Jack Schmidling wrote:
Is it convenience or the laws of physics that modulators are always two
tubes in push pull?

I have an extra 811 and socket and was wondering if there is a way to
use this to plate modulate my 811 cw transmitter.

pics of 811 project at http://schmidling.com/radio.htm

With one audio amplifier tube you'll have to run in class A. A class A
amplifier always has the same current flowing into it. With no audio
applied all that power gets burned up in the tube; at maximum output
(with a sine wave) 1/2 of the power goes to the output.

So the best audio power you'd get would be 1/2 the dissipation of the
tube. IIRC an 811 is only good for 65 watts, so you're talking 32 watts
of audio, which isn't much.

With two tubes in push-pull you can run class AB. A class AB amplifier
has significantly less standing current, and consumes more as the power
output goes up. Not only could you get significantly more sinusoidal
power out of the amp without burning up the tubes, if you want to push
things a little bit you can take advantage of the fact that the tubes
will only dissipate heat when you're making noise. Taking that into
account a pair of 811's will (IIRC -- check up on this) be good for
something between 120 to 200 watts.

Or you can just run a PA amplifier from the Rat Shack into a step-up
transformer...

--

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

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