tube synths /VCA

Rene Schmitz uzs159 at uni-bonn.de
Thu Jul 2 22:33:42 CEST 1998


At 12:04 01.07.1998 -0700, you wrote:

>>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.

I just thought, since in most poweramp schemes I've seen so far the 
screengrid was directly connected to the supply and the plate via
the output transformer, thus the plate voltage is lower than the 
screengrid.

>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.

As I see it the voltage of the screen grid determines the current
thru the tube, right?
But won't then the plate voltage vary with the CV? 

If that is the case would it be better to use either a choke or a 
transfomer instead of the plate-resistor.

>Try it, Rene..........
Yes I'll give it a try.

-René
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