1977 JMP 2204 tone issues

Discussion in 'The Workbench' started by Fearzero, Jul 17, 2014.

  1. Fearzero

    Fearzero Member

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    Hi there. I have a '77 JMP 50 watt. It is all original, verified to be cascading. My amp tech checked it thoroughly and said it is in perfect working order. It was re-tubed and biased back to the original 6550s and still no good.

    The problem is, the tone is flabby and the distortion is really gritty, not "nice" smooth distortion that I have heard from other amps of this ilk.

    Any ideas out there about why the amp sounds this way? Caps maybe? They are getting on 40 years old after all.

    Any other ideas (short of modding) on how to get this to sound how it should sound?

    Thanks in advance.
     

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  2. mott555

    mott555 Well-Known Member

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    Bad caps will usually add a lot of buzzing and humming to the output, even when not playing.

    Can you post a clip of how it sounds? I also have a cascaded 1977 JMP 2204 though it's on EL34's, still should be close enough to compare.

    Any idea what that 1/4" jack to the right of the output impedance selector is? Mine doesn't have that.
     
  3. mickeydg5

    mickeydg5 Well-Known Member

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    How is the idle bias set (mA/mV) and at what plate voltage?
     
  4. jgab

    jgab Well-Known Member

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    Was the tone ever to your liking, or has this buzz bad distortion just recently occurred? As Mott said, a clip would really help us out here.
     
  5. Fearzero

    Fearzero Member

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    Thanks for the replies.

    Well, there is not a lot of buzzing or humming so maybe that is not it.

    I will post some clips when I get some time on Sunday.

    Not sure about the "mains" 1/4" jack, must be some Canadian thing. Apparently there were several different versions of this amp.

    As for tone before, I never got to try it before I bought it so it has been like this since I bought it, I just figured new tubes and a bias would fix it but neope.

    As for idle bias, not sure but the plate voltage is 373v according to my amp tech. He said that is within normal range.

    Stumped.
     
  6. Joshabr1

    Joshabr1 Well-Known Member

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    I see a 330 uf capacitor at the far left of the board. It should not be there if your amp is cascaded. I can't tell if the resistor on v2 socket is 100k or 330k. Looks like a 100k. Your amp is the earliest version it will need to be modded to the later version with a cascaded v1(not big job at all). With the 2204 the earliest ones do not sound in the ball park of the later. Make sure ur tech has the latter schematic.
     
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  7. Joshabr1

    Joshabr1 Well-Known Member

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    [​IMG] Here's one of mine. U see the extra resistor and cap on the volume pot and jack? U can also see the difference on the board. It's not a big deal and would not hurt the value. Change it and u will here that baby roar.
     
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  8. Fearzero

    Fearzero Member

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    Great, thanks so much for the info.

    I did the cascade test and found signal from both cables but when I brought the amp into the tech he said it was already cascading. Doh! I guess I won't be taking my Marshall to him any longer lol!

    Do you have a schematic w parts I can bring to an amp tech?

    Thanks again!
     
  9. Joshabr1

    Joshabr1 Well-Known Member

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    The 2203 schem would work for the changes(as they are all in the pre amp). Or the jcm800 50 watt schem I was gonna say if ur tech told u that was cascaded to look for a tech. Sounds like u figured that out. Like I said its not a hard job. And the amp will be massively improved. 2204 all musturd caps oh yea. It will yield righteous tone
     
    Last edited: Jul 18, 2014
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  10. Fearzero

    Fearzero Member

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    Awesome, looking forward to getting this mod done. Thanks again for the help!
     
  11. Oatie

    Oatie New Member

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    I have one of these 50 watt MV amps, a 1977 I think it is. Mine is still stock and sounds like crap. Looks like I will be doing the mods as well.

    I bought the combo amp for the cab and the Brown-Back Marshall Celestion Speakers, I removed the MV Chassis and put one of my old Plexi's in it. The Lar=-Mar has changed the world.
     
  12. Oatie

    Oatie New Member

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    I have a Step By Step Conversion for this amp, I will post the links to DL the PIcs instructions, it looks well done. I found on a internet link a while ago. Just DL each pic, this way it will lose the advertising, copy to your computer.

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    This step by step shows how to convert the earliest "oddball" master volume 2204 or "M 1987" amps (circa 1976- early/mid1977) to the later/greater 2204 circuit used in them from early/mid 1977 onwards. This step by step varies a bit from the original post '77 amps, but only in that shielded wires are used at 2 critical spots. The shielded wires greatly reduce the chance of instability/squealing, thus likely will save you from the need to "chopstick" and glue/silicone wires in place where things are stable. Circuit itself is still exactly the same as all post “oddball” 2204s, the shielded wires only serve to make the conversion easier and (most likely) problem free once done.

    LOOK THROUGH ALL STEPS BEFORE STARTING!!! SOME “oddball” 2204s may have the correct part values in certain spots already (like the two 470K voltage divider resistors for example), as these amps did apparently evolve a bit in their short run before the 2204 circuit was finalized as we know it today. So, look carefully through all steps to verify if your particular amp may or may not need a few bits changed here or there to potentially save you some time doing the conversion.

    Use this info at your own risk, tube amps have very high and potentially LETHAL voltages inside them!!!! If you don't know how to safely work on such equipment, nor how to work on printed circuit boards, then please do not attempt this conversion yourself - take amp to a qualified tech.
     
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  13. Fearzero

    Fearzero Member

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    Thanks, I actually found a zip of this mod on another site. It looks easy enough but steps 4 and 5 are nebulous (the brown wire is not really shown where it is supposed to go) so I will have to take it to a tech I guess. Too bad, if only he was more detailed in that step I could do this myself. :(
     
  14. Fearzero

    Fearzero Member

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    Had the cascading mod done to my amp and the amp sounds killer now. Definitely worth the mod price.
     
  15. Point 2 Point

    Point 2 Point New Member

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    Reaching back on this thread.
    I have a ‘77 2204 on the way.
    It appears to be very clean and un modded I’m told.
    Is it possible to get an idea as to weather or not it is cascaded by it’s serial # ?
    Thanks
     
  16. pedecamp

    pedecamp Well-Known Member

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    If it's unmodded then its not cascaded. Sounds like the OP was very happy with the mod you may be too! :yesway:
     
  17. Point 2 Point

    Point 2 Point New Member

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    Not necessarily
     
  18. Point 2 Point

    Point 2 Point New Member

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    Just got the serial # from the seller,
    It’s from July ‘77 so chances are it’s cascaded
     
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  19. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Does it have that resistor and cap running from the first volume pot back to the low input jack? If it doesn’t then it isn’t cascaded.

    30E4CDA4-1471-4803-A79A-2B74A849034D.jpeg
     
  20. Point 2 Point

    Point 2 Point New Member

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    Great thanks, it comes Tuesday.
     

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