All of my tube circuits have that, it's to adjust the high voltage. The supply I use can go from 35V to 450V, so it's nice to not have to worry about redesigning it every time I want a new voltage. I can run a starved tube at 35V, 5672 at 90V, 6N21B at 200V, and 12AX7 at 240V all with the same circuit, just adjust the trimpot.What's the trim-pot for?