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Thread: Twin Reverb 100 to AB763 Conversion

  1. #1
    Supporting Member Robert Technology's Avatar
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    Twin Reverb 100 to AB763 Conversion


    I'm working on a Twin Reverb 100 (NOT the ultra-linear version, but it does have the push-pull Master Volume and hum-balance), that has been butchered by some former lost tone-soul. Almost every solder joint in this thing is suspect and it has some poorly executed mods that I haven't fully traced out yet as well as a runaway feedback in the reverb circuit that is causing it to blow fuses (with the reverb tank disconnected fuses do not blow).

    The owner has asked that I put it back to stock, and since so much work is needed has requested that I convert it to a AB763 circuit while I'm in there. Looks like this will just require a few component value changes compared to the stock Master Volume era circuit (phase inverter resistors, and a few coupling caps) in addition to the "hum-balance" circuit being converted to a proper bias circuit.

    Anyone out there have experience with these amps that would lean towards keeping it as a stock Master Volume "Twin 100" vs. converting it to the AB763 circuit? Any other major circuit revisions that I'm overlooking at this point from going through the two schematics?

    Given the current condition of the amp it will be about the same amount of work either way, so I'm inclined to just deliver what the customer is asking for (the AB763).


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    Last edited by Robert Technology; 02-11-2019 at 10:58 PM. Reason: Attached Schematics

  2. #2
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    I would personally restore the amp to stock first, w. Master & pull boost. After that, let the owner give it a thorough test.

    One thing that changed alot was the wiring dress. Having tried converting a later MV Bassman to an AB165 once, I learned the hard way that an improperly located ground wire can cause issues. I say this to caution against simply wiring to AB763, and make sure you follow the layout also, preferably by looking at an actual unmodded AB763 Twin Reverb.

    Good luck, hope all goes well!


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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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