Re: Audio Levels for AM (Audio Modulation) for VTTCs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 12:24 01/04/03 -0700, you wrote:
>Original poster: "Mccauley, Daniel H by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
>
>
>I'm working on my audio driver for my audio modulated dual 833A coil.
>
>My questions are in regard to what amplitude should my audio input (which is
>coupled into grid circuit via step-up transformer) in respect to the grid
>signal for best results.  I've been playing a bit some transformer design
>using PSPICE.

Grid modulation won't work well in a Class-C RF amplifier, which is what a 
well-designed VTTC is. You're driving the tube like a switch, with such a 
high grid drive that it goes from fully cut-off to fully saturated. The 
effect of the modulation will be quite small, non-linear, and it will upset 
the grid bias if you're using the traditional self-bias circuit. From a 
plasma speaker perspective, it'll sound like poo.

To get high-quality linear grid modulation you would need to redesign for 
Class-B and take the resulting hit in efficiency/output power. With no 
audio input the coil should run at 25% of the power you used to get in 
Class-C so you have room for the positive peaks of the modulating waveform. 
The DC bias issue still holds, unless you replace the RC circuit with a 
zener diode or something.

A slightly better technique is series modulation where you leave the 
oscillator in class-C but bring the supply voltage in through a series-pass 
tube. By modulating the grid of that tube you make the B+ go up and down in 
time with the audio. For maximum undistorted output, you make the quiescent 
supply voltage half of what your power supply gives: 25% power again. If 
you were regulating the power supply anyway, you could modulate the 
regulator tube.

Of course the deluxe approach is high-level modulation with a separate 
audio power amp and modulating transformer in series with the plate supply. 
If you leave the plate voltage the same as it was, you'll quadruple the 
peak output power, unless something blows out, which it probably would. For 
a dual 833A coil, you're talking serious iron and audio power: 1kW at 
whatever the lowest audio frequency you want is.

Anyway, you're basically designing an AM radio transmitter, and there's 
plenty of literature on the subject. Try the ham radio handbooks. Maybe you 
and Steve Ward should get together to bring us the world's first plasma 
subwoofer?

Steve C.