Gain boost question.

Discussion in 'Shock Brother's DIY Amps' started by Bitsleftover, Mar 26, 2020.

  1. Bitsleftover

    Bitsleftover Tele-Meister

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    Hi all. I was just studying the schematic from an amp I used to own. A Marshall JVM. In a nutshell it has about 6 modes. Starting from a Plexi gain structure. ie 2 gains stages, cathode follower, PI, Power Amp. (then increasing one tube stage at a time with the addition of a gain boost. All controlled by relays.
    It’s the gain boost around this gain stage I’m both intrigued and confused by in equal measure.
    Can someone please explain how this works?
    Just for my own education or understanding.
    I’ll post a close up of the stage in question, then a schematic of the preamp only question so you can see it in context.


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  2. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  3. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  4. Bitsleftover

    Bitsleftover Tele-Meister

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    CN202 just heads off to a gain pot with a bright cap around it[​IMG]


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  5. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    R74 22k and C45 1uF make an RC filter allowing most of the signal to go to ground when RL2 is engaged. R76 1 meg is there to reduce a "pop" when the relay switches.
     
  6. Bitsleftover

    Bitsleftover Tele-Meister

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    Ahh I see. Nothing more complicated than that.
    Thank you


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