Identifying 60's Fender Twin Reverb Circuit

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tomcurrie, Feb 6, 2015.

  1. tomcurrie

    tomcurrie Member

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    Hello Friends.

    I recently picked up a 60's "drip edge" Fender Twin Reverb. Seems to be completely stock except for the bias cap and filter caps. I'm trying to identify which Fender circuit this is. The sticker inside the cab says "AB763" but to the best of my knowledge this is a '69. What puzzles me more is that there are 2x 220uF/250V filter caps. I'm pretty sure Fender never used 220uF/250V caps in Twin Reverbs.

    The amp works and sounds pretty good. I'd like to identify which circuit this really is and get it back to stock.

    I'm open to and would appreciate any suggestions.

    https://www.flickr.com/photos/34258368@N03/sets/72157650687374425/

    Thanks
     
    Last edited: Feb 7, 2015
  2. tomcurrie

    tomcurrie Member

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    It's not a true AB763 circuit judging from the schematic. I also noticed that the 220uF/250V caps would have 400+ volts on them. Not sure how they haven't failed yet.

    My guess would be either a AC568 or AA769.

    Looking for some clarification and thoughts.
     
  3. Ampedusa

    Ampedusa Member

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    Not AB763. It's had a cap job. It doesn't look like it has the transformer in the 135 Watt schematic. It looks to be stock except for the cap job. It looks like a combo of these 2 schematics, might be a transition model.

    http://www.thetubestore.com/lib/thetubestore/supersixreverb100.gif

    http://www.thetubestore.com/lib/thetubestore/supersixreverb135.gif

    If you are going to blackface it to AB763 specs, listen to my old boss at the 7:00 minute mark. That model is close to what you have and you can get a good idea of what to change.

     
    Last edited: Feb 7, 2015
  4. tomcurrie

    tomcurrie Member

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    It's not either of those.
     
  5. Ampedusa

    Ampedusa Member

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    It's sort of a combo of the 2. The power supply filter circuit is similar to (unknown what values were originally there before the cap job) the 135w schematic. The rest of it (going by resistor values and layout) is the same as the 100 watt schematic, except for the phase inverter values. I can't find any schematic with 470k P.I. resistor values, usually it's 1 meg or 330k. And it doesn't have a push/pull volume pot. The transformer codes are the same. There may be a different schematic with exactly the same circuit. See if you can go find it, I couldn't.

    I'll also point out that your transformers are dated to early 1969, so it was built probably mid to late 69. That 100 watt schematic was approved in 1971 and was Rev F. So Rev A could have been around the time of that build.
     
    Last edited: Feb 7, 2015
  6. gldtp99

    gldtp99 Member

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    Look closer at the 220uF/250V filter caps------ they are connected in series with 220k balancing resistors across them------ this gives the equivalent of a single 110uF/500V cap.
    The 10uF/450V pairs that are connected in parallel are the same as a 20uF/450V single cap.
    As for the rest of the circuit, no way is it an AB763.................. gldtp99
     
  7. pdf64

    pdf64 Member

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    It looks pretty close to the aa769 http://bmamps.com/Schematics/fender/twin_reverb_aa769_schem.pdf with the only (originally fitted) discrepancy seemingly being the 470k LTP grid leaks.

    Whereas the later models linked in post #3 had evolved a fair bit, with master volume, pull boost etc.

    What's the VB+? The schematics indicate that it varied quite a bit in that era.
     
    Last edited: Feb 7, 2015

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