Mofeta Pedal Sweet Spots?

zgrav

Well-known member
the bias options seem to be:

1. set all of the 2n5457s at about 4.5 to 4.7 volts. sounds good here.
2. some discussions about the sunn t jfet builds suggest setting the bias at 2/3 or the max voltage you can get on the trimmers. have not tried that yet.
3. others suggestions are to bias at half of the supply voltage for all trimmers.
 

droneshotfpv

Active member
Thanks for this, I was just now building / finishing my Mofeta build, and was about to ask this question as well. I assumed if would be the 4.5v'ish range, but was curious about the half supply voltage , as that is how I did my Blue Shoe Gai Pan (which I later changed anyway, and followed @PedalPCB advice of "adjust to taste :) )
I am going to set it at 4.25v and go from there. I will report anything I see / experience if different!

@PedalPCB . side question, does the 3rd trimmer adjust the BIAS of the Drain on Q3, but the Gate of Q4 simultaneously?
 

yeeeargh

New member
This might be a silly question but,... did you measure 4.25V with the charge pump installed or out of circuit?

I've finished mine today, and the trimmers don't have the range to set the drain voltage that low. I've measured the voltage between drain and ground. At the lowest setting of the trimmers the voltage at drain was 10V if I remember correctly.

I took the charge pump out and checked the range of the trimmers again. Now I was able to set the bias to 4.25V. After installing the charge pump again the bias settled at 11.9V.

edit: After reading through some other thread it looks like my JFETs are fake or otherwise inadequate. I had a couple of others and those get in the 4.25V range. Guess I'll have to wait till Mouser or other reputable sources have them in stock again.
 
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