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1968 Bassman Amp, can anyone tell what's been done?

Discussion in 'Amp Central Station' started by halfsmith, Apr 16, 2021.

  1. Wally

    Wally Telefied Ad Free Member

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    The serial number chart is good except for the cusps of the years, so a more definite number would be of interest. http://www.ggjaguar.com/fendamp6.htm
    That pot date from the 40th week of 1968 is a strong hint that the amp left the factory as an AC568.
    The cathode circuit for V1P3 is on the far right end of the board...find pin3 on V1 and follow the yellow wire. As I noted, the AC wiring needs to be best and safest. Here is a diagram...https://studylib.net/doc/18063589/converting-2-prong-to-3-prong-grounded-plug
     
  2. tele_savales

    tele_savales Tele-Holic Platinum Supporter

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    @Wally I'm a little confused. I'm scouring over both the AA and AB165 and the AA864 and they all look like it's just half of V2 that's wired. They're not identical but it looks like there's a vacant triode. Can you clarify the difference in the wiring of V2? There's some serious gaps in my knowledge base and I just don't get it.
    Also: would you personally rather have 3 preamp stages in each channel or the ability to jumper the two channels? I've had 2 AB165 and jumping the channels was "the kind".
     
  3. printer2

    printer2 Poster Extraordinaire

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    Yes they did not use the extra triode. Both channels go through the third stage tiode so both channels are in phase.
     
    Last edited: Apr 19, 2021
    tele_savales and Wally like this.
  4. Wally

    Wally Telefied Ad Free Member

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    There are mods for utilizing that unused triode in V2. One can have a four gain preamp if wanted.


    printer2, I have to think that you wanted to type triode instead of diode there in the second sentence??
     
  5. E5RSY

    E5RSY Doctor of Teleocity

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    Is this a BM50, or BM100?
     
  6. Wally

    Wally Telefied Ad Free Member

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    This amp s probably an AC568 that has been modified back to an AA864 bias circuit. The Bassman 50 is a descendant of the AB165 Bassman as is the Ac568. The Bassman 100, which is a descendant of the Super Bassman, is a different circuit with only two gain stages per channel.
     
  7. printer2

    printer2 Poster Extraordinaire

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    Damn diodes get in everywhere if you let them. Fixed it.

    I would rather go with a 5F6 channel and a BF channel. I have a '68 sitting downstairs, just no use for it.
     
  8. halfsmith

    halfsmith TDPRI Member

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    Is it correct to think that if the circuit is AA864 or AA165 then jumpering the Bass side and Normal side will put them out of phase (because the Normal side only has 2 gain stages, and the Bass side has 3)?
    I am still working out how to determine where my Normal channel meets up with the Bass channel. I'm just not adept enough to follow the board.
     
  9. printer2

    printer2 Poster Extraordinaire

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    Follow the signal through the plate capacitors.
     
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