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

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

On Aug 31, 2:17 pm, K7ITM wrote:
On Aug 30, 7:38 pm, "Harold E. Johnson" wrote:





"COLIN LAMB" wrote in message

... Push-Pull allows you to run in other than class A, which is about 35%
efficient. However, class A can result in very good audio.

Some high end audio amplifiers are single ended class A. High power puts
out a lot of heat.

Colin K7FM

Back in the year one, Bill Orr, W6SAI, published an article (When QST was
still a technical magazine) on a single 304TL Heising modulator. I built one
to modulate a 4-400A.

Ran class A when you were talking to it, ran class C when you weren't.
Shifted the bias at a syllabic rate by rectifying a bit of the preamp audio
and applying it as forward bias when talking. Worked very well, quite
efficient, and the origin of EBC. The Electronic Bias Control bias shift
seen in 1970 and 80 Alpha RF Amplifiers to hold down plate dissipation for
class AB and B operation.

W4ZCB

Which Year One was that, Harold? Where I became a ham many years
back, there was a creative fellow in our club who designed a compact
AM transmitter using a couple 6146s, one for the RF PA and one for the
audio output. They were coupled not by a choke as in Heising, but by
a common audio output transformer, available back then much more
readily than a modulation transformer, and having the advantage that
the DC in the center-tapped winding was (nearly) balanced, avoiding
saturation effects in the transformer and allowing the use of a
relatively small transformer. I think Mitch designed that thing in
the mid 50's.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

I understand that the answer to the basic question is 'no'; not
necessarily push pull for an audio modulator. Just a big hefty audio
amp and some means of plate modulating the RF PA! There are other
methods of AM modulation etc.

BTW the 6146 design above sounds ingenious; have a mental picture of
the total DC plate currents divided in opposite directions one for the
RF tube and the other for the modulator tube. Then I suppose the
modulator tube as it received audio, varied the plate voltage/current
of the RF tube thus modulating it. A sort of variation of Heising
(choke) modulation? Interesting.

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.

terry wrote:
On Aug 31, 2:17 pm, K7ITM wrote:
On Aug 30, 7:38 pm, "Harold E. Johnson" wrote:





"COLIN LAMB" wrote in message
... Push-Pull allows you to run in other than class A, which is about 35%
efficient. However, class A can result in very good audio.
Some high end audio amplifiers are single ended class A. High power puts
out a lot of heat.
Colin K7FM
Back in the year one, Bill Orr, W6SAI, published an article (When QST was
still a technical magazine) on a single 304TL Heising modulator. I built one
to modulate a 4-400A.
Ran class A when you were talking to it, ran class C when you weren't.
Shifted the bias at a syllabic rate by rectifying a bit of the preamp audio
and applying it as forward bias when talking. Worked very well, quite
efficient, and the origin of EBC. The Electronic Bias Control bias shift
seen in 1970 and 80 Alpha RF Amplifiers to hold down plate dissipation for
class AB and B operation.
W4ZCB
Which Year One was that, Harold? Where I became a ham many years
back, there was a creative fellow in our club who designed a compact
AM transmitter using a couple 6146s, one for the RF PA and one for the
audio output. They were coupled not by a choke as in Heising, but by
a common audio output transformer, available back then much more
readily than a modulation transformer, and having the advantage that
the DC in the center-tapped winding was (nearly) balanced, avoiding
saturation effects in the transformer and allowing the use of a
relatively small transformer. I think Mitch designed that thing in
the mid 50's.

Cheers,
Tom- Hide quoted text -

- Show quoted text -

I understand that the answer to the basic question is 'no'; not
necessarily push pull for an audio modulator. Just a big hefty audio
amp and some means of plate modulating the RF PA! There are other
methods of AM modulation etc.

BTW the 6146 design above sounds ingenious; have a mental picture of
the total DC plate currents divided in opposite directions one for the
RF tube and the other for the modulator tube. Then I suppose the
modulator tube as it received audio, varied the plate voltage/current
of the RF tube thus modulating it. A sort of variation of Heising
(choke) modulation? Interesting.

While modulation transformers are getting a bit rare and expensive these
days there are ways to get around the problem. Years ago the ARRL
handbook used a filament transformer as a modulation transformer. The
center tapped secondary was used as the primary with high power
germaninum transistors (this was a mobile rig). The 115v primary was
the modulation secondary.

One could also use two tube type output transformers back to back.

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