tube synths /VCA

[Eric at Svetlana Electron Devices]

>Would that mean that I could use a transformerless voltage multiplier as 
>supply for your circuits? (A high frequency square fed into a poweramp and 
>then into a diode/cap cascade)
>Would you recommend that or a switcher? 

Yes. Such devices (using step-up transformers) are made commercially
by firms such as Pico Electronics and ERG. Expensive, though. 
I believe the Paia Tubehead makes its 40v plate supply with such a scheme.

You could just use a 555 oscillator and a small, cheap radio-type audio
output
transformer to get HV--the kinds used in pocket radios are usually 100-500
ohms
to 8 ohms--drive the 8-ohm secondary from the 555 output
(perhaps using a power transistor or LM386 as a buffer), take the
voltage off the primary, and rectify and filter it. Should be good for 1 or
2
milliamps. (Have to breadboard the circuit--there are losses in 
such transformers which make the final voltage difficult to determine.)

>How much current does your VCA draw at 50-100V?

Less than 0.5 mA. At 250v it will draw about 1.5 mA.

>I thought pentodes need a high voltage on the screen grid?

Huh? The screen voltage MUST always be LESS than the plate
voltage! if not, the screen will draw all the plate current,
possibly (in power tubes) causing damage or (in small tubes
like we are talking about here) screwing up the tube's operation.

The gain of a pentode is controlled by the screen-grid voltage.
If it is zero, the gain will be zero. If you set it to about
half the plate voltage, the gain will be maximum--from 100
to as much as 500 depending on the device.

Try it, Rene..........