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'73 Superlead to Plexi Mod?

Discussion in 'Let's Talk Vintage' started by GOAT, May 17, 2021.

  1. GOAT

    GOAT New Member

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    Hello,i own a 1973 Superlead and now have a good tech. Is there such a thing as modifying the Superlead amp to late 60's Plexi circuit? I understand the values are not that far off .
    If so, is it a good idea or bad idea? Thanks.
     
  2. Deftone

    Deftone Well-Known Member

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    I'm no expert but any mod to a vintage Marshall seems like a bad idea to me.
     
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  3. neikeel

    neikeel Well-Known Member

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    The differences are minimal (depending on which era plexi you mean)
    67 shared cathode v1 low filtering - Superbass preamp
    68 split cathode v1 low filtering - Lead preamp
    69 split cathode v1 higher filtering - Lead preamp.

    If your 73 is turret board (vs late 73 which would be early type pcb) then mods are easy and reversible:

    Add 0.68uF axial (160v mustard normally) but any low voltage cap across the 820R on V2a.
    Increase NFB from 100k via 4ohm tap to 100k on 16ohm tap (close enough to 47k on 8 which is plexi spec)
    remove the jumper wire between anodes on the screen filter cans so you have just 50uF in series with 50uF to make 25uF on the screens.

    That will give you a taste of plexi tones form your amp (assuming you are running EL34s).
     
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  4. JP2036

    JP2036 Well-Known Member

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    Don't touch that amp.
     
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  5. matttornado

    matttornado Well-Known Member

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    I have 1974 Superlead with a few mods. I changed the NF R from 100K / 4 ohm tap to 47K / speaker jack & added a .68uF Orange Drop bypass cap on V2. I also changed from 6550s to EL34s.
     
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