> How is it that AM transmitters use class C?

Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
modulation usual in broadcast afaik. You simply modulate the plate DC with
audio and you get AM 30% (they could have chosen up to 80% AM without
problems otherwise). If you search the web for homebrew small AM
transmitters (favored by old radio collectors etc) you will find schemes
that exemplify this. The simplest scheme (which I've tried myself) can be
made from parts from a tiny AM radio.

You use the L.O. (complete with variable), substituting the ferrite
antenna for the AM LO tuning circuit (red core coil). You may have to
rewind the ferrite with a tapped coil - standard tap is 0% power 30%
collector 100% tuning cap - i.e. 25+55 turns on a normal ferrite antenna
that works with a 270pF tuning cap, using the original wire). Then you
wire the audio amp output secondary (which must be transformer type) in
series with the collector power of the LO, instead of the IF1 coil
(yellow) if it's a self-oscillating converter. Audio in is at the hot side
of the volume pot. That's it.

Please note that this may be slightly illegal in some countries. Do not
attempt to run high power into this, it will not work. Mine barely worked
across a room on the original 3V batteries. Man this was *years* ago.

Peter

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics