Our old 4.5" Tesla Coil. Input power is 900VA with a 15kV 60mA "Jefferson" Neon Sign Transformer. Capacitor is .0106uF MMC rated 20kVDC. Primary is 12 turns of 1/4" copper tubing, 1/4" between turns. Primary is mounted on notched lexan standoffs epoxied to 1/2" lexan baseplate. Secondary coil is 4.5" x 21" winding of #24, wound onto 22" section of white PVC pipe. Winding was first sealed with epoxy, then polyurethane varnish. Spark Gap is RQ style cylindrical, 10 copper tubes, 9 gaps. A gap of .4" was used throughout coil's (MMC's!) lifetime. Gap is quenched by 63CFM fan mounted on one end. Max sparklength ever achieved with this system: only 36". Much more could have been achieved with more tank capacitance and a better gap, however this was a very reliable system, and we were pleased with it. No problems whatsoever on continuous 5-minute runs.