Spark Gap: Unusual 9V Hybrid Valve Overdrive

[rankot]

Just built this one, but used true bypass switch instead of transistor switching.

[rankot]

Also added simple voltage control for filaments (potentiometer + BC337). Works fine and the results are clearly visible on an o-scope, but not too audible. Also tried to add a tone control pot in series with C12 (10n), but it wasn't much useful.

[merlinb]

Love it! Always gives me a warm fuzzy feeling inside when I see people adapting my circuits.
I never saw your previous question. Simple answer is, I'd never tried BJT switching before and wanted to see how well it worked, since you hardly ever see it mentioned in the textbooks. I'm going to revisit it at some point, purely for my own continued learnings.

[rankot]

I have also compared those tube diodes to Si and Schottky, and noticed that Schottky (1N5819) have big loss of volume, while Si (1N4148) sound a little bit brighter than tubes. Also tried to remove tubes, and sound was similarly distorted as with them!

What is the purpose of C12 (10n)? Some kind of low pass filter?