Spark gap tesla coil are definitely one of the scariest you can make. Not only are they super illegal, but they’re also very dangerous and violent. I made 3, and they’re soooooo cool. They also absolutely rust the heck out of the spark gaps on account of the excessive amounts of ozone generation. I have them powered from various size flyback generators. Since its a pulsed power source, the resonance calculations are not as important meaning its not too difficult to scrap one together without a ton of analysis.