Marshall JMP-1 tube locations

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Roodboy, Dec 31, 2005.

  1. Roodboy

    Roodboy Supporting Member

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    I have read that the two preamp tubes in the JMP-1 work one each for clean and the OD channels. I am swapping tubes arond and would like to minimize the swapping as they are mountes direct to the flimsy board, so I would like to know which tube corresponds with clean and which one is the OD?
    I am thinnking Tungsram ecc83; JJ ECC 803 or Mullard 12at7 for the clean and JJ ecc83; Mullard ECC3 or early 80's chinese for the OD.
    Thanks for the help.
    Happy New Year!
     
  2. loverocker

    loverocker Member

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    I've got a schematic somewhere, but I'm not sure it shows which is which. Try taking them out one by one and see which channel disappears in each case.

    FWIW, I wouldn't expect much of a tonal change for the overdrive channel, no matter what valve you use. It comes after the diode-clipping stage and (IMHO) so it's effect is much reduced. I don't even know which valves are in mine - it souds fine the weay it is. :)
     
  3. Roodboy

    Roodboy Supporting Member

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    I've been playing around with the tubes and am getting pretty much the initial results I expected ... I like Mullards even in this preamp. Maybe I'm insane and need to have someone else swap the tubes for a more objective result. The Chinese are very aggressive lack a bit of mid detail, the Tungsrams are great but like a little oomph. I'm generally not a big JJ fan but I was surprised by the ECC803 JJ. The JJ ECC803 has a lot of character on the clean settings CL1 & CL2 but is a little brittle in OD2. The JJECC83s is generic sounding.
    I haven't played aound much with EQ to adjust for the characteristics of the tubes and am jonesing for some RFT's to try.
    I have only tested identical pairs so far so I haven't made the V1/V2 channel determinations yet.
    I have read that the OD is all diode driven but from what I'm not hearing that, however, I thought that Mullards improved a JCM900 4502 DR.
     
  4. John Phillips

    John Phillips Member

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    Tubes will affect the sound even in something that uses diode clipping - especially if they come after the diodes. That allows you to hear the tone characteristics of the tube working on the frequencies produced by the distortion stage. It's the other way round that I would expect to make less difference, particularly on higher-gain tones where the diodes will basically obliterate everything that comes before.

    FWIW I have never heard one single product that uses a tube anywhere in the signal path where the tone is not improved by old-production tubes, where I've tried different ones - even stuff like the Mesa V-Twin pedal which is probably about as much a 'tube' preamp as the JMP-1 is. To me, the old tubes have a richer and warmer but also clearer and more transparent tone - modern tubes seem to sound flatter and harsher, and fuzzy or lacking in detail as Roodboy describes. Not all to the same degree of course, and there is a big tone range in both types, but that's what I've heard across the board.

    Just my opinion.
     
  5. Roodboy

    Roodboy Supporting Member

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    Thanks John, I can't read a schematic are the JMP-1 tubes after the diodes?
    Alos what tubes are you using in your V-Twin?
    Let me know when you get a chance.
    Thanks
    Rudy
     
  6. John Phillips

    John Phillips Member

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    I don't know where the tubes are in the JMP-1 - I've never looked inside one or at the schematic, since I've never come across a broken one (unreliability is not one of their faults in my experience!) - but if loverocker says they come after the diodes I would assume it's so. It would make more sense - I would take a (very) small bet that the tubes are used in the DC-coupled cathode-follower stage that drives the tone stack in the traditional Marshall circuit, and is relatively far up the signal chain... this is where a big part of the 'Marshall crunch' lives, which is why they used it in the hybrid-preamp JCM900s (like your one) too.

    I don't have my V-Twin any more, but I tried several types of tubes in it and found (as usual with Mesa products IMO) that it sounded best with old RCAs or GEs, with Mullards pretty nice and maybe a bit richer, but less clear. It was far from the most tube-sensitive device I've ever messed about with though, and I didn't really think any of them made enough of a difference to cure the general muddiness at higher gain that it has. The V-1 is a whole different ballgame though - it's radically more tube-sensitive and far better-sounding with good tubes, and is in fact all tube. I have GEs in mine.
     
  7. Roodboy

    Roodboy Supporting Member

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    I didn't mean to diss Loverocker, I'm sorry, I missed that he said the tubes were after the diodes.
    Thanks to you both for the help.
    Rudy
     

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