AW: SCR VCO?

[Haible Juergen]

	>After reading about the thyratron VCO, I thought: "Why not an
SCR
	>vco?"
	>I have quite a pile of them of various ratings, and I wondered
if
	>anyone has tried this?  Assuming a saw waveform, the question
(for me)
	>is will an SCR discharge the cap fast enough to make a VCO with
decent
	>range?

Most Korg VCOs actually use SCR circuits !

(MS-20, Polysix, Trident, to name just a few.)

Only that they don't use an SCR *component* - they
build their own SCR circuit from two bipolar transistors.
But it works exactly the same way: It starts to conduct
when a certain gk voltage is reached, and it stops when
the current thru the device reaches zero (i.e. when the
capacitor is discharged).
The first Moog VCOs used unijunction transistors - not
the same, but similar in operation.
An actual SCR component I have only seen in Roland 
System 700 docs. Not in the VCO's, but in the envelopes,
if memory serves.

JH.

PS: The Korg circuit looks very elegant, as you don't need
any voltage comparator (it's buit in), nor hysterestic switch
or monoflop (the end condition is "current == 0").
Does this circuit also have some drawbacks ?
Tolerances of trigger voltage ?
Temperature dependance ?
Any idea why it wasn't used by everybody?