One significant factor regarding the G2DAF is the time delay on the screen
voltage. If the dc created by the rectified rf charges a capacitor in the
screen circuit and that charging causes a delay in the rise of screen
voltage, then the screen will not be following the grid and distortion will
occur. So, if someone builds the G2DAF with a big screen bypass capacitor,
it will distort. This is a problem similar to alc circuit time constants
which delay the gain reduction voltage from increasing until after "the bull
is out of the corral".
The RCA Transmitting Tube Handbook shows a recommended Class B circuit which
connects the screen grid to the driver transformer and the control grid
through a resistor to the screen - thus operating the tube as a triode. In
this case, the screen voltage is not regulated - yet the stage is linear.
So to categorically dismiss the G3DAF as a bad circuit because the screen
voltage is not regulated is in conflict with RCA's recommended circuits -
and they pretty much wrote the book on vacuum tubes.
Colin K7FM
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