Samuel Hunt wrote:

Hi all.

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

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.


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?


Sam


Sam:

If you want to run an RF amp in a linear manner then one cannot bias the amp
in class C. Class C is not a linear mode of operation and will give all
kinds of trash if running in SSB. Now if you are running either CW or FM,
then you can run a class C amplifier.

The Most linear class of amplifier operation in the classic modes is class A
mode. This mode means the active device is always conducting even when the
input is not being fed any signal. In an ideal world efficiency of class A
is only going to be 25 percent.

Most amateur class amplifiers run in a mode that is either ABone or ABtwo.
I can't recall the differences between the two at the moment but they are
clean under normal operation and don't put out a lot of noise.

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