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VTTC - audio modulation

Original poster: "Herwig Roscher by way of Terry Fritz <twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>

Hi all,

I am running a VTTC, driven by a power tetrode. Plate voltage is 
filtered DC. As I assume that I have maxed out the system, I am 
aiming at audio modulation now.

I've tried some kinds of control grid modulation circuits already. The 
result always was loud but rather distorted.

Therefore I would like to ask the more experienced coilers:

1.  Does the tweeter become louder if the volume of the plasma ball 
became larger? If yes, how should the toroid/corona shield look 
like? Crown? Brush of copper wires?

2. Armstrong oscillators are self regulating and should resist any 
type of modulation therefore. I have reduced the control grid current 
already, but this helped a little only. Would it be better to operate 
the tetrode in A-mode and to drive it from an external oscillator? 
The control grid bias voltage could be provided by a fixed source 
and would be superimposed by the audio signal for modulation.

Any advice would be appreciated very much.