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Old February 13th 12, 08:17 AM posted to rec.radio.amateur.homebrew
K7ITM K7ITM is offline
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First recorded activity by RadioBanter: Jul 2006
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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 è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è 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