Tubescreamer schematic question..

[JP19]

Hi. I am just looking at the excellent Electrosmash tubescreamer analysis and I have a question. I'm sure it's a simple electronics question but just trying to understand what's going on. Hopefully someone can enlighten me...  Schematic is attached.

I am trying to understand capacitor C8. The points before and after it are both pulled up to 4.5v, so what is it doing?

Thanks!

[POTL]

it blocks DC current and forms a high pass filter together with R12

[JP19]

Ok thanks, I guess I was wondering really why DC current would flow if both points are at 4.5v?

[GGBB]

Although it does form a high-pass filter with R12, that's not the main purpose of C8 (knee frequency is below audible). The main purpose is as a coupling cap to keep the bias on Q3 stable. Without it, other parts of the circuit can change that bias. For example the level pot would be a path to ground from Q2 gate through Q2 when the circuit is engaged.

[antonis]

To be more precise, corner frequency is set by C8 and R12//[(R13//RC) X h_FE], neglecting intrinsic emitter resistor & RB..

[PRR]

> points before and after it are both pulled up to 4.5v

Base of Q3 is not at 4.5V. It is surely somewhat less because a base sucks.