tube synths /VCA

[cyborg0 at GlobalEyes.net]

WOW!!! thanx for the insights!! gotta go look into the giant cabinet o
old tubes at work and get something going..
rob

Eric at Svetlana Electron Devices wrote:
> 
> >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..........