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


js


--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

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


js


Reminds me of a phono oscillator I built as a teenager out of old TV and
radio parts. Had a 6F6G oscillator modulated by another 6F6 coupled by
an old TV filter choke. The oscillator coil was wound around a
cardboard tube from a roll of toilet paper. With 300 volts on the
plate, It had a range of a few city blocks.

On Thu, 30 Aug 2007 17:54:46 -0600, 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


js
Finding a plate modulation transformer may be the hardest part. They
have always been expensive.

Cathode modulation is my next choice with grid modulation as a last
resort.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

On Sep 5, 12:24 pm, John Ferrell wrote:
On Thu, 30 Aug 2007 17:54:46 -0600, 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 athttp://schmidling.com/radio.htm

js

Finding a plate modulation transformer may be the hardest part. They
have always been expensive.

Cathode modulation is my next choice with grid modulation as a last
resort.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

Screen grid modualtion it it was a tetrode PA! But the 811 is a triode
I think?

terry wrote:
On Sep 5, 12:24 pm, John Ferrell wrote:
On Thu, 30 Aug 2007 17:54:46 -0600, 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 athttp://schmidling.com/radio.htm
js
Finding a plate modulation transformer may be the hardest part. They
have always been expensive.

Cathode modulation is my next choice with grid modulation as a last
resort.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

Screen grid modualtion it it was a tetrode PA! But the 811 is a triode
I think?

You can always grid modulate a triode tube. The results are similar to
screen modulation of a tetrode tube.

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 at http://schmidling.com/radio.htm


js


--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

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

--
PHOTO OF THE WEEK:http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silverhttp://schmidling.com

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. ;-)

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. ;-)

Tom

August 1956 QST has a class B modulator that works without a modulation
transformer! Two tubes, which goes against the original posters question,
but what the hay. If you don't have to buy the transformer, you can afford
the second tube.

Still looking for my class B/A article on the 304TL Heising bias shift
modulator. Am certain that the author was Bill Orr, W6SAI, cause I called
him on the phone when my 304TH didn't work as advertised. Very nice and
helpful to an almost beginner, but that article doesn't exist in any QST to
back before I would have read it. As I now recall, I think maybe Bill wrote
it up in CQ magazine and got paid for it. Don't have those on CD's.

Regards
W4ZCB

On Sep 5, 12:33?pm, "Harold E. Johnson" wrote:

Still looking for my class B/A article on the 304TL Heising bias shift
modulator. Am certain that the author was Bill Orr, W6SAI, cause I called
him on the phone when my 304TH didn't work as advertised. Very nice and
helpful to an almost beginner, but that article doesn't exist in any QST to
back before I would have read it. As I now recall, I think maybe Bill wrote
it up in CQ magazine and got paid for it. Don't have those on CD's.

Regards
W4ZCB

Harold

The article was "The Bias-Shift Modulator", by Bill Orr, CQ, April
1954, pp 32-38, 68. At the top of the article he was described as
being a contributing editor.

HTH and 73

John KC0G

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

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html