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.

---
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?

No, that's plate modulation. Class C is when the active element
conducts for less than 180 degrees of the cycle. A lot of times
when they plate modulate, they'll also apply the modulation to the
screen grid or even the control grid or previous stage. :-)

Cheers!
Rich

On Sat, 28 Aug 2004 09:23:40 GMT, Rich Grise wrote:

No, that's plate modulation. Class C is when the active element
conducts for less than 180 degrees of the cycle. A lot of times
when they plate modulate, they'll also apply the modulation to the
screen grid or even the control grid or previous stage. :-)

Can't blame 'em! Given that the audio power level needs to be
commensurate with the transmitter's RF power output level, using final
plate modulation with a 500KW TX might be a more than a little
inefficient. :-)


--

"What is now proved was once only imagin'd." - William Blake, 1793.