NFB resistor question, AA764 Vibro Champ

Discussion in 'Shock Brother's DIY Amps' started by zoran, Feb 25, 2020.

  1. zoran

    zoran Tele-Meister

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  2. sds1

    sds1 Tele-Afflicted

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    The 47R forms a voltage divider with the 2700k NFB resistor. Roughly a 60:1 ratio. The output of the voltage divider is the amount of NFB signal applied.

    Double, or half, that 47R you still have a very low value. But look what it does to the ratio of the voltage divider -- big change to the amount of NFB...

    Also, we need that resistor to be a low value, otherwise look what it does to Rk and therefore the bias of that tube.
     
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  3. zoran

    zoran Tele-Meister

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    aaah, a voltage divider. interesting. it makes sense that it's there for a reason. thanks for clearing that up. still wrapping my head around a nfb loop as a concept. it's incredible.
     
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  4. zoran

    zoran Tele-Meister

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    so if the value of the NFB resistor (2.7k on the schematic) changes, is it important to change the value of the 47 ohm resistor to keep the 60:1 ratio?
     
  5. sds1

    sds1 Tele-Afflicted

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    Yep, to maintain the same amount of NFB you need to keep that ratio.
     
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  6. King Fan

    King Fan Friend of Leo's

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    And OTOH, different amps can have different ratios.
     
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  7. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The 47R divider allows for a bypass cap & gain stage that wouldn't work without it. If you check Rob's site about adding NFB to a 5e3, part of the deal is that it doesn't work with the bypass cap in place. So, his mod uses a 3-way switch that allows NFB but disconnects the bypass cap for it.

    Using the voltage divider allows the bypass cap to remain in place and function properly and have NFB. I used that setup on my 5e3, and on a current build I'm working on.

    *** - I think this is correct, but don't take my word as gospel. ;)
     
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