Biasing JFets for Chop Shop

J

jonlucasman

New member
Hi,

New pedal builder here. First non kit build. Trying to understand how to Bias the JFets in the Chop Shop. I think in a very basic sense I have an understanding of what the biasing does. I am looking for a detailed explanation of how to do it.

From my understanding the Sag knob will reduce the voltage and that it should be biased around 6v's. I am wondering:

Where do I measure the bias: black to ground and red to ?
Should the sag knob be turned all the way up CW or all the way down CCW or in the middle when biasing?
Should the Volume or Gain knobs be set at a particular level?
Anything need to be plugged in to the input or output?
And of course anything else I may be missing.


Thank you. Appreciate it!
 
Hi. Revisiting this pedal and looking for some assistance. When I bias this pedal I turn sag control all the way CCW. When I bias the jfets I can only get them to 3.782 vdc. Both of them are 3.782 with the bias trimmer turned all the way CW, so it is at the end of its range.

The pedal has some useable sounds but as it is biased really cold not great. Kinda spitty. About half way CW on the sag knob it starts to massively lose volume. Same for the drive knob.

Any thoughts? Different JFets. I got the original ones from amplified parts.
 
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Measure the voltage on pin 1 (marked in red). It should be the same as on pin 3 (marked in green). Full CCW means minimum voltage (max sag).
Bias should be set as if there was no SAG potentiometer at all (full CW).

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Well that seems like it will clear it up. I'll pop it open and give it a go. Do you have a voltage you aim for? I hear a lot of people set it around 6v.
 
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