[AMRadio] Biasing a Class C modulated final

Larry Szendrei ne1s at securespeed.us
Wed Feb 18 06:53:38 EST 2015

On 2/17/15 1:56 PM, Paul Baldock wrote:
> 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?
I see no reason why this wouldn't work, although the effective plate 
voltage, which is the plate-cathode voltage, will be reduced (plate 
supply voltage - cathode bias voltage). Strap each 813 filament 
connection to chassis right at the tube socket with a 0.01mf disc 
ceramic or better. Also, of course, the Zener needs to be able to handle 
the peak current under modulation.

What's a pseudo-Zener, and why not bias the grids conventionally?


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