On Sun, 12 Mar 2006 11:30:43 +0000 (UTC), "Samuel Hunt"
wrote:

Hi all.

I'm trying to find a good circuit for a Class C 4CX250B linear.

Good luck. "Linear" and "Class C" are mutually exclusive.


I've found a few for multiple valves in Class A, but I'm after just a simple
circuit that shows a 4CX250B linear for 2m with Class C biasing.

Idea is to use it for a little project where I want to run about 100W, but
run it cleanly without all the RF hash that transistor amps give.

Then you want to run it class AB1, which is much easier on the fragile
grids, BTW.


Can anyone point me in the right direction?

Also, how would you go about varying the RF power output on a 4CX250B linear
in class C? Is it by varying the 1.2kV supply voltage, which would make
sense, or is there some other recommended way?

If you would just forget the Class C part, you would simply vary the
input (drive) power and get a linear change in output power. Varying
the drive power to a Class C amplifier will change the output power
too, but in a "non-linear" way. Changing the anode voltage on a
tetrode to vary (DC) input power is fraught with trouble and doesn't
make sense.

If you insist on Class C bias, (grid cut off with no drive) and you
can't vary the rf drive level for some reason, with excess drive
power, you can bias the grid further into cutoff. This will increase
the harmonic content, but the plate capacitance of these tubes
(valves) is high enough that the Q of the plate tank will be
necessarily high so this may not be an issue.

You might consider changing the screen voltage. Be advised, however,
the screen grids in the tubes are very delicate and you will have to
insure that dissipation limits are not exceeded.