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Help with High Voltage in Blackface Vibro Champ

Discussion in 'Amp Central Station' started by Josey, Jun 27, 2018.

  1. peteb

    peteb Friend of Leo's

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    One way to see if the caps are working right, that the DC is where it is supposed to be and the AC is where it is supposed to be is



    Check the DC on all grids of all tubes. Should be very close to zero.

    Check for AC on the filter caps, that should also be real low, a couple, or few volts max.



    Make sure that you turn the amp volume way down when measuring pins on v1 and v2, or any tube.
     
    Josey likes this.
  2. peteb

    peteb Friend of Leo's

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    Champ and Vchamp power transformers typically have an unused filament center tap, cut and taped off.

    I think I see it in this picture of Josey’s Amp.

    It should be a green/yellow covered wire.



    Has anyone ever heard of anyone rewiring the filaments on a champ by using the filament centertap?


    And to what affect?




    A8510355-7FF7-413B-9A2B-2E99580519A9.jpeg
     
  3. peteb

    peteb Friend of Leo's

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    Speaking of twisted pairs:


    Elpico recently had a thread about grounding, that became a thread about grounding and hum. It was interesting, informative and put a lot of emphasis on twisted pairs to reduce loop area, to reduce transmission and reception of EMI, electromagnetic interferance. On the BF/SF champs and vchamps, Fender chose not to use twisted pairs on the filaments, even though the centertap is there, but they went to considerable trouble, to me at least, to twist the wires going over the board, and it is not documented on the layout or schematic.




    What are these wires that are twisted together and what loop area size is reduced, and how does it figure into overall hum reduction theory?



    The red wire connects both plate resistors for the two preamp stages to the B+.

    The yellow wire delivers the preamp signal to the grid of the power tube.




    Not exactly a loop or a complete loop, but two partial loops.



    Even though Fender treated the champ like a practice amp, they must have considered twisting these wires as being fairly important.









    B15BE19F-8146-4503-8FCB-36A388D0D35D.jpeg
     
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