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Re: VTTC - audio modulation

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

Dr. Duncan Cadd wrote:

>> The exact class A operating point is not suitable for modulation, 
>> as a nonlinear part of the VT characteristic needs to be used 
>> here.
> Um, why?  I would expect any rf oscillator to be capable of being
> modulated linearly, regardless of its class of bias.

Hi Duncan, All!
IMO(!) modulation means superimposing two signals onto a 
nonlinear device. Linear(!) modulation is obtained only, if both 
signals are not clipped. Therefore the operating point of the tube 
should be placed between A- and B-mode. Please correct me, if 
I'm wrong.

>>Wouldn't modulation cause the oscillation to stop?
> It all depends on the modulation depth
- So overmodulation would cause the oscillations to stop and a low 
modulation depth means low loudness?

> Rainer, DC7BJ, who runs the site has a German radio textbook 
from around 1920 or so by Paul Fischer on CD rom, which you 
might find very exciting!)
- Very interesting stuff! Thank you for your hint.

> you'll find plate modulated oscillators are used.
- Yes, plate- (and screengrid-) modulation is the only way (IMO 
again) to modulate a class-C-oscillator linearly. Unfortunately this 
is not possible in my case, because of the huge audio power I 
would need and because I do not have an access to a modulation 
xformer which tolerates the large plate current pulses and the high 
plate voltage. I'm stuck to grid modulation therefore.

> why a class C power oscillator with suitable modulation should
>not produce good quality amplitude modulated rf output.  This, of
>course, is exactly what Ulrich's original circuit does, so it is 
- Ulrich obviously is about to rework the schematic. He used a 
transistor modulation stage with bad results and notes that the 
distortion could be lowered drastically by using a class-A-driver. 
The lower cutoff frequency of his system is 4 kHz, but hopefully a 
larger plasma ball will generate lower audio frequencies as well.
The provided schematic shows a modulator stage for screen grid 
modulation. I'll need to visit the sites of other builders, he is 

> Spot-on!  The requirements for say screen modulation are less severe,
- See above.

> but I think I'll just make it for Saturday's Teslathon :-)
- Have fun!