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  • Marshall JCM 900 mod

    Hey guys,
    I got an offer to pick up a JCM 900 100W Higain dual reverb combo (2x12) at a ridiculously low price. I tried the JCM-900 before but remember not really falling in love with the distortion of it. Started googling around and it turns out a lot of people feel the same way. There is a diode clipping stage that handles distortion instead of tubes? I figured this would be the right place to ask for suggestions on mods to make it feel more "valve-ish" when overdriven.. Also, other mod ideas are appreciated to get the most out of it! It's meant to be a recording amp and a stage amp. Good low volume sound would be awesome for recordings and micing up.

    EDIT: Oh and I believe that particular model is 4100.

    I know some will feel that it's not right to buy an amplifier that isn't right in the first place but I don't mind tinkering around with it as I consider it fun and interesting. Also there is a pretty tight budget.

    Thanks in advance!
    Last edited by sibo; 05-31-2014, 11:21 PM.
    “Women are meant to be loved, not to be understood.”

    ― Oscar Wilde

  • #2
    FWIW it´s quite usable as-is, somewhat transistory sounding (that´s what it is anyway) and you won´t think it´s a JCM800 or something in a blind test, but a lot of people have managed to coax good sound out of it.

    Forget modding unless you fully gut it and build something better inside, but using it full blast or at least on Master volume set to 6 or higher, the classic Marshall power stage takes off and sweetens everything.

    Forget it in bedroom or garage situation.
    Juan Manuel Fahey

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    • #3
      Originally posted by J M Fahey View Post
      Forget modding unless you fully gut it and build something better inside,
      Really? Why is that?
      “Women are meant to be loved, not to be understood.”

      ― Oscar Wilde

      Comment


      • #4
        Because it´s a full solid state preamp, up to and including all important distortion, driving a real tube power amp.

        Modding the preamp would be modding an Op Amp based, diode clipping preamp.
        Changing a few resistor and cap values here and there won´t turn it into a full tube preamp.

        Just check the schematic

        The only tube which might distort and add tube flavour (so it would sound like, say, a hybrid Valvestate) is "handcuffed" so as not to distort *at all*.
        Juan Manuel Fahey

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        • #5
          Mkay I see. I just noticed all those small tubes in the back and thought they were preamp tubes but I get it now! I guess I'll have to Google around and hopefully find what I'm looking for. I am more of a player than a modder so I kind of need a good guide to be able to pull all that off. Thanks!
          “Women are meant to be loved, not to be understood.”

          ― Oscar Wilde

          Comment


          • #6
            Yeah, there is no goofing with the 3 preamp tubes.

            1/2 of V1 is the Send jack.
            The other half is a buffer to the Phase Inverter (V3) on the power amp.

            V2 is used for Reverb.

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            • #7
              Speaking of the reverb, it sounds fantastic to my ears!
              “Women are meant to be loved, not to be understood.”

              ― Oscar Wilde

              Comment


              • #8
                Just in case I was not clear, I didn't send you somewhere else looking for Mods, but "don't waste time" , use it as is, preferrably LOUD .
                Sound is quite usable, reverb as you found is usable too.
                The day you want fire and brimstone out of it at a lower volume, build and use a full tube clone of :

                The project is in the Net and not hard to build
                Juan Manuel Fahey

                Comment


                • #9
                  pretty simple!


                  Edcor XPWR194 ($20) might work
                  Last edited by tedmich; 06-01-2014, 08:16 PM.

                  Comment


                  • #10
                    Meh! Just as I decided to settle for stock sound you show me this Sounds awesome, is it a tube distortion stomp box? I feel like I already got enough pedals.. I think the way to go for me is to use the amp as-is until I can gather enough dough for a substitute. I might build that pedal if I stumble upon some kind of "BYOC-kit" with instructions or at least a casing for that sexy circuit. Maybe I'll just tweak the amp little to my taste later, I read that some guys mess around with changing caps and resistors for more bass and less treb etc.. Seems a very fast easy job and IMHO the EQ doesn't seem very responsive.

                    Is the eq chained to the preamp side? I ask this because I figured I should try to use as high volumes as possible on power amp side for tube fatness, leaving perhaps some tonal loss? Or am I approaching this the wrong way?
                    “Women are meant to be loved, not to be understood.”

                    ― Oscar Wilde

                    Comment


                    • #11
                      I had 3 900's back in the day, 2 dual reverbs and 1 was the slx single channel high gain. I've said this b4 and i'll say it again....they are GREAT amps in a live situation because of the way they cut thru a mix. But, you have to have a good one. They seemed to vary wildly, at least the ones i had did. One was horrible. Had to keep the treble at 0 just to be useable and the tubes were known good and the same i used in the other ones. The one i used for years that sounded great beat out all the other marshalls i had including 800's of which i also had 3, and even beat or at least worked as well as a silver jubilee i had. Not doubt the jub was a great amp, but the way the 900 cut thru the mix just worked great. You could play it solo next to a good 800 and you'd be wanting to choose the 800 no doubt. But take em to a gig and i would choose the way the 900 cut every time.

                      Now the SLX 900 was a different story. Even at low gain settings, which it wasn't made for to begin with, turning your guitar down to clean up was impossible. I mean, you COULD, but it never sounded very clean or very good. That amp i DID mod by eliminating the diode clipping and it helped immensely. It still never sounded near as good as the dual reverb model tho.

                      Comment


                      • #12
                        Okay so I've gotten familiar with my new amp and now I know what it will and wont do for me. To my ears the amp can do medium gain stuff at high volumes very well, also the clean sound is alright which kinda surprised me.
                        I am using a lot of pedals and it seems to take them pretty well, especially when the master volume is cranked.

                        The problem is, to reach that sweet spot the volume needs to be too damn high (I knew that before I bought it too) so I am thinking there should be an easy mod for that. I know there's attenuators but they cost as much as the amp did

                        Is there any smarter/easier/cheaper way to approach this problem? I am not talking bedroom levels here but I do want to hear the drummer from time to time.. TYVM
                        “Women are meant to be loved, not to be understood.”

                        ― Oscar Wilde

                        Comment


                        • #13
                          they're okay, sorta dated sounding, here's the deal

                          Originally posted by sibo View Post
                          Okay so I've gotten familiar with my new amp and now I know what it will and wont do for me. To my ears the amp can do medium gain stuff at high volumes very well, also the clean sound is alright which kinda surprised me.
                          I am using a lot of pedals and it seems to take them pretty well, especially when the master volume is cranked.

                          The problem is, to reach that sweet spot the volume needs to be too damn high (I knew that before I bought it too) so I am thinking there should be an easy mod for that. I know there's attenuators but they cost as much as the amp did

                          Is there any smarter/easier/cheaper way to approach this problem? I am not talking bedroom levels here but I do want to hear the drummer from time to time.. TYVM
                          basically the combos have ALOT of bass removed, think of it this way, there's a couple of opamp solid state stages in that amp, and they each have a 'pre' eq, that is a preset tone before the tone knobs. you can modify the tone response of these stages, it involves modding the feedback of the opamp stages, sometimes just clipping one lead of a couple caps helps. I will tell you that having balanced diodes in that lead channel helps a lot. they can be opened up and made to sound closer to the jcm800, it just takes some experimenting in that opamp section.

                          Comment


                          • #14
                            Originally posted by supawah View Post
                            basically the combos have ALOT of bass removed, think of it this way, there's a couple of opamp solid state stages in that amp, and they each have a 'pre' eq, that is a preset tone before the tone knobs. you can modify the tone response of these stages, it involves modding the feedback of the opamp stages, sometimes just clipping one lead of a couple caps helps. I will tell you that having balanced diodes in that lead channel helps a lot. they can be opened up and made to sound closer to the jcm800, it just takes some experimenting in that opamp section.
                            Interesting, where do I begin?
                            “Women are meant to be loved, not to be understood.”

                            ― Oscar Wilde

                            Comment


                            • #15
                              Originally posted by sibo View Post
                              Okay so I've gotten familiar with my new amp and now I know what it will and wont do for me. To my ears the amp can do medium gain stuff at high volumes very well, also the clean sound is alright which kinda surprised me.
                              I am using a lot of pedals and it seems to take them pretty well, especially when the master volume is cranked.

                              The problem is, to reach that sweet spot the volume needs to be too damn high (I knew that before I bought it too) so I am thinking there should be an easy mod for that. I know there's attenuators but they cost as much as the amp did

                              Is there any smarter/easier/cheaper way to approach this problem? I am not talking bedroom levels here but I do want to hear the drummer from time to time.. TYVM
                              Yeah.. Sell it and buy a used AC 15 or something off of CL, lol. No matter how you mod it, it will still be loud.

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