
     
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  10. ProfRhino

    ProfRhino Member

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    Overthinking - it depends.
    For live use, just give me any 1986/87 circuit, ideally with the NFB rotary in place of the 2nd speaker jack, and I'll be fine.
    The idea behind that 3way hybrid was a flexible studio amp, without sacrificing the simple topology, and it's turned out incredible. :drool
    Without doubt, the switchable PSU and the NFB switch are the major character changers, and their effect is huge.
    The remaining 3 switches are more subtle on their own, but combined they do make a difference in (upper mid) gain, compression and feel as Mike said, not unlike the treble shift on a Mk2 Boogie to give you an idea.
    These switches kinda were a result of "damn, we have 3 empty holes now :omg, how can we fix that with added benefit ...".
    Once you have them, you really start appreciating them, and the extra cost was like €30, so what's not to like ?
    Btw, the amp is whisper quiet and the tone is pure, I don't see the switches having any negative impact. :dunno
    Certainly not for everybody, but a wonderful tone machine in its own right, good enough for me. :love:
    ymmv,
    Rhino
     
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  11. music321

    music321 Member

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    How about this idea:

    Start with a JMP bass circuit

    Use KT66 output tubes

    shared/split cathode switch

    switch to approximate sag across solid state rectifier

    Adjustable negative feedback.

    All of this is certainly pretty straight forward. Do you think that this would more or less get things done?
     
  12. Roe

    Roe Member

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    no need for split cathode or .68uf on v2a if your after a 1986 and jtm45
     
  13. music321

    music321 Member

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    What would this mismatching of the load do? I thought mismatching load was a sure way to damage the amp?
     
  14. DGDGBD

    DGDGBD Member

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    Sure you can 95% there by having a bright cap switch and tweaking a few signal cap values.
     
  15. Shiny_Beast

    Shiny_Beast Member

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    I think having variable feedback is just awesome, it's my new toy :)

    I've got that and

    split cathode switch
    v2 cathode switch

    I use the bright tone stack with low screen filtering into a 4.2k OT

    I don't care much about tube rectifier sag or variable tone stack, I'd rather have a 270k/470k mixer switch or something there-other. I think the mixer network really differentiates early marshalls.
     
  16. Shiny_Beast

    Shiny_Beast Member

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    The jtm 45s put a high impedance load on the output tubes. I don't know the whole story, but picture this as holding the tubes back a bit. You get less of certain harmonics, and the tone typically isn't as bright and edgy. If you go too high power output suffers. This is why the jtm45 and 100 amps were really closer to 35 and 70 watts respectively.

    The low impedance iron is also part of what gives the later plexis their bite. Some of the early plexis had output iron not as bold as the later super leads. EVH #1 is rumored to have a 2.2k OT instead of the usual 1.8k, which would be less in the direction of a lower primary like the later super leads. The Fender bassman, which was sort of the forerunner to the jtm45 had 4.2k I'm pretty sure as apposed to the eventual 50w Marshall standard of 3.4k.

    Some quick googling should turn up good information on this.


    Mismatching the load, most notably, can damage the output transformer. Putting a higher load on a lower tap can cause high voltage spikes that can short out the windings. This is why you don't want to ever run most tube amps without a speaker connected. A lower load can stress the power section of the amp and possibly cause heat damage. They say you are usually safe going 2x in either direction. IMO high voltage vintage style marshalls being run wide open into a high load might not last forever.

    If the transformer is designed to reflect a high load to the tubes then everything is fine assuming the design is good.
     
  17. fusionbear

    fusionbear exquirentibus veritatem Gold Supporting Member

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    There is another reason it is hard to switch between the both circuits: There is one more B+ drop in a JMP Bass circuit between the cathode follower and the preamp. This makes another significant difference between the circuits
     
  18. Roe

    Roe Member

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    It increases the reflected primary impedance from 3k5 to 7k and reduces power and bandwidth. Its pretty safe
     
  19. alvagoldbook

    alvagoldbook Member

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    The reason for the different impendices on the OT is because the amps used different power tubes. The load resistance on the plate of a power tube varies. EL34s have a low load resistance of about 2k which is why Marshall used a output transformer with a 1.7k primary. JTM45s (reissues) used a 4.2k because 5881s have a higher load impendence. The reason why the JTM45 only got 30 watts is because big bottle power tubes suck a lot of current and a single tube rectifier can only deliver so much. This is why Mesa uses two rectifier tubes. The JTM100 had the jtm45 circuit but with a solid state rectifier, so depending on how its biased could get around 100 watts.
     
  20. Shiny_Beast

    Shiny_Beast Member

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    My understanding is the jtm 45/100 is really a 70 watt amp for the same reasons the 45 is a 35 watt amp, and nothing to do with the rectification. My understanding of power tubes and output sections isn't detailed enough to specifically back this up, but if it's wrong it's one hell of an urban myth.
     

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