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Thread: JCM 900 Mod questions

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    JCM 900 Mod questions

    Hi, I know there has been loads of threads about modding JCM 900's but I just wanted to ask a quick few questions. I wanted to find out about modding my mid 90's dual reverb to a more traditional Marshall sound and came across this mod on you tube:
    Marshall JCM900 dual reverb, all tube preamp mod:
    Here's what I changed in the amp:
    1] C27, (4n7)snipe one leg off
    2] C35, (220pF) to 100pF
    3] R42, (10k) piggy back another 10K over it.
    4] D10, (diode) snipe one leg off
    R60, (470k) Pickybag 47K over it
    R13, (220k) snipe one leg off
    https://www.youtube.com/watch?v=dOyCfyDcVHQ

    The results of the mod on the video sounded good. I just wondered, electronically speaking, what the mods actually do to the amps circuit and if there are any dangers in doing this to the circuit. I'm not a complete novice, I have built a few 5E3 and JTM-45 circuits in my time. Any help would be greatly appreciated.

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    Lifetime Member Enzo's Avatar
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    Why not make a copy of the opriginal schematic, than make those changes to it. That will show you how the circuit was altered.


    JCM900 is a series, not a model. What model amp do you have?

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    Education is what you're left with after you have forgotten what you have learned.

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    I have the 100w Dual Reverb 4100.

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