hi! yes i did. also, just recently i did a test and it turned out that the 12cm primary and 16cm primary doesn't make any difference. but as i know, the sstc must have as tight coupling as possible. (first problems)

just add a zener, it hasn't too many capacitance to mess something up. my design uses two 12v zeners back-to-back. not a supressor, it has a big capacity. also, with 600v d-s you will be able to supply it 70v max, my irfp260 drain sees 350v on it at just 35v, i still don't know why it doesn't burn. anyway, it should be good enough. zeners must be as close to gate and source as possible. also, about design improvements, make the gate voltage divider work from separate 12v source, that way you can interrupt it, ramp it and make a lot of nice stuff. the tuning will be much easier with some scope. good luck:D