Ah, ok.
I built it with an astable multivibrator (with discrete transistors, haha) and at 50% duty cycle at around 100 Hz, it kinda sorta sparked. I'm guessing the 10uF (440V) dual A/C cap was too big.
I wanted to just take the cap off, but I know I risk barbecuing my irf530.
I tried just manually touching the 12V wires to the cap, and got nice sparks (but also got impressive sparks just on the 12V wires too).
I need 1 uF, huh.
Michael