Which is the suitable toroid ring to wind the gate drive transformer of a SSTC?
Toroidal cores you find in old power supplies and whatnot which are used for chokes will NOT work for Tesla coil gate drive transformers. They tend to be made up of iron filings or iron powder and their permeability is VERY low. You really need to purchase known cores from a trusted distributor. The permeability must be very high, > 8000 and preferably > 10,000. For my GDTs and CTs which I use on my DRSSTC I used TDK B64290L0674X038 and they work great.