In one test I ran a PT powered 600 watt ballasted 120 bps coil. Using
my homemade ballast the coil drew 600 watts, but using a 20 amp
variac as a ballast, the coil drew 1200 watts. That's when Dr. Resonance
suggested cutting a slot in the variac to prevent saturation of the core.
I think with STR operation the slotting may not quite as important? It
may be very important at 120 bps. (I'm not sure if every 120 bps coil will
behave the same in this sense.) Slotting seemed to be very much needed
because only a few turns of the variac were needed at 120 bps.
Cheers,
John