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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
> 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
>>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!