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


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Push-Pull allows you to runn 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

"COLIN LAMB" wrote in message
...
Push-Pull allows you to runn 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

On Aug 30, 7:38 pm, "Harold E. Johnson" wrote:
"COLIN LAMB" wrote in message

... Push-Pull allows you to runn 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

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

Hi Tom. Must have been around 1954, cause I'm sure it was after I came back
from Sunny Athens Greece. (SV0WX) I have the complete QST on disk, if you'd
like to peruse the article, I can go find it for you.

(Yes, and there was another one. Mobile Tube rig, winding up in a 10 Watt
2E26 I think, with something like a 25 Watt transistor audio amplifier for a
modulator. Part of THAT was rectified and used for the B+ for the rig. I
started on building that and then found a vibrator supply.)

Regards
W4ZCB

On Aug 31, 9:40 am, "Harold E. Johnson" wrote:
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

Hi Tom. Must have been around 1954, cause I'm sure it was after I came back
from Sunny Athens Greece. (SV0WX) I have the complete QST on disk, if you'd
like to peruse the article, I can go find it for you.

(Yes, and there was another one. Mobile Tube rig, winding up in a 10 Watt
2E26 I think, with something like a 25 Watt transistor audio amplifier for a
modulator. Part of THAT was rectified and used for the B+ for the rig. I
started on building that and then found a vibrator supply.)

Regards
W4ZCB

Oh, the good old days when you could mention vibrators in polite
company and not get odd looks, and when the whir of a dynamotor likely
meant someone had keyed up to transmit.

I suppose Mitch got some of his ideas from articles in that era, but
for sure the design was his own, with quite a few innovations as far
as anyone around there knew.

No need to look any of the old QST stuff up for me. I'm too busy
having fun with all this new-fangled stuff, trying to get IIP3s above--
well, above some pretty large numbers.

Cheers,
Tom

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 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. ;-)