FMing of VCO

Eric Barbour ebarbour at
Thu Jul 9 23:25:28 CEST 1998

>I wonder if there is a way to do FM on the tube synth vco (by Eric)
>posted here a bit ago?

Yes. Just:
--build two VCOs (their outputs are weird-looking sawtooth waveforms)
--connect input of each to an op-amp driver so you can mix 2 or more CVs together.
--Then take one VCO's output, run thru a 10-1 voltage divider (to protect the
      op-amp from damage by the high voltages the VCO thyratrons make)
--and run it into the input op-amp of the other VCO. 
--Do vice versa for cross-coupling.

>Tubes are non-linear to some degree

That, sir, is a FALLACY. You have been reading the incoherent and
ignorant statements made by half-baked designers who don't know
any better. Many of them have never actually tested a variety of
tubes for distortion. They make this kind of statement based on the
cheap, low-quality 12AX7s (which is the only tube type they are familiar with)
which they tend to use in their cheap, sleazy audio processors.

In fact, some small-signal triodes are MORE LINEAR THAN ANY

	I CAN PROVE IT!!! Any takers? 

The tubes used in these VCOs are nonlinear, which makes them
useful for unique FM behavior. A low-distortion tube VCO IS possible.......

>combining this with crossed-patched FM might lead to a
>good sounding chaotic sound system generator.

In fact, I've done this with my experimental modular system. It works
amazingly well, and VERY strange things happen as you change the
tunings of the two VCOs.

>Can the existing sync in be used some how?

Yes, but you would have better results by mixing CVs with
an input op-amp, then going to the grids of the thyratron.

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