invitro-FET VCA

[Eric Barbour]

At 12:24 PM 9/26/97 +0200, you wrote:

>>3) you can drive the screen grid with a CV and CONTROL THE TUBE'S GAIN.
>
>Does this mean one could build a VCA this way? Which gain range would be
>obtainable?

When the screen voltage is below ground (negative), the signal is COMPLETELY
cut off. With the screen approaching 1/3 to 1/2 of the plate voltage, the
gain maximizes. With typical sharp-cutoff pentodes like 6AU6 or EF86, this
voltage gain is on the order of 150 to 250.

Thus, you get control from the noise floor (usually -80 dB or more depending
on the plate power supply) to 45 dB above the noise floor. Or 125 dB. It 
varies with the tube type and the actual quiescent DC plate voltage.

Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA