>
>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".
? This is the tip of the iceberg. What about the seriously non-linear
relationship between screen potential and peak anode current?.
>
>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.
If this is the case, there can be no grid/screen amplification. At RF
frequencies such a circuit is quite like to result in oscillation.
>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.
RCA does not recommend a circuit that is anything like the DAF design.
>
cheers, Colin
- Rich..., 805.386.3734, www.vcnet.com/measures.
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