I have a '77 F-2b and I noticed the blue dial inside when changing the tubes today.  It looks like it might be a gain adjustment, but does anyone know for sure?  I checked the FAQ page and did not see a reference to this dial.  I didn't fool with it, since the preamp sounds good with the dial where it is currently set.  Thanks!

My wild guess is that it's related to the lower voltage side of the power supply and not the audio chain (or there would be two).  Mica can confirm.

Jimmy J

Thanks, Jimmy.

Trimmer pot going to the tube heaters, I wonder?

Hum balance control. You only might need to change when you change the tubes, since each set of tubes is a little different.

Awesome, thanks, Mica!