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green board blues jr screen grid resistors

Discussion in 'Amp Central Station' started by RICKYV, Aug 18, 2015.

  1. RICKYV

    RICKYV TDPRI Member

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    I'm installing a Billm input jack. Since I have the board out, I've read that the 100 ohm 1/2 watt screen grids should be bumped to 3 watt, is this the case and should I change the value on R32? Thanks?
     
  2. mcabe

    mcabe Tele-Meister

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    Is this a recent Blues Jr. III or a green board from the 90s? Fender recently started using green boards again, so the green board/cream board distinction is not as meaningful as it once was.

    R32 on the old green boards is 2 watt voltage dropping power resistor. I had it burn up on me when there was a short in a power tube. Burned the board a little too. Increasing the wattage on R32 is not necessary, but elevating the resistor to keep it away from the board is helpful. New Blues Jr's use standoffs to keep the power resistors away from the pcb.
     
  3. RICKYV

    RICKYV TDPRI Member

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    It's an old green board Rev. C
    R32 should be 2.2k, but I see someone changed it because there are burn marks underneath, hard to tell the value though. It's not 2.2k.
     

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  4. mcabe

    mcabe Tele-Meister

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    That's what happened to mine too. Seems to be a common problem when a power tube fails. Rather than change the value, you can elevate the resistor to keep the board from burning.

    I have a hard time seeing the color bands in that photo, but red-red-red would be 2.2K.

    http://www.michaels-electronics-lessons.com/resistor-color-code.html

    If you have a multi-meter you can try measuring it in the circuit. I am not aware of mods that involve changing the value of that resistor.
     
  5. RICKYV

    RICKYV TDPRI Member

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    How weird, it measure 2.23 yet the color code is not red red red?
     
  6. mcabe

    mcabe Tele-Meister

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    That is strange. Looks more like red-red-green which would be 2.2M! Well if it measures right, don't sweat it.

    In terms of the screen resistors, I extensively modded my amp, but didn't touch those resistors. Increasing the wattage would help with heat dissipation and maybe improve reliability. I think there would be no audible effect on tone.
     
  7. RICKYV

    RICKYV TDPRI Member

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    Thanks!
    Just curious, are you using .1 on C11 & .015 on C13?
     
  8. mcabe

    mcabe Tele-Meister

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    I'd have to go and check. I installed the fromel kit because billm's store was down. I think the fromel kit uses those values, and they are recommended on billm's old site: here .
     
  9. RICKYV

    RICKYV TDPRI Member

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    Well, I installed the input jack and reverb mod, everything was nice and neat and now the amp has low volume and distorted! Went back and reversed mods and still the same! Should have left it alone!
     
  10. mcabe

    mcabe Tele-Meister

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    Ug. That sucks. Try building a signal tracer and follow the signal. Find where the distortion is entering. Billm has a video to show how.
     
  11. mcabe

    mcabe Tele-Meister

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    The ribbon cables are a frequent problem. Taking the board out can break a ribbon cable connection. Take a look at those. Use the multimeter to test for continuity everywhere you were working. You'll find the problem.
     
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