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Thread: JCM900 Tone stack mod 4101 50watt combo

  1. #1
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    JCM900 Tone stack mod 4101 50watt combo

    Hello

    I have a customer who wants me to mod his JCM900
    His words below
    "The issue that I would like fixed is an eq problem ... there’s almost no bass and there is a nasty spike in the upper treble that I can’t dial out. "
    4100_4101_4102_4500_4501_4502_Pre-amp_CCT.pdf
    He only uses the clean channel, finds the drive channel to be to much, if i could reduce the gain
    This is not normally my strong point Hence why i ask the experts
    I had seen a post on here about doing some mods I'm not 100% sure if i would get any benefit from these ones
    http://music-electronics-forum.com/t43511/

    So what do you experts think

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    Old Timer Leo_Gnardo's Avatar
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    A common problem with lots of amps. Once you tame the hi frequency boost it seems the mids & lows come blooming thru. My best guess is reduce C37 on the master eq from 470 to 220 pF, maybe swap out the treble control for an audio taper pot. There's also an enormous 1 nF bright cap C34 on the clean channel's volume VR1, you could reduce or eliminate that.

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    Thanks Leo. I shall look into that. I was thinking of upgrading the tone stack caps to Orange drop or Mallorys

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by blindboybenton View Post
    I was thinking of upgrading the tone stack caps to Orange drop or Mallorys
    You can do that but I really don't think it's going to alter the general tone of the amp.

    When there's way too much top end, it distracts the ears (OW!). Take that away and hear the rest of the audio spectrum, it's much more satisfying. And in this case cheap and relatively easy.

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    Old Timer J M Fahey's Avatar
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    Unfortunately it is not a tone stack problem (its circuit is basically the same used in all Marshalls) but whole amplifier EQ and architecture.
    Itīs a buzzy nasty amplifier, *designed* for early 90īs Thrash, itīs in the nature of the beast.
    "Clean" and "JCM900" donīt belong in the same phrase.

    And BOTH 2 channels are dirty.
    Notice the Boost channel, built around IC1b and IC6b is clipped by a diode bridge BR2 and diode D10 BUT the "Normal" (IC1a + IC6a) one is *also* clipped by 4 Leds LED1-4

    If you want ton clean up and somewhat fatten the Normal channelyou can:

    1) remove the clipping LEDS.

    2) increase C25 which currently cuts everything under 440Hz .
    Try doubling it to 0.1uF ; if not enough to .22uF but not further, sound can become farty/muddy at high volume.

    3) C34 is a "bright" cap, always on; you may try removing it .... you might lose sparkle but on the other side, amp might become more "pedal friendly".
    If your friend uses pedals for distortion, they can sound less buzzy.

    **Plan B**
    Mod nothing, except perhaps removing clipping LEDs, and insert a graphic EQ pedal in the effects loop, then tweak at will

    **Plan C**
    Bypass (most of the) Preamp, plug whatever he wants from a Sansamp Classic to any pedalboard to a Notebook running some Amp emulator to line out from any nice and clean 15W amp he likes (not kidding) into Effects Return .
    Reverb and Master selection still work, now providing 2 different levels of same sound.

    -----------------------------
    Commenting on Mods shown on other post, where modder admits he does not know what he did:
    1] C27, (4n7)snipe one leg off
    Boost channel includes a fixed setting tone stack, he clips the treble section off.
    2] C35, (220pF) to 100pF
    Do-nothing mod, just shifts the RF cutoff somewhat higher.
    3] R42, (10k) piggy back another 10K over it.
    Will slightly increase gain on Boost channel.
    In theory by 6 dB, in practice less because the Op Amp is already running close to maximum (audio) gain.
    4] D10, (diode) snipe one leg off
    Fully removes diode clipping.
    Try it and see if you like it or not.
    R60, (470k) Pickybag 47K over it
    Another WTF? mod: cuts bass from *one* reverb recovery stage, not sure what they want to accomplish with it.
    R13, (220k) snipe one leg off
    Increases gain on the Tone Stack driving stage, will let amp clip more, not what I guess you are needing now.

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    Juan Manuel Fahey

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    Yes upgrade the C34 value to something smaller.
    It ain’t the cap brand that’s causing a massive treble boost.

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    Many thanks I will try a couple of theses and let you lot know how it works

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    Has he tried using other speakers? What speakers is he currently using? To me it sounds like he's not unhappy with the overall tone of the amp and going all mod-happy might change the amp more than he'd like.

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