John,
The short answer is: It depends.
Once upon a time it was standard procedure to run the final amplifier in
an AM transmitter in class C. This works fine when the final amplifier
is the stage that is modulated. A class B push pull modulator modulates
the class C final RF amplifier and it produces a good AM signal. This is
the way that most AM transmitters, both for broadcast and communications
used to work.
If an earlier low power stage is modulated, then all of the subsequent
RF amplifier stages need to be linear. They could be class A, class AB,
maybe even class B (probably not), but absolutely NOT class C. If the
rig runs SSB as well as AM, then it almost certainly generates the SSB
at a low level, and does not use a modulated final RF amplifier stage
even when it is in AM mode. In that case the final RF amp must be
linear, and is not class C.
DE N6KB
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