These spectacular display devices produce arcs and sparks quite unlike the damped spark gap driven Tesla coils. Operation does not require a noisy spark gap that produces copious amounts of RFI radio frequency interference but operates efficiently at the quarter wave frequency of the secondary coil.
Vacuum tubes while large and requiring filament power do offer robust and forgiving operation and are intended as a good starter approach to solid state Tesla coils. This approach is far less temperamental than the solid state MOSFET or IGBT drivers. However once these devices are debugged and tweaked one can obtain some excellent results. Our lab FET coil generates over 6 foot sparks using only 3 kilowatts. Output frequency tracks within 5% of the resonant point using one our proprietary circuits.
The circuit of the vacuum tube device is nothing more than a high powered Hartley radio frequency oscillator tuned to the resonant frequency of the secondary coil. The circuit uses a medium powered 833A triode transmitting tube that inherently has a high grid to plate capacitance and is readily available. The plate section of the coil is wound with 1/4 inch copper tubing. The grid feed back coil is wound with #16 litz wire. The secondary is wound with #24 litz wire on a properly prepped plastic form.
The output of the oscillator is relatively closely coupled to the secondary coil designed for high Q performance and self resonant to the 1/4 wave of the oscillator frequency. The voltage distribution is now that of a quarter section with a current node at these base and voltage node at the top. Input power to the coil is raw unrectified AC at 3000 volts rms with a current of .5 amps. Peak voltage is over 4000 volts and is supplied by a conventional plate transformer being fed by a voltage adjustable variac for power output control.
We also have built coils using 304TH, 450TH and parallel 2500TH’s with a full 5000 watts of plate dissipation. This coil produced spectacular hot flaming arcs up to 8 feet with 8 kw input.
A pulse signal is also shown that controls the grid of the tube allowing a wide range of spark texture variation by changing the duty cycle and frequency.