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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 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/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. |
#2
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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 è Radioamatore, se non gli fuma il saldatore! - Campagna 2006 "Il Radioamatore non è uno che ascolta la radio" it.hobby.radioamatori.moderato http://digilander.libero.it/hamweb http://digilander.libero.it/esperantovenezia |
#3
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Quote:
You could use an audio amplifier tube as the modulator with its matching audio output transformer for choke modulation. Here's how I did it. http://nandustips.blogspot.com/2011/...ansmitter.html 73 Nandu. |
#4
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Looking for one low-power tube modulation transformer
"it9xxs" wrote in message ... Hi, I'm looking for a low-power modulation transformer. I'd like to build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s, IT9XXS The guys presented a lot of great ideas.. If a conventional plate modulation transformer is found, and 100% modulation of the final RF stages plate voltage is achieved, the next logical step (I think) is to introduce "Negative Peak Clipping". A highly controversial method that clipped a little of the negative modulation envelope, and added that same amplitude to the positive envelope peaks. 100% PLUS modulation! Oh, how the purists did cry! Fact is, that oscilloscope observations were convincing, over the air observations while switching the "clipper" in and out were totally convincing. It worked! There was a little commercial AM broadcast testing, with FCC strictly against such abhorrent practices, but it did work. I swear. On the receiving end, the audio sounded much like the current practice of Television stations "enhancing" commercial advertising. The received audio amplitude would increase startlingly. Detractors "proved" mathematically that this distortion of the AM envelope introduced undesirable sidebands, but hey, with my 10 watts input to a 2E26 final, nobody ever complained. Time frame, AM glory days 1955 to 1958. Old Chief Lynn, W7LTQ |
#5
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Looking for one low-power tube modulation transformer
On 01/20/2012 05:08 PM, it9xxs wrote:
Hi, I'm looking for a low-power modulation transformer. I'd like to build a QRP (5W?) tube AM transmitter for the 10m band. Thanks, 73s, IT9XXS Use a filament transformer. Primary winding to the plate circuit, secondary winding driven by the output of a PA amplifier. |
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