On Sep 5, 8:39 am, Straydog wrote:
On Thu, 30 Aug 2007, 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.

The most practical approach (someone else mentioned this) would be Heising
modulation. The modulator 811 would be, like, in parallel with the RF amp
811 and both fed with DC through a fairly big choke (10 Henry or more).
There has to be a dropping resistor that eats up some of the DC voltage
going to the 811 RF amp, and you'll need many watts (5+?) of audio to
drive the grid of the 811 modulator.

Modulation transformers are still made by Peter Dahl (?) in Elpasso,
Texas, and will be several hundred bucks at least. I think I have never
seen a single ended modulator circuit unless it was for very low power.

pics of 811 project athttp://schmidling.com/radio.htm

js

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Of course, Heising is single-ended. In normal Heising, the choke must
handle the DC current of the PA and the modulator, and not saturate in
the process. By using instead a center-tapped winding you can
(nearly) balance the DC, so the choke saturation isn't so much a
problem. If you use an audio output transformer for the job, say 4000
ohms CT to 8 ohms, that's an 11:1 turns ratio from half the primary to
the secondary, and you can add the secondary to the PA side to get a
little higher (a little closer to 100%) modulation. Not that audio
output transformers of an appropriate size are a dime a dozen, but
there's at least some hope of finding one "kicking around" somewhere.

Or--maybe you can find someone taking an old plate-modulated AM
broadcast transmitter out of service and get a really good set of
modulation transformer, modulation choke, and coupling capacitor. ;-)