Paul, the fixed bias is there to protect the tube from excessive
cathode/screen/plate current if drive power is lost.
Another name used is protective bias. Your idea of using a a zener or
equivalent should work just fine. Since the operating bias will be mostly from
the grid leak source, the protective bias need not be more than that needed
to actually protect the tube. Using the minimum needed protective bias
voltage will also minimize the heat generated by the zener diode or
equivalent.
The only downside I can see is that the effective plate voltage will be
reduced by the zener voltage but that is such a small amount that it would be
hard to see an actual effect.
73,
Gerald K5GW
In a message dated 2/17/2015 12:12:02 P.M. Pacific Standard Time,
paul@paulbaldock.com writes:
I am building a 2x813 class C final modulated by 2x813.
The typical way to bias the class C final is with a combination of
fixed bias applied to the control grid and bias derived from the rf
drive through a grid resistor.
Rather than fixed bias applied to the grid it seems that I could
achieve the same effect by using a pseudo zener in series with the
cathode, similar to what is used in cathode driven linears.
Any thoughts on this approach?
- Paul KW7Y
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