Vacuum tube VCO with expo control.

[jhaible]

>As the thresholds will only change the linear mA/Hz scale factor of the CCO
>(which ideally should go straight thru the origin) any ageing of the neon
>tube should only result in a pitch shift. Current doubling still means
>frequency doubling.

Yes, that's the big advantage of V/Hz VCOs. Even when they drift, you just
have to slightly re-tune them, like a guitar string. (If you have to adjust
the
scale factor, that's more like redesigning the fretboard, to stay with the
guitar analogy.)

JH.