In my spare time I like to build little guitar/bass amps.

I'm looking for schematics for my next micro bass head build. I wanted to keep it super simple. I found one that I think I like, which is based off of the Alembic F2b Tube Bass preamp (however with a J201 jfet in lieu of a tube).


Looking over the preamp, it all looks pretty do-able for me, but what in the sam-hell do these two 100k trim pots do? They aren't labled like the rest of the pots. Are they knobs for selecting your frequency? If so, for which band? Mid? Ultra-Lo? Kinda stumped here. But I'm a n00b at schematics and whatnot.

Any advice would be greatly appreciated

 

The pots are trimmers (Google 'trimmer pot image') for pre-setting the DC bias points of the drains of the two FETs to be about half the supply voltage, i.e you adjust them until the drain voltage is about 4.5V then leave them at that setting. This ensures that the drain AC voltage has maximum undistorted swing.

 

JFETs have a rather wide variation in their specifications so this DC level adjustment is required.

 

Thanks so much for your replies! So correct me if I'm wrong, this would sort of be like adjusting the bias on a tube amp? Any recommended viewing on youtube to give me a crash course on trimmers and how they interact with your circuit?

 

this would sort of be like adjusting the bias on a tube amp?

Yes. It's just a screwdriver adjustment. You hardly need a course for that .