John Fields wrote:
On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
wrote:



Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.


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Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

In a tube setup the RF amplifier should be operating in class C and the
power audio amplifier should provide nice linear modulation to the RF
amp's plate supply. In fact* one needs to provide sufficient excitation
to the RF final, too, lest the thing go into a current limited mode on
the modulation peaks.

Presumably you could make a REALLY EFFICIENT setup with transistors by
operating the RF final in class E, but you get that pesky capacitance
problem back...

* So I understand, I'm just a tube wannabe.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com