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Please explain this Marshall preamp circuit

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  • Please explain this Marshall preamp circuit

    The inputs and volumes are doing strange things on this amp. It has been hacked, and I am pushing my way through it. But, I'm not sure if there is an input wiring problem because I'm not sure I grasp how this circuit is supposed to work. I can see when plugged into J2 Low, the jack ungrounds ant the signal is fed to the preamp volume and into the grid of V1b. But when plugged into J1 Hi, it is sent thru extra gain stage at V1a to C3, but tif the other side of C3 is grounded as drawn, what happens then?



    Here is a shot of the ugly jacks. I always get into trouble with these.

    Click image for larger version  Name:	002.JPG Views:	0 Size:	406.2 KB ID:	909252

    On the left jack lower left red wire goes to C3, lower right goes to the 470K resistor and .0047 cap to the volume control.
    Right side jack lower right red wire goes to R3 68K, and the black wire is ground.

    I don't know why it won't take the correct link, but it is a Marshall 2204 preamp.
    Last edited by Randall; 07-15-2020, 02:07 AM.
    It's weird, because it WAS working fine.....

  • #2
    Hopefully this works.

    https://www.thetubestore.com/lib/the...-Schematic.pdf

    With the new forum software, you can just copy and paste the link right into the text field. It's no longer necessary to build a link.

    C3 is not grounded if something is plugged into J1. See the extra switching part of the jack on the J1 ground? The signal path is through the switch contacts of the J2 jack to the same place as if you were plugging into J2.

    Edit: (Adding Picture)

    Click image for larger version  Name:	Marshall-2204-Input.jpg Views:	0 Size:	47.6 KB ID:	909255
    Last edited by The Dude; 07-15-2020, 11:40 PM.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      These are dual switchjacks, and the schematic doesn't really show that. So with no plug in the jack there is continuity from the left to right side of the jack, one contact for the shield (upper in your photo) and tip (lower). The contacts on the right side contact a plug when inserted, the ones on the left are then open.

      The arrow down from the the Low input jack tip to the shield of the High input really is going through the shield switch on the High input. You can see this in your picture, the Low jack (on the left), lower left corner (red wired connected to C3) is wired over to upper left corner of the High jack via the green jumper wire. The upper left of the High jack is only connected to ground (black wire, upper right) when there is no plug in the High jack. When an input is plugged into the High jack that upper left contact goes open, and that green wire is no longer connected to ground, so the red wire connected to C3 is no longer connected to ground.
      Last edited by glebert; 07-15-2020, 03:11 AM.

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      • #4
        Thanks gents, that makes sense now. I wondered about that down arrow, but didn't realize how it worked. Marshall drawings sometime give me fits. I think these jacks were actually wired correctly, but they were so poorly done, I went ahead and replaced them, and the picture became clearer upon doing so. Things are still strange in the HI channel involving the pre volume. It will screech and then drop right off in volume as it gets turned up, SOMETIMES. Then a slight reduction in volume, and it comes right back, as if it was a bad pot, but I don't think it is, the LO channel doesn't do this. It will oscillate if turned full up. New preamp tubes makes no difference. So far, three controls wired wrong, along with those hacked input jacks, so I am chasing after someone's bad work, always a jolly good time!

        I'll attack it tomorrow with a fresh head.
        It's weird, because it WAS working fine.....

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