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Looking for one low-power tube modulation transformer



 
 
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  #11  
Old February 12th 12, 10:57 AM posted to rec.radio.amateur.homebrew
Ian Jackson[_2_]
external usenet poster
 
Posts: 389
Default Looking for one low-power tube modulation transformer

In message
,
K7ITM writes
On Jan 22, 1:33*pm, "i3hev, mario" wrote:
Mark Zenier wrote:
One technique was to use only the primary of a center tapped (push pull)
audio output transformer. *...


I tried it (many years ago, when I was a boy...) but it doesn't work
well: most push-pull output transformers are designed for a null total
cc flux (currents in the two halves of primary winding should be equal
and opposite) and, when driven in single-ended, the core saturates and
severe degradation of audio response occurs.

--
73 es 51 de i3hev, op. mario

Non Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non uno che ascolta la radio"


it.hobby.radioamatori.moderatohttp://digilander.libero.it/hamwebhttp://
digilander.libero.it/esperantovenezia


A solution to that is to make it more like they used to do AM
broadcast transmitters: feed the RF stage its DC through an audio
choke, and capacitor-couple the audio into the RF deck end of the
choke (from one of the plate connections on the audio transformer).
If the audio amplifier and the RF deck use the same plate voltage, the
capacitor doesn't need to handle the full DC voltage, but it should be
non-polar. The problem then becomes one of finding (or winding) an
audio choke with enough inductance and that can handle the current.
In AM broadcast transmitters, the modulation choke was typically the
largest component. Since for voice you should only need to get down
to 300Hz or so, 10 henries inductance should be OK (about 20k ohms
impedance), and you might be OK with less. A 4.7uF coupling capacitor
should work OK, as it would be just over 100 ohms reactance at 300Hz.
4.7uF film capacitors aren't unreasonable to find. You wouldn't ever
get to 100% modulation, since the audio side doesn't go to zero volts
on the plate of the conducting side, but you could add the voice coil
winding in the proper phase to get a bit more modulating voltage.

I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. That allowed reasonable balance of
the DC in the transformer, and worked decently.

Have a look at the Codar AT5 schematic:
http://www.vmarsmanuals.co.uk/new/at5.htm
http://www.vmarsmanuals.co.uk/new/at5.pdf
--
Ian
  #12  
Old February 13th 12, 07:17 AM posted to rec.radio.amateur.homebrew
K7ITM
external usenet poster
 
Posts: 643
Default Looking for one low-power tube modulation transformer

On Feb 12, 2:57*am, Ian Jackson
wrote:
In message
,
K7ITM writes







On Jan 22, 1:33*pm, "i3hev, mario" wrote:
Mark Zenier wrote:
One technique was to use only the primary of a center tapped (push pull)
audio output transformer. *...


I tried it (many years ago, when I was a boy...) but it doesn't work
well: most push-pull output transformers are designed for a null total
cc flux (currents in the two halves of primary winding should be equal
and opposite) and, when driven in single-ended, the core saturates and
severe degradation of audio response occurs.


--
73 es 51 de i3hev, op. mario


Non Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non uno che ascolta la radio"


it.hobby.radioamatori.moderatohttp://digilander.libero.it/hamwebhttp://
digilander.libero.it/esperantovenezia


A solution to that is to make it more like they used to do AM
broadcast transmitters: *feed the RF stage its DC through an audio
choke, and capacitor-couple the audio into the RF deck end of the
choke (from one of the plate connections on the audio transformer).
If the audio amplifier and the RF deck use the same plate voltage, the
capacitor doesn't need to handle the full DC voltage, but it should be
non-polar. *The problem then becomes one of finding (or winding) an
audio choke with enough inductance and that can handle the current.
In AM broadcast transmitters, the modulation choke was typically the
largest component. *Since for voice you should only need to get down
to 300Hz or so, 10 henries inductance should be OK (about 20k ohms
impedance), and you might be OK with less. *A 4.7uF coupling capacitor
should work OK, as it would be just over 100 ohms reactance at 300Hz.
4.7uF film capacitors aren't unreasonable to find. *You wouldn't ever
get to 100% modulation, since the audio side doesn't go to zero volts
on the plate of the conducting side, but you could add the voice coil
winding in the proper phase to get a bit more modulating voltage.


I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. *That allowed reasonable balance of
the DC in the transformer, and worked decently.


Have a look at the Codar AT5 schematic:http://www.vmarsmanuals.co.uk/new/at...uk/new/at5.pdf
--
Ian


Thanks for sharing that, Ian. The webpage says "1960's." You
wouldn't happen to know any closer than that, would you? The one I'm
thinking of was a homebrew unit that as far as I know was developed by
Dick "Mitch" Mitchell in Washington state. I forget his call. He
developed it before I became a ham, so it must have been mid 1950's.
He used a 6146 RF PA, and 6146 modulator output. It was a minimalist
design, using a carbon mic with a mic transformer to directly drive
the modulator grid. I think he had a couple tubes in the RF chain
before the 6146, but I don't remember. It was very compact, built
entirely inside a 3" high aluminum chassis box, probably 10" x 12".
It used an external power supply: a dynamotor in mobile applications,
typically. We typically used one on Field Day, and given how much
power was dissipated inside that small box, I was a little surprised
that we pretty much never had any problem with it. I don't recall now
if he built them as single-band units, or had some band switching.

Cheers,
Tom
  #13  
Old February 13th 12, 08:50 AM posted to rec.radio.amateur.homebrew
Ian Jackson[_2_]
external usenet poster
 
Posts: 389
Default Looking for one low-power tube modulation transformer

In message
,
K7ITM writes
On Feb 12, 2:57*am, Ian Jackson
wrote:
In message
,
K7ITM writes





I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. *That allowed reasonable balance of
the DC in the transformer, and worked decently.


Have a look at the Codar AT5
schematic:http://www.vmarsmanuals.co.uk/new/at.../www.vmarsmanu
als.co.uk/new/at5.pdf
--
Ian


Thanks for sharing that, Ian. The webpage says "1960's." You
wouldn't happen to know any closer than that, would you?


No really. It was just that the mention of the centre-tapped modulation
choke immediately reminded me of the Codar AT5 and the matching
receiver, the matching receiver, the T28. I used the pair mobile back in
the 1960s, and although such equipment is hardly up to today's
more-exacting standards, I had great fun with them. You occasionally see
the odd one (or pair) on sale at the various amateur radio rallies, for
around $50.
--
Ian
  #14  
Old February 15th 12, 10:48 AM posted to rec.radio.amateur.homebrew
i3hev, mario
external usenet poster
 
Posts: 5
Default Looking for one low-power tube modulation transformer

K7ITM wrote:

A solution to that is to... feed the RF stage its DC through an audio
choke, and capacitor-couple the audio into the RF deck...


this is of course a perfectly good solution, at least from a technical
point of view, but... audio chokes are nowadays precious rare modern art
masterpieces and the only practical way to get a suitable one is to
wind it yourself, which is more or less the same as winding your own
transformer

...I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. That allowed reasonable balance of
the DC in the transformer, and worked decently. ...


been there, done that - it sure works, in a fashion... but not very
satisfactorily. At the time when I tried it, I reasoned that using a
single ended audio tube with a plate current more or less similar to
that of the RF amp tube, the total core flux in the transformer should
be more or less balanced - what I did not know then (I was *very*
young...) was that average plate currents mean very little, saturation
occurs because of peaks!

--
73 es 51 de i3hev, op. mario

Non Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non uno che ascolta la radio"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia
  #15  
Old February 24th 12, 12:02 PM posted to rec.radio.amateur.homebrew
highlandham[_3_]
external usenet poster
 
Posts: 11
Default Looking for one low-power tube modulation transformer

On 12/02/12 10:57, Ian Jackson wrote:
In message
,
K7ITM writes
On Jan 22, 1:33 pm, "i3hev, mario" wrote:
Mark Zenier wrote:
One technique was to use only the primary of a center tapped (push
pull)
audio output transformer. ...

I tried it (many years ago, when I was a boy...) but it doesn't work
well: most push-pull output transformers are designed for a null total
cc flux (currents in the two halves of primary winding should be equal
and opposite) and, when driven in single-ended, the core saturates and
severe degradation of audio response occurs.

--
73 es 51 de i3hev, op. mario

Non Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non uno che ascolta la radio"


it.hobby.radioamatori.moderatohttp://digilander.libero.it/hamwebhttp://
digilander.libero.it/esperantovenezia


A solution to that is to make it more like they used to do AM
broadcast transmitters: feed the RF stage its DC through an audio
choke, and capacitor-couple the audio into the RF deck end of the
choke (from one of the plate connections on the audio transformer).
If the audio amplifier and the RF deck use the same plate voltage, the
capacitor doesn't need to handle the full DC voltage, but it should be
non-polar. The problem then becomes one of finding (or winding) an
audio choke with enough inductance and that can handle the current.
In AM broadcast transmitters, the modulation choke was typically the
largest component. Since for voice you should only need to get down
to 300Hz or so, 10 henries inductance should be OK (about 20k ohms
impedance), and you might be OK with less. A 4.7uF coupling capacitor
should work OK, as it would be just over 100 ohms reactance at 300Hz.
4.7uF film capacitors aren't unreasonable to find. You wouldn't ever
get to 100% modulation, since the audio side doesn't go to zero volts
on the plate of the conducting side, but you could add the voice coil
winding in the proper phase to get a bit more modulating voltage.

I've also seen a design where the modulator was single-ended but used
a push-pull transformer; the RF amp was fed its DC through the other
side of the center tapped winding. That allowed reasonable balance of
the DC in the transformer, and worked decently.

Have a look at the Codar AT5 schematic:
http://www.vmarsmanuals.co.uk/new/at5.htm
http://www.vmarsmanuals.co.uk/new/at5.pdf

================================================== =
To my knowledge it is called Heising modulation.As a young
(radio-unlicenced) boy I made a single 6K7 valve(tube) oscillator for
the 80m band with the anode fed by the anode voltage of a single EL3
output valve 4 Watts audio amplifier meant to feed a loudspeaker. With
a long wire antenna the results were an amazingly clean AM signal
getting out well in excess of 100 km
Those were the days before SSB.

Frank GM0CSZ / KN6WH only licenced since 1985.
  #16  
Old June 1st 12, 04:06 PM
Eric W. Nyman Eric W. Nyman is offline
Junior Member
 
First recorded activity by RadioBanter: May 2012
Posts: 3
Default

I have used the Vertical Output transformers from Tube-type TV sets.
They are designed to handle quite a bit of current.
Not 'Hi-Fi' tho, but Very cheap..
You can connect the modulator tube to the tap to increase the modulation for the PA tube, OR connect the modulator tube to the end of the winding and the PA tube to the tap to prevent possible over-modulation.
Both ways work.
73 de Eric / WA4HIS.
 




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