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!