In the "good old" tube days, typically the transmitter's final amplifier
was the keyed stage, or at least one of them. So the key shaping was
not degraded by a subsequent amplifier. It's OK if the PA is a class C
amplifier as long as the key shaping circuit takes that into account.
One way to get away with using a separate class C amplifier it to use a
two-stage grid bias circuit. A fixed voltage biases the tube so that it
draws a little resting current (like a linear class B amplifier) and the
rest of the bias is developed from a grid-leak resistor, which only
kicks in when RF is applied to the input. So as the RF input ramps up
at the beginning of a dit, there is no abrupt transition from off to on.
It still increases the slope of the rise and fall, so it does increase
key clicks, but not as bad as with pure fixed bias. You can compensate
by slowing down the rise/fall times of the key shaping circuit.
Alan N1AL
On 07/27/2015 04:57 PM, Tom Azlin W7SUA wrote:
Interesting discussion, but I'm now confused.
In all the old handbooks the discussion of amplifier design says ( page
76, 1941) "In amateur transmitters, and r.f. amplifier is invariably
operated Class C ( see Chaptr 3)."
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