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Looking for one low-power tube modulation transformer
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
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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 Are there any old tube type CB radios around? I suppose size might matter, but sometimes old tube type power transformers can come up with a "close enough" impedance ratio. Another old time solution was to find a tube type speaker output transformer, with the secondary on an outside winding which could be stripped away, making room to add a new secondary. (keep a log of turns removed on the old secondary, so a good estimate of new turns required) (to get impedances fairly close.. precision not required) Old Chief Lynn, W7LTQ |
Looking for one low-power tube modulation transformer
On Fri, 20 Jan 2012, coffelt2 wrote:
"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 Are there any old tube type CB radios around? I suppose size might matter, but sometimes old tube type power transformers can come up with a "close enough" impedance ratio. I barely see solid state CB sets around, so I suspect the tube ones have long disappeared. Besides, he's not in North America, and CB was mostly a North American thing. Another old time solution was to find a tube type speaker output transformer, with the secondary on an outside winding which could be stripped away, making room to add a new secondary. (keep a log of turns removed on the old secondary, so a good estimate of new turns required) (to get impedances fairly close.. precision not required) Or find a 400Hz transformer, nobody wanted them so at one time they were not only available but cheap. Various articles said they worked fine, since voice didn't have to go down to 60Hz. Michael VE2BVW |
Looking for one low-power tube modulation transformer
In article ,
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 One technique was to use only the primary of a center tapped (push pull) audio output transformer. The modulator tube, running class A, was on one side, and the other side was used as the supply for the RF output stage, with the B+ to the center tap. Mark Zenier Googleproofaddress(account:mzenier provider:eskimo domain:com) |
Looking for one low-power tube modulation transformer
On Jan 21, 9:56*am, Michael Black wrote:
On Fri, 20 Jan 2012, coffelt2 wrote: "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 Are there any old tube type CB radios around? I suppose size might matter, but sometimes old tube type power transformers can come up with a "close enough" impedance ratio. I barely see solid state CB *sets around, so I suspect the tube ones have long disappeared. Besides, he's not in North America, and CB was mostly a North American thing. Another old time solution was to find a tube type speaker output transformer, with the secondary on an outside winding which could be stripped away, making room to add a new secondary. (keep a log of turns removed on the old secondary, so a good estimate of new turns required) (to get impedances fairly close.. precision not required) Or find a 400Hz transformer, nobody wanted them so at one time they were not only available but cheap. *Various articles said they worked fine, since voice didn't have to go down to 60Hz. * * Michael *VE2BVW Get a single ended triode type of audio output transformer and drive it from a hi-fi amp. |
Looking for one low-power tube modulation transformer
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.moderato http://digilander.libero.it/hamweb http://digilander.libero.it/esperantovenezia |
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. |
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 |
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. |
Looking for one low-power tube modulation transformer
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. Cheers, Tom |
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 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 -- Ian |
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 |
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 |
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 |
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. |
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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