Modified Fender Twin Reverb AB763 schematic: Am I doing this right?

Discussion in 'Amp Tech Center' started by Diverted, Oct 17, 2019.

  1. Diverted

    Diverted Tele-Meister

    Age:
    48
    Posts:
    299
    Joined:
    Nov 11, 2016
    Location:
    Bristol RI
    Hi all,

    Would love some extra eyes on this schematic. I would like to build a single channel Twin circuit with no tremolo.
    How does my schematic look? Anything I did wrong or am missing? The original schematic is linked below for reference.

    I have read that a resistor needs to be added to make up for the gain increase due to the lack of tremolo. I have read a bunch on this issue but it seems there are a million opinions. Can anyone clarify?

    Thank you!

    Original circuit:
    https://schematicheaven.net/fenderamps/twin_reverb_ab763_schem.pdf Twin schematic.jpg
     
  2. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,121
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    This whole page should be useful reading for you, and especially toward the bottom where there is a layout for AB763 including reverb excluding tremolo.
    https://robrobinette.com/RR763_BLACKVIBE.htm

    The schematic above might make a working amplifier but it has some choices that look to me like the person who deleted the trem wasn't sure which parts belonged to the trem and which to the signal path, and which to both, doing gain structure stuff. You can improve on this I think.

    For example, the 0.1 microfarad cap and 220k resistor between the final gain stage and the phase inverter aren't performing any service now that the other parts they used to interact with are gone. Good luck



    1,000,001 opinions and counting...

    Edited to remove excess, unhelpful criticalness. I misread the first post and assumed that this schematic came from the schematic heaven site from some amp guru file. Didn't realize it was a work in progress.
     
    Last edited: Oct 17, 2019
  3. Diverted

    Diverted Tele-Meister

    Age:
    48
    Posts:
    299
    Joined:
    Nov 11, 2016
    Location:
    Bristol RI
    Thanks, didn't know Robinette had this.
    True, I am more used to the bias shifting tremolo and not the one used in AB763 models.
     
  4. 63 vibroverb

    63 vibroverb Tele-Holic

    Posts:
    779
    Joined:
    Nov 4, 2014
    Location:
    Lititz, PA
    I’ve tried removing the tremolo from an AB763 amp before and while you certainly do get more gain/push, you get more noise and bass flub too. Depending on what you want, you’ll have to put in some resistors of certain values to tame down the tubbiness.
     
  5. Diverted

    Diverted Tele-Meister

    Age:
    48
    Posts:
    299
    Joined:
    Nov 11, 2016
    Location:
    Bristol RI
    Thanks, yes that's what I'm trying to avoid. I don't want more gain with the trem removed. Thanks!
     
  6. 63 vibroverb

    63 vibroverb Tele-Holic

    Posts:
    779
    Joined:
    Nov 4, 2014
    Location:
    Lititz, PA
    If you like the tone of an AB763 with the tremolo turned off, just put in a resistor equal to the value of the “intensity” pot plus the optocoupler (roach). I just pull the V5 tube and call it a day.
     
  7. schmee

    schmee Poster Extraordinaire

    Posts:
    7,505
    Joined:
    Jun 2, 2003
    Location:
    northwest
    Removing the trem should work out fine. Just try plugging into the normal channel to see if you like it. Are you trying to retain the Reverb section though? I've done single channel with no Verb or Trem, but not with Verb only.
     
  8. Diverted

    Diverted Tele-Meister

    Age:
    48
    Posts:
    299
    Joined:
    Nov 11, 2016
    Location:
    Bristol RI
    Yes, going for basically a very stripped down version of a twin: Single channel, TMB controls, and reverb. That's it. Thanks!
     
  9. Ten Over

    Ten Over Tele-Meister

    Posts:
    400
    Joined:
    May 13, 2015
    Location:
    Central California
    Hoffman AB763 1 Channel.jpg

    The 820R resistor on the cathode of the second gain stage should be changed to 1.5K. You also might consider separate cathode circuits for the reverb recovery and mixer stages. As 63 vibroverb said, put a 47K resistor to ground after the 0.1uF coupling capacitor on the mixer stage. You can remove the 220K resistor as snfollhat said, but the 0.1uF must remain. You could put a switch on the 47K to have it both ways.
     
    schmee likes this.
  10. Diverted

    Diverted Tele-Meister

    Age:
    48
    Posts:
    299
    Joined:
    Nov 11, 2016
    Location:
    Bristol RI
    Thank you! This schematic looks perfect.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.