Train Wreck Mod?? Or other master volume mods??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by northern1, Sep 21, 2004.


  1. northern1

    northern1 Supporting Member

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    Just inquiring, could anyone tell me if they have the "train-wreck" mod for a RI Marshall 1987x head or other master volume mods? Pic's or drawings, parts list, and instructions would be greatly appreciated! Thanks
     
  2. TheAmpNerd

    TheAmpNerd Member

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    What is the mod supposed to do?
     
  3. northern1

    northern1 Supporting Member

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    From what I have been told the train wreck mod is a master volume mod, that is apparently better than the "rich mod" which is the mod that most use for the Marshall plexi's.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Where are you finding information about these mods?
     
  5. Blue Strat

    Blue Strat Member

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    I'm guessing that you're asking about the "Ken Fisher" (father of Trainwreck) post PI dual pot master volume.

    Not sure if it's correct to attribute this to Ken Fisher or not, but his name is attached to it in Gerald Weber's first book which shows a schematic.

    If this sounds like what you're looking for, try a websearch of "post PI master volume" or similar words. I'm pretty sure I saw this online somewhere.

    This is generally considered a better master volume circuit than the single pot Pre PI circuit used in many amps.
     
  6. northern1

    northern1 Supporting Member

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    Thanks alot for the info, I will try that search. If I find anything I will let you know if you are interested.
     
  7. hear and play

    hear and play Member

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    Both the Rich mod and the Trainwreck mod are post-PI MVs.

    Rich himself posts (or used to post -- I haven't been over there in a while) on the LP Forum. You may want to inquire over there.
     
  8. TheAmpNerd

    TheAmpNerd Member

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    And the rich mod differs from
    the trainwrek mod how?
     
  9. 8nthatK

    8nthatK Supporting Member

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    FYI...



    "The reason that the PPIMV works so well in most Marshall circuits is
    because a significant component of the distorted tone in a classic Marshall
    style amp comes from PI distortion, not output section distortion. It fails
    to achieve the same degree of success in BF and SF Fender amps because most
    of their distortion occurs elsewhere in the circuit.

    This actual PPIMV circuit was included in some of the Marshall JCM800 amps
    that featured a master volume control. Its first use as a "mod" for
    Marshall style amps is attributable to a fellow named Joe Varela. It was
    first published as a "mod" for Marshalls, and credited to Joe Varela, in
    Craig Anderton's "Electronic Guitar" column in Guitar Player magazine in
    December 1982. The publishing date was 12/1982, page 113.

    The PPIMV actually pre-dates Rich, who apparently copied it from one of
    the MV-equipped JCM800 Marshalls and has made a name for himself, at least
    at the LPF, by applying it to older Marshalls.

    Interestingly, when copying the MV circuit from the MV JCM800, Rich
    apparently failed to recognize the safety value of the final pair of
    coupling caps and omitted them from his version of the PPIMV mod. By
    omitting these coupling caps, his implementation of the PPIMV mod fails to
    isolate the master volume control (and the front panel of the amp) from DC
    voltages, which is a major safety faux-pas.

    For safety reasons, I'd recommend using Joe Varela's circuit, not Rich's
    abbreviated version of Joe's circuit, as the last pair of coupling caps
    serve to keep DC voltage off of the master volume control, and off of the
    front panel.

    Instead of calling it the Rich Mod and giving credit to the wrong guy, it
    looks like we should really be calling it the Joe Varela Mod, or the Joe
    Mod. ;)"
     
  10. 8nthatK

    8nthatK Supporting Member

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    I first used this "mod" as described in Guitar Player around 1990...it works very well, though I've tweaked it for my own ears over the years. If you would like a copy of the original GP article in a JPEG format describing how to perform this (which is what I believe some refer to as the "Trainwreck mod"), e-mail me and I'll send it on.
     
  11. northern1

    northern1 Supporting Member

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    Thank you for the offer for the info on the mod, I sent you an email. Hope to hear from you soon!
     
  12. TheAmpNerd

    TheAmpNerd Member

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    This is pretty interesting.

    If we were to classify the three types
    of amp distortion, and their qualities
    what would the qualites be, and how would we recognize it?

    Pre amp distortion:




    Phase inverter distortion:




    Power tube distortion:
     
  13. 8nthatK

    8nthatK Supporting Member

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  14. sws1

    sws1 Member

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    The "Rich" mod was actually designed by a Electrical Engineer friend of Rich who knew nothing about guitar amps. As told by Rich himself, this guy took his meters and probes and played around with the circuit till he came up with something that Rich liked. Rich kept using it throughout the years. Only once he posted it on the LP Forum, did someone coin the term Rich mod.

    Aside from the missing caps, the other difference between Fischer MV and Rich MV is that the Fischer MV is referenced to ground (i.e., 3rd lugs of pot go to ground), whereas the Rich MV has the 3rd lug referenced to the bias circuit.

    I have the Rich mod in a Super Bass / Lead conversion, and while it works when the amp is turned up, it does not sound like a good MV at very low volumes. Actually gets buzzy. Then again, so do the JCM800s unless they are up loud.
     
  15. John Phillips

    John Phillips Member

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    The DC voltages involved in a PPI MV are only those of the bias supply circuit (approx. -30 to -60V, with very limited current capability), not the full B+ DC supply, and are not usually considered dangerous - I certainly wouldn't call it a 'major safety faux pas', and I am extremely safety-conscious. In any case, the power and standby switches carry highly dangerous voltages and are mounted on the same panel.

    In fact, one big reason the Ken Fischer mod (I'll call it that because I first saw it refered to as such in Aspen Pittman's book - and this is the non-grounded, referenced-to-bias-supply kind) is preferable is because when the control is up full the circuit is exactly stock - you have simply replaced the bias feed resistors with pot tracks. Introducing coupling caps alters the circuit, however slightly.

    In fact, the real issues with this mod are: 1. Because there is DC on the pot tracks, turning the pot is always noisy; 2. If a pot contact fails (which does sometimes happen), the bias supply to the power tube in question is removed, which will run it wide open and kill it very quickly.

    FWIW, the first Marshall to use a PPI MV was the Club & Country combo of 1976 (I think - pre-JCM800 anyway) - which is an excellent-sounding amp IMO - and does use coupling caps.


    I also like the 'cross-line' MV - the same type as used by Matchless and Bad Cat among others, which is also PPI, and involves no actual 'mod' to the circuit at all - just the addition of two wires and one pot to the existing circuit. It's the least invasive and simplest of all MVs to fit. It works by allowing the two halves of the push-pull signal to cancel each other out. It's also not isolated from the bias supply, although in a cathode-biased amp it is effectively at ground potential.
     
  16. 8nthatK

    8nthatK Supporting Member

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    Just to be sure it is clear :)

    The post was placed in quotations as they are not my statements, and as stated, FYI, take it for what it is worth... to you.
    It doesn't necessarily reflect my views one way or another, though I do have an opinion, but have learned to keep those mostly to myself, at least when I can help myself ;)
     

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