VTTC (Vacuum tube tesla coil) is a Tesla coil with vacuum tubes as a switch.
I used self-oscillating circuit with two russian (soviet) tubes GU-81M connected in parallel.
The source of high voltage
are two MOTs (Microwave oven transformers) connected in parallel and with the secondary connected to the top of primary (it increases the voltage a little).
They are fed through a variac, but the operation without variac is also possible.
MOT secondaries are connected to the doubler with microwave oven capacitors (MOCs).
High-frequency current is
blocked by parallel connected capacitor 10n/10kV. Heating voltage provides
rewound MOT with 14 turns.
Heating is switched on in
two steps - first through a ballast, and then directly (the ballast is shorted with a switch).
The tubes are operated in triode mode. As the grid resistance i use 100W/230V lamps.
The secondary has a 200turns with of wire diameter of 1mm on a 11 cm diameter PVC (novodur) pipe. Winding height is 23cm. Resonance at 1.3 MHz. The primary has 22 turns of thick insulated wire and wound on 16 centimeters diameter. The primary has many taps to connect the resonance capacitor (TC is tubed by switching taps). As a resonant capacitor I used the pulse capacitor 470pF 15kV 40kVAr. The best results i got with the tap at 9 turns. Feedback winding is 7 turns and is located below the secondary. Sparks (streamers) are 45 cm.
I tested this VTTC also with second secondary, wound in the sprawling spiral with 50 turns and resonant frequency 4.6 MHz. For this secondary seemed the best 9 turns on primary and feedback winding with 4 turns. It worked without a resonant capacitor.