I have a mid-'60s Airline GVC-9045A that uses the bias-vary tremolo circuit (separate 12AX7). I have the amp up and running, but the tremolo doesn't work. There is a single speed control (no intensity). What is the likely culprit and what's the best way to diagnose it? There is a schematic link here: http://www.tdpri.com/forum/amp-central-station/225574-airline-identification.html Thanks for your wisdom!