general question about JFET-based amp-sim pedals

[zilla]

Just a general question about JFETs as used in amp sim pedals like the chunk chunk.

i'll use the gausmarkov dr boogey schematic for reference here:



this question pertains to R2 (1k8) and C1 (1u) (and i guess to an extent R6 & C6 and R12 & C9)

What/how do these values affect the gain coming out of q1?    I've fooled around with adjusting C1 and i see that when i drop this value the amount of gain/saturation drops, but i'm not sure why it's doing this?  do these only have an impact on the overall gain of Q1?

[jubal81]

You're on the right track. The resistors and caps on the sources of the JFETs determine the gain. The caps down there also affect the frequencies that get boosted - bigger caps down there, the bigger the boost as it boosts more lower frequencies.


Try a 22uF cap in place of C1.

[zilla]

what about the resistor that's in parallel with the cap?

[jubal81]

what about the resistor that's in parallel with the cap?

To increase gain, you'd lower it, but you have to watch out at running the risk of it just sounding more farty than more brootz.


Another mod you could try is putting a pair of germanium clipping diodes in parallel with R10 and possibly raise R10 value to 1M.