tube screens

[Eric Barbour]

>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 difference is only a few volts. And it was cheaper to
connect the output tubes that way--no need to derive a separate
screen supply. And guitar amps are VERY hard on the screen
grids of their output tubes. This is why they have 470 ohm
or 1000 ohm resistors in series with each screen---to limit the
current! Otherwise the screens will get white-hot and melt.

Please. Stop looking at old 6L6 guitar amps, they have very
little to do with small EF86 pentodes in a VCA. 

>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.

It's always better to use a inductive plate load on a tube.
The issue is always the high cost of a suitable choke or
transformer. High inductance is needed (more than 100
henries for a small preamp tube). You see it done in
exotic high-end audio equipment, rarely in other equipment.
Yes, the plate voltage varies with the screen voltage-
it is slight if the screen voltage is only going 0-5v
of change, as with a typical synthesizer CV.

Another possibility is to use two tubes in balanced
push-pull. Add their outputs with an opamp, thus
canceling the DC common-mode change.

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