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Re: VTTC - audio modulation

Original poster: "David Trimmell by way of Terry Fritz <twftesla-at-uswest-dot-net>" <davidt-at-pond-dot-net>

Hi, I guess I will bite.

At 11:22 AM 5/13/01, you wrote:
>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.

How distorted? I like to compare my best results to a old AM radio, not 
HiFi, but ok.

>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?

Yes, as I have increased the voltage to the plates with a variac, the 
discharge increases, and so does the volume, but only to a degree.
I never had your problem of worrying about a corona shield. I have, 
however, experimented with several discharge points in order to change the 
tone successfully. It helps to slightly increase base response (reduce 
distortion). This may also help reduce the need for a large corona shield, 
three or four point crown may be interesting.

>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.

I know you have seen the method I have used with Triodes, so I can't really 
comment, except that by increasing the component count, one will 
exponentially affect the level of complexity. Possibly a interesting idea 
would be to take Ulrich Haumann's (sp?) design to a more powerful tube and 
see where that leads.

I have found that my design works "well enough" with the 833's.

Sounds like you are having loads of fun. Good luck.

David Trimmell

>Any advice would be appreciated very much.