Fender Blues Junior Negative Feedback R Value for 16 ohm

Discussion in 'Amp Tech Center' started by Joe Guitar, Sep 18, 2020.

  1. Joe Guitar

    Joe Guitar TDPRI Member

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    Hello. I just recently replaced the stock Output Transformer in my Fender Blues Junior with a Hammond P-T1760F with 4/8/16 ohm taps. The stock Negative Feedback R value is 75k for 8 ohm. Do I need to adjust this value for 16 ohm?
     
    Last edited: Sep 18, 2020
  2. Jon Snell

    Jon Snell Tele-Holic

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    As you are increasing the voltage across the speaker by 1.414 times then a 1.414 multiplier can be used, making 100k about right, from 75k.
     
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  3. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Or... you can use the original resistor on the 8ohm tap and still use the 16ohm tap for the speaker.
     
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  4. Joe Guitar

    Joe Guitar TDPRI Member

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    Just to clarify, are you saying the current R value will work for 16 ohm tap?
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    As long as the original resistor is still connected to the 8-Ohm tap, you are free to use any of the 3 taps (4/8/16) with no affect on the NFB. Go for it.
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    ^^^^
    What Andy said.
     
  7. andrewRneumann

    andrewRneumann Tele-Holic

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  8. Joe Guitar

    Joe Guitar TDPRI Member

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    I only have one 1/4” jack for the speaker out. The OT has wired for 4/8/16, but I am only using the 16 ohm for the one 1/4” jack with no other option. So the 8 ohm tap is unused.
     
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  9. andrewRneumann

    andrewRneumann Tele-Holic

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    It’s not unused. It’s going to your negative feedback. :)
     
  10. Joe Guitar

    Joe Guitar TDPRI Member

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    Maybe I’m not understating and I’m sorry for the confusion.

    The 8 ohm wire from my OT secondary is literally disconnected and taped off, so it’s not going to the negative feedback. The only wire connected to the speaker out is the single 16 ohm tap which is connected to the negative feedback. I just want to make sure that the R value is still correct at 75k or if I need to change it when only using 16 ohm.
     
  11. andrewRneumann

    andrewRneumann Tele-Holic

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    I’m sorry, I was being a little unclear. Unless you want to change the resistor, you can bring the 8ohm tap back on the playing field and run that to your feedback resistor, instead of using a wire from the speaker jack to the resistor. If you want to keep the wire from the speaker jack /16 ohm tap, then you will need a different value resistor.
     
  12. Joe Guitar

    Joe Guitar TDPRI Member

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    I think I understand. I take the 8 ohm tap and solder it to the NFB wire (which connects to the 75k R). Then use the 16 ohm tap on the output jack with no other changes or connections to that NFB. Is this right? I just want to make sure I’m doing it right.
     
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  13. Joe Guitar

    Joe Guitar TDPRI Member

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    Awesome. Thank you for your help!
     
  14. andrewRneumann

    andrewRneumann Tele-Holic

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    Correctamundo!
     
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