[Amps] Biasing a Class C modulated final

[Vic Rosenthal 4X6GP/K2VCO]

I'm not sure what a 'pseudo zener' is, but it would need to dissipate 
about 45 W when transmitting to produce the portion of bias equal to 
cutoff, which is usually what the fixed bias supply does. And of course 
a plain zener could not cut off the plate current completely in standby, 
because there would be no bias generated unless some current was flowing.

The traditional clamp tube circuits that others have mentioned drop the 
screen voltage by pulling a lot of current through a dropping resistor 
from the plate supply. This also wastes a lot of power, especially with 
more than one 813. For example, a pair of 813's running at 2250 V plate 
and 400 V screen would require a dropping resistor of about 23000 ohms 
which would dissipate 147 watts under normal operating conditions (80 mA 
screen current). Assuming that the circuit includes a couple of VR tubes 
or equivalent to cut off the screen voltage completely, it would be 
necessary to drop the screen voltage to about 140 V, which would require 
92 mA and 194 watts!

One 6L6 would not be able to handle it; probably two would be required.

Yes, I'm sure you could come up with sophisticated solid-state circuits 
to achieve the same results without all the heat, but I think the 
traditional grid bias supply is simpler. Operating bias is -155 V with 
the above parameters and if I remember correctly, cutoff is around -48 V 
or so, which is what will be required from the bias supply.

On 18 Feb 2015 00:14, KA4INM wrote:
>    On 02/17/2015 03:11 PM, Paul KW7Y 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 am guessing common cathode RF amplifier, a more standard way to do
> it would be an adjustable bias supply and a clamp tube on the screen
> grid.  (6L6 or similar) look into any pre-1960 ARRL hand book.

-- 
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/