# JFET's Trim-pot value



## alienas (Jul 24, 2021)

Hi,
for J201, the trim pots are usually 100k, but there are out of stock in 1/4" here, so do you know if a 50k would be enough? I have J201 from different sources, not tested yet...
Thanks


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## temol (Jul 24, 2021)

What circuit? What's the source resistor value?


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## alienas (Jul 24, 2021)

Boogie monster for instance, between 1k and 5k....


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## temol (Jul 25, 2021)

With 9V supply 50k should be ok. Only with Q3 you get closer to 50k. Q1,2,4  - probably below 30k. 
All of this assuming you have transistors with Vp around -0.7 to -0.8V.


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## alienas (Jul 25, 2021)

Great thanks Temol^^


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## alienas (Jul 25, 2021)

If the B+ is higher, for the same source resistor, how goes drain res?


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## temol (Jul 25, 2021)

If  you want to keep  drain voltage @ 1/2 supply you have to increase value of the drain resistor. 
Here's an example (Q1). From left to right:
9V supply, V drain = 1/2 supply,
24V supply - same R drain as above,
24v supply, V drain = 1/2 supply





But there's more. Sinewave @ Q1. Green - 9V supply, blue - 24V, no R adjustment, red - 24V, R adjusted.





Seems ok, right? But then there's first stage frequency response - not so right.
Colors as above




It would be necessary to recalculate each stage in order to keep frequency response similar.


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## alienas (Jul 25, 2021)

Great thanks man!


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