Blue dial inside F-2B - gain adjustment?

[rv_bass]

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!

[JimmyJ]

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

[rv_bass]

Thanks, Jimmy.

[gtrguy]

Trimmer pot going to the tube heaters, I wonder?

[mica]

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

[rv_bass]

Awesome, thanks, Mica!