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AB668 Pro Reverb Questions

Discussion in 'Amp Tech Center' started by CroMagnonDan, Jun 10, 2020.

  1. Ten Over

    Ten Over Tele-Holic

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    That guy didn't know what he was doing and he must have been getting money under the table from suppliers for using way more parts than necessary.

    The cathodyne does have a very low output impedance at the cathode so a very long cable can be used for the send. But the cathodyne has half the headroom as a cathode follower for about the same output impedance. His cathodyne is nowhere near center-biased so that it uses more current than necessary and the bias will wander with large signals.

    The amp will function properly with no device in the loop, but it won't with a line level or less device in the loop. You would have to turn the master volume way down for line or instrument level and there is no return stage to bring the signal back up to fully drive the 6L6's. To make matters worse, there are two voltage dividers after the return jack.

    R33, C17, and R34 do nothing for the frequency response, but they do make the first voltage divider more severe. They may be an artifact from the reverb version.
     
    Wally likes this.
  2. CroMagnonDan

    CroMagnonDan TDPRI Member

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    Im never going to even mod this amp. It was my first and has been my main amp for most of the time i have been alive.
    I am getting parts together to make a CLONE of this amp (at least as close as i can) and i would like to remove all the things i dont use, in exchange for some modern features.
     
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  3. CroMagnonDan

    CroMagnonDan TDPRI Member

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    The plan is to build an amp from scratch. I literally just know this amp so well, i cant use anything else. I have tried. For like 15 years. And i am ready to give it back to my grandfather before i finally hurt it.
     
    corliss1 likes this.
  4. CroMagnonDan

    CroMagnonDan TDPRI Member

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    Do you guys think it would be feasible to run the line out from the amplifier to the line or drive input of my Nova, then jump that back into the unused channel on the front of the amp? Would that give me a wet/dry mix? Or would it just be a mess? Or would it just flat out not work at all?
     
  5. CroMagnonDan

    CroMagnonDan TDPRI Member

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    I tried this, it did not go well. Noise fest.
     
  6. CroMagnonDan

    CroMagnonDan TDPRI Member

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    Transformer codes:

    Left of power tubes
    A 015895
    U 1411
    EIA606-051

    Center
    013897
    EIA 606 117

    Right
    022921
    EIA606-136
     
  7. Wally

    Wally Telefied Ad Free Member

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    in order as you describe....power transformer, output transformer and reverb driver?????? If one is going to get into cloning, modifying, or any kind of tech work/talkit would be best to be able to identify components. The power transformer number is one that I don’t immediately recognize....that may be because it is an Ultralinear PT. I have one out of Brother homesick345’s UL Pro Reverb...I’ll have to refresh my memory by giving that one a look.
     
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  8. CroMagnonDan

    CroMagnonDan TDPRI Member

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    I Believe those are correct. I am absolutely fresh to any of this. Im just trying not to state anything incorrectly if i can avoid it. I know the specifics of the tone and performance I want, but am completely unsure what is needed to get there.
     
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  9. Wally

    Wally Telefied Ad Free Member

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    Cro, check that PT number again....the one I have reads 013895. And....we are all Somewhere on the learning curve.
     
  10. CroMagnonDan

    CroMagnonDan TDPRI Member

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    I had to guess whether it was a 3 or a 5. It is either scratched or covered in gunk. Did my best to wipe it off, im sure it is supposed to be a 3 though
     
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