Mofeta Bias Question

[dahnke26]

Hey y’all, kinda new to the whole pedal building game - I’m building a Mofeta and I’m not sure where on the PCB should i place the hot lead of my multimeter for setting transistor bias. Thanks in advance for the help y’all!

 

[temol]

You can measure the voltage either on a smd or through-hole pads. Schematic is supposedly similar to the Moder T (Black Hole?). If that's the case then Q4 drain should read more or less Vcc voltage (after charge pump).

 

[dahnke26]

You can measure the voltage either on a smd or through-hole pads. Schematic is supposedly similar to the Moder T (Black Hole?). If that's the case then Q4 drain should read more or less Vcc voltage (after charge pump).

View attachment 16773

thanks so much! as a followup, I’ve used through-hole J201s from mouser and i’m getting readings between 16-18V at these locations and can’t bring these values down to the reccomended 4.25-4.5V. do you happen to have any ideas as to what has gone wrong there?

 

[temol]

There's a charge pump in the circuit, boosting supply voltage (9V I assume) to .. higher value. I do not know exactly what's the voltage coming out from a supply circuit here, but probably between 20-24V (build doc calls for electrolytic caps rated 25V min). So "recommended" 4.5V (which is 1/2 of the Vcc) is true when you have 9V going into the circuit. With 24V going into the circuit it's 12V. There's no biasing information provided with the build document so you can start with 1/2 of the voltage measured before the trimmer or coming out from the supply circuit (after the caps and diodes). Because there's no schematic available you have to figure it out on a pcb. You can try to measure Vcc voltage at the marked points (green).




edit

I've just noticed you have J201 instead 2N5457 .. well.... your circiut may sound different that with 5457's.

 

[temol]

One more thing. With J201 instead of 2N5457 there may be a problem with setting right bias voltage (because of the parameters difference between the two models). If you have 24V coming into the circuit then 20k trimmer is probably too low in value to get 12V on a drain of the thansistors. That's why you can set only 16 to 18V. Replace 20k with 50k or 100k. Or get the 5457's

 

[dahnke26]

One more thing. With J201 instead of 2N5457 there may be a problem with setting right bias voltage (because of the parameters difference between the two models). If you have 24V coming into the circuit then 20k trimmer is probably too low in value to get 12V on a drain of the thansistors. That's why you can set only 16 to 18V. Replace 20k with 50k or 100k. Or get the 5457's

This makes a lot of sense - i’ll order some and try it out. Thanks so much for the help!