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Audio VTTC - Audio modulated vacuum tube tesla coil

     I tested a simple Audio VTTC (audio modulated Tesla coil). You might argue that I should rather call it Audio plasma flame :). The principle of operation is simple: Power tube oscillator, generating high frequency high voltage, is amplitude modulated. Changes in the intensity of the discharge is heard as sound.Thus, if oscillator is modulated by the audio signal, The discharge or plasma flame actc as a speaker.
     I used a self oscillating circuit with a vacuum tube GU-81M, then these two tubes in parallel. Modulation is introduced to the cathode using MOSFET. The best seems IRFS630 on a sink from PC power supply. The high voltage of the MOT (HV transformer from the microwave oven) in the first version is half-wave rectified, in the second version bridge-rectified. 3 capacitors from the microwave oven (in parallel) provide filtering of the high voltage. High-frequency current is blocked by a pulse capacitor 7n5/4kV​​ connected in parallel. You can control the MOT input voltage by a variac to regulate the anode voltage, but it is not necessary. Heater voltage ensures rewound MOT with 13 or 14 turns. Heater is switched on in two steps - first through limiting ballast, then directly. The vacuum tubes are operating in triode connection. As a biasing resistor is used incadescent bulbs 60W/230V. Secondary builds on top of the primary. All windings are wound with 1mm diameter wire on a 105mm diameter pipe. As the source of signal, use a battery-powered device - the circuit is electrically connected to the mains!

     WARNING !!! Tesla coil is extremely dangerous device! Without knowledge of the principles of working with high voltage, you should not construct it. All parts are connected directly to mains. MOTs produce high voltage with current of several ampers. Tesla coil is causing a broadband radio frequency interference. Electromagnetic radiation can damage electronic devices or storage media. During the operation it produces ozone (O3) and other toxic gases, must be ventilated! Everything you do at your own risk! For any injury I do not take any responsibility.

Schematic of the Audio VTTC with one GU-81M

Schematic of the Audio VTTC with two GU-81M

With two tubes.

With one tube.

With one tube again.

With one tube, modulated and unmodulated

During construction

Almost complete

Source of signal and modulation mosfet IRFS630 on heatsink.

Audio VTTC operating

Audio VTTC with two GU-81M power tubes.

High voltage bridge with 4x 12 diodes BY255.

Detail of the diodes