"sustain bias" and other organ amp ?s

So, I'm starting to work with my old Conn Caprice spinet organ after wanting to junk it. It's actually got a very pretty sound—the flutes are pure sines, but with that warmth that only comes from tubes.

On my amp, of which I'll try to get some pictures of, I've got three screws. Two are located next to the two pedal RCA input cables, and another, which is labeled "sustain bias" by the diagram on the side of the amp, is located between the two other screws. Both of the input screws each have a tube next to them (12AX7s, if memory serves), but the other doesn't.

Turning the sustain bias screw seems to have no effect on the organ, whereas turning the other two controls the volume of the flutes and reeds/strings, respectively. What does that screw do?