Bias Feed resistor Question

Discussion in 'The Workbench' started by matttornado, Nov 20, 2020.

  1. matttornado

    matttornado Well-Known Member

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    I was reading the Trainwreck Pages and it mentioned that by changing the bias feed resistors from 220K to 100K will increase tube reliability.

    Anyone here do that?
    Isn't that the same as turning a dual 250K PPIMV pot down about half way?

    I also read this from "Marshall Amp Circuits 101" (not sure where this originally came from)


    Bias Splitter Resistors


    The bias splitter resistors, although we call them that for obvious reasons, they're really not just to adjust magnitude of the fixed bias voltage as I've sometimes heard; they also serve as grid resistors for the power section, required for the particular operating characteristics and appropriate functioning of the output tubes. The bias splitter resistors, along with the rest of the bias supply, controls the DC bias voltage of the (input) grid with respect to the plate voltage. Lower values of splitter resistors give more grind to the sound (browner).


    Also the 70's Marshalls saw different bias splitter values. 82K, 100K, 150K and the common 220K. The smaller values will load down the preamp and roll off the top end. The 82K/100K was usually used when 6550's came stock in the amp. A lot of people had the 6550's switched over to EL34's and most techs didn't change the bias splitters which resulted in a little more preamp crunch and a "browner" sound to the top end. The 220K's are the classic value for use with EL34's."


    I was thinking about trying this.
     
  2. South Park

    South Park Well-Known Member

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    The PPIMV rolls off signal of the PI . The bias feed resisters work more like grid stoppers in the input to stop oscillation . The bias pot bias range resister control grid current .
     
  3. RickyLee

    RickyLee Well-Known Member VIP Member

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    The lower values there should load down the signal a bit. But lower value grid loads usually make the tone brighter. Hmmm.

    My old JCM 800 Marshall Studio 15 has an odd tone stack, that uses an adjustable grid load as a bass control. When the pot is dialed in for more bass, the grid load value resistance is increased. It also increases the audio signal a bit there as well doing this.

    I would like to hear your observations when you do the swap.
     
    matttornado likes this.

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