VCO experiments.

[Rene Schmitz]

Hi all !

I have done some further experiments with my tube VCO.

I hooked it up to my 75 volts supply, which I use to power the tube VCA.
It turned out that this is insufficient to fire the neonlamp. 

So I've replaced the neonlamp by a DIAC and raised the integration cap to
4.7n. 
(I also did some other minor mods.)

The result is a VCO which produces a good sawtooth. It tracks over 6-8
octaves (unlike the neon version, which is limited to 1-2 octs).
It could probably do better than that if I would include a HFT. The tuning
curve seems to get flatter on higher octaves. But I think that is
sufficiently good.

I made the same observation like with the neonlamp, the DIACs thresholds
are dependant on ambient light (and probably temperature too). I put it
into a black heat shrinking tube.
 
Soundwise there is almost no reason to do this, since it sounds pretty much
like my other sawtooth VCOs. But this holds only for a single VCO: The
DIACs thresholds "float" against the supply voltages. So there shouldn't be
softsync between several of these. Consider the HV-part runs out of an
unregulated supply! And the hum is not FMing the VCO. I think this is an
advantage of such a simple two terminal oscillator.

I think I will try this idea in a solid state circuit as well, replace the
cascode tube by a (mos)FET, should make a very compact oscillator core for
VCO banks. Of course I will need a higher supply then, the DIAC does need
some 35-40 Volts to trigger, and gives an oscillation of about 20V.

Bye,
 René





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