Carbon black trim biasing

[WheatAndBarley]

Apologies in advance if this has already been asked before but I couldn't find the answer in the search.

What do I need to set the trimmer on the carbon black to? I'd thought it'd be like the benson preamp where you have to have the trims exactly right otherwise it doesn't work but mine is working fine and the trimmer seems to just be adjusting the maximum gain. Is there a voltage I need to set it to or what?

 

[benny_profane]

Try starting at 1/2Vc and adjust to taste. It’s setting the bias for the third transistor. This circuit is basically an Si tonebender MKII with a RC filter tacked on at the end. You could try edging the bias higher to the supply rail to be more inline with the vintage Ge units. But, whatever works for you is good.