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Modulation Inductor?
I've acquired a high power 50 watts plus tube type hifi output
transformer. I'd like to try modulating a pair of 6146's with a hifi amp. The circuits I see use a modulating inductor in series with the B+ to the final to keep single ended dc off the transformer secondary. Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts? I was thinking of checking out the secondary of a tv transformer? (Not many left used to be common whatever happened to those tv's). Anyone have any ideas on winding one? Seems to me taking an old transformer apart and scramble winding as much wire as one can afford might do the trick? Ideas? Thanks, Tony, WA6LZH |
Modulation Inductor?
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Modulation Inductor?
I assume you are talking AM modulation and that you really want plate
modulation. Otherwise you are discovering why screen modulation was so popular with 6146 finals! de W8CCW John On Thu, 18 May 2006 19:56:21 -0700, Tony Angerame wrote: I've acquired a high power 50 watts plus tube type hifi output transformer. I'd like to try modulating a pair of 6146's with a hifi amp. The circuits I see use a modulating inductor in series with the B+ to the final to keep single ended dc off the transformer secondary. Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts? I was thinking of checking out the secondary of a tv transformer? (Not many left used to be common whatever happened to those tv's). Anyone have any ideas on winding one? Seems to me taking an old transformer apart and scramble winding as much wire as one can afford might do the trick? Ideas? Thanks, Tony, WA6LZH John Ferrell W8CCW |
Modulation Inductor?
Yes... I think he should seriously consider low-level modulation
instead of high-level modulation. www.telstar-electronics.com |
Modulation Inductor?
"John Ferrell" wrote in message ... I assume you are talking AM modulation and that you really want plate modulation. Otherwise you are discovering why screen modulation was so popular with 6146 finals! de W8CCW John I suspect he is talking Heising modulation, which normally is used with a single-ended Class A modulator whose plate is directly tied to the PA plate (the choke is used to provide a high impedance to the filter chokes. But, what I wonder, a 50 watt tube audio amp is very substantial, and is probably running Class B P-P. I don't see how Heising modulation would work in that application. Pete |
Modulation Inductor?
Tony
What you need to do is to find a universal modulation transformer and replace the audio output transformer used on the audio amplifier. That is the most practical solution to your quest. Pete |
Modulation Inductor?
wrote in message ... On Fri, 19 May 2006 15:12:53 -0400, "Pete Bertini" wrote: "John Ferrell" wrote in message . .. I assume you are talking AM modulation and that you really want plate modulation. Otherwise you are discovering why screen modulation was so popular with 6146 finals! de W8CCW John I suspect he is talking Heising modulation, which normally is used with a single-ended Class A modulator whose plate is directly tied to the PA plate (the choke is used to provide a high impedance to the filter chokes. But, what I wonder, a 50 watt tube audio amp is very substantial, and is probably running Class B P-P. I don't see how Heising modulation would work in that application. Pete Thats one form. The other is to use a huge plate inductor and couple with a cap (usually 8 to 20uf) to the plate side of the inductor. The modulator can be either single ended or PP and the plate side of the Mod amp couples through the cap to the mod inductor. Reason for doing this. At 6146 power level you seeing 600-800V DC power for plates and modulation will take this to over 1200. Those numbers are manageable. For bigger tubes (say a pair of 3-500s) the plate voltage can be 2000V or higher and with modulation peaks hit 4000. By using an inductor the PP transformer is isolated from the really high volts. Choke with required insulation are/were common in big transmitters for broadcast and HF. However mod transformers that can stand 1000V on one side (say PP plates) and 2-5000V on the other side can be hard to come by. Gets the big DC off stuff even though there are some impressive AC voltages with modulation. The trick for 6146 size amps is a big choke on the final plates DC and a good 8uF 1000V cap and you can then use a tube amp of suitable power if you couple the cap to the plate side of the output transformer. You can even use a solid state amp with a 8ohm to few thousand ohm transformer that doesnt have to stand 600-900V dc. Cheap 8ohm to plate(say 2000ohm) transformer? Try a 6V to 220 control transformer backward or maybe a transformer out of an old (tube before transformerless) TV. Allison Agreed, but the choke must be able to stand the high DC standing current without going into saturation. Also, the choke method somewhat limits the maximum modulation unless a few additional steps are taken. I'd be leary about some of the suggestions regarding the use of pwr. transformer windings to accomplish the task. Pete |
Modulation Inductor?
"Tony Angerame" wrote in message ... I've acquired a high power 50 watts plus tube type hifi output transformer. I'd like to try modulating a pair of 6146's with a hifi amp. The circuits I see use a modulating inductor in series with the B+ to the final to keep single ended dc off the transformer secondary. Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts? I was thinking of checking out the secondary of a tv transformer? (Not many left used to be common whatever happened to those tv's). Anyone have any ideas on winding one? Seems to me taking an old transformer apart and scramble winding as much wire as one can afford might do the trick? Ideas? A blast from the past: Oftentimes the voltage rating of chokes was more a limitation of breakdown from conductor/windings to the core or case rather than interwinding breakdown. So........... a quick method of cheating on the rating was to mount the choke on standoff insulators or at least insulating the choke from any metal in the chassis or frame of the RF or modulator deck. Only thing then is to be sure to treat that choke core or case as if it might be at high voltage potential......yikes! Although Choke modulation (or Heising as one respondent mentioned) is "high level" modulation if applied to the RF final plate/s, it takes a lot of tinkering to achieve 100% modulation (balanced, above and below zero). Almost always a compromise in Amateur gear, and some broadcast gear came close, but at great expense. Many will throw rocks at me for this, but "low level" modulation is a huge waste of RF power, and in the good (?) old days, was just another weakly modulated hetrodyne the howling AM section of the bands. OH, OUCH, OWEE, HEY! Old Chief Lynn W7LTQ |
Modulation Inductor?
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Modulation Inductor?
Tony,
I did exactly what you are describing. I used a solid state audio amp, and drove a tube type audio output transformer backwards to step up the impedance from 8 ohms to several thousand ohms. I used a 4 uf oil capacitor to couple that across a HV choke that carried the RF final amp plate current. For a choke I used a 125 watt Stancor poly-pedance modulation transformer where I used as much winding on the primary as available. It worked very well. It might be easier to consider cathode modulation, and in this case the transformer turns ratio will be much less, and the HV problem will also be less. Maybe a 110/220 vac to 12vac 5 amp filament transformer will work out here. This is just a guess, but it might work. Cathode modulation is a combination of plate and grid 1 modulation, so the % eff is somewhere between 66% (plate) down to 33% (grid) , and might end up at about 50%. Consider the 220 vct winding where the grid return goes to the CT, and the cathode goes to the top of the 220 v winding, and the bottom of the winding goes to ground. Your turns ratio need may vary, but the concept proposed is workable. Jim WD5JKO "Tony Angerame" wrote in message ... I've acquired a high power 50 watts plus tube type hifi output transformer. I'd like to try modulating a pair of 6146's with a hifi amp. The circuits I see use a modulating inductor in series with the B+ to the final to keep single ended dc off the transformer secondary. Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts? I was thinking of checking out the secondary of a tv transformer? (Not many left used to be common whatever happened to those tv's). Anyone have any ideas on winding one? Seems to me taking an old transformer apart and scramble winding as much wire as one can afford might do the trick? Ideas? Thanks, Tony, WA6LZH |
Modulation Inductor?
On Thu, 18 May 2006 19:56:21 -0700, Tony Angerame
wrote: I've acquired a high power 50 watts plus tube type hifi output transformer. I'd like to try modulating a pair of 6146's with a hifi amp. The circuits I see use a modulating inductor in series with the B+ to the final to keep single ended dc off the transformer secondary. Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts? I was thinking of checking out the secondary of a tv transformer? (Not many left used to be common whatever happened to those tv's). Anyone have any ideas on winding one? Seems to me taking an old transformer apart and scramble winding as much wire as one can afford might do the trick? Ideas? Thanks, Tony, WA6LZH It looks like AM is really becoming a "lost art". Of all the responses here, only one or two guy knows what you are talking about. The purpose of the choke inductor like you are describing is to keep the DC off the secondary of the modulation transformer. This allows the use of a modulation transformer with much less iron, which will give better low frequency response. If the finals DC current is run through the modulation transformer, as is commonly done, there needs to be an air gap in the transformer to keep the core from saturating from the DC current. The bigger the air gap the less inductance, the greater amount of DC current it can handle before saturation and the poorer low frequency response. To increase the inductance with a larger air gap requires more iron (larger core) to bring the inductance back up. By using a choke and capacitivly coupling the audio from the modulation transformer to the final plate there is no DC on the secondary of the modulation transformer to cause saturation. The modulation transformer then can be any audio transformer. It doesn't need an air gap. The transformer can have much less iron in it for the same amount of inductance as one with an air gap. Very good low frequency response can be had this way. By the way the screens of the final tubes need to be modulated along with the plates too. The easy way is to supply the screens through a dropping resistor from the top of the choke, the same point that feeds the plates. THIS IS NOT HEISING MODULATION. It is regular old plate modulation just like you would do with a conventional modulation transformer. Yes power transformers can be used quite successfully as a modulation transformer with this type of setup even though there is no air gap in the transformer. There is no worry about core saturation because there is no DC current on the transformer. A large enough choke for this application can be had by putting several chokes in series to obtain the required inductance. Regular power supply chokes work fine. Do not use a swinging choke! It has no air gap and will saturate. A TV transformer will not work well as a choke because it has no air gap and the core will saturate quickly with DC going through it. Cathode modulation is very similar to grid modulation in performance. After all audio is placed between the grid and cathode just the same as it is with conventional grid modulation. The amount of cathode to plate modulation is minimal. Screen modulation is similar in performance to grid modulation also. Efficiency is around 35% carrier efficiency. Very tricky to tune up properly. Low level modulation with a linear amplifier behind the modulated driver also has an efficiency of around 35%. Excellent AM can be generated this way as is done with some SSB transmitters in the AM mode. If properly set up one can not tell the difference between it and a high level plate modulated transmitter except that the plate modulated transmitter will probably have higher distortion. 73 Gary K4FMX |
Modulation Inductor?
"Gary Schafer" wrote in message ... It looks like AM is really becoming a "lost art". Of all the responses here, only one or two guy knows what you are talking about. The purpose of the choke inductor like you are describing is to keep the DC off the secondary of the modulation transformer. This allows the use of a modulation transformer with much less iron, which will give better low frequency response. If the finals DC current is run through the modulation transformer, as is commonly done, there needs to be an air gap in the transformer to keep the core from saturating from the DC current. The bigger the air gap the less inductance, the greater amount of DC current it can handle before saturation and the poorer low frequency response. To increase the inductance with a larger air gap requires more iron (larger core) to bring the inductance back up. By using a choke and capacitivly coupling the audio from the modulation transformer to the final plate there is no DC on the secondary of the modulation transformer to cause saturation. The modulation transformer then can be any audio transformer. It doesn't need an air gap. The transformer can have much less iron in it for the same amount of inductance as one with an air gap. Very good low frequency response can be had this way. By the way the screens of the final tubes need to be modulated along with the plates too. The easy way is to supply the screens through a dropping resistor from the top of the choke, the same point that feeds the plates. THIS IS NOT HEISING MODULATION. It is regular old plate modulation just like you would do with a conventional modulation transformer. Yes power transformers can be used quite successfully as a modulation transformer with this type of setup even though there is no air gap in the transformer. There is no worry about core saturation because there is no DC current on the transformer. A large enough choke for this application can be had by putting several chokes in series to obtain the required inductance. Regular power supply chokes work fine. Do not use a swinging choke! It has no air gap and will saturate. A TV transformer will not work well as a choke because it has no air gap and the core will saturate quickly with DC going through it. Cathode modulation is very similar to grid modulation in performance. After all audio is placed between the grid and cathode just the same as it is with conventional grid modulation. The amount of cathode to plate modulation is minimal. Screen modulation is similar in performance to grid modulation also. Efficiency is around 35% carrier efficiency. Very tricky to tune up properly. Low level modulation with a linear amplifier behind the modulated driver also has an efficiency of around 35%. Excellent AM can be generated this way as is done with some SSB transmitters in the AM mode. If properly set up one can not tell the difference between it and a high level plate modulated transmitter except that the plate modulated transmitter will probably have higher distortion. 73 Gary K4FMX Oh, my gosh, you're right, Gary! It is not Heising modulation. For 60 years now I've had the mistaken impression that "Heising" implied simply that a series choke was used in the plate/screen supply to allow ANY modulation to be applied, either by modulator tube directly, or through a suitable capacitor. I am embarrased. "Googling" brought up a whole lot of other aspects of high level modulation that I didn't know (or hopefully just forgot!) Does anyone remember negative peak clipping that was the rage about 1958 or so? (Amateur only, as far as I know)...... If the scope wasn't lying, one of my home brew "high level" AM rigs was capable of modulation of over 100%. There was an awful lot of controvery at the time, and our nearest FCC chief engineer said that he didn't care if it did, it was still illegal. Sigh. Old Chief Lynn, W7LTQ |
Modulation Inductor?
Low level modulation is a huge waste of RF power?? Hey, _AM_ is a huge
waste of RF power, and of spectrum to boot! Except with unusual modulating waveforms, 2/3 of the power is in a useless carrier. Now, compared with plate modulation of the PA, low level AM modulation followed by a linear amp may be somewhat less efficient (though a waste of DC power, not RF power), but remember, the modulator, presumably being a linear audio amplifier, isn't all that efficient either. In addition, there ARE ways to generate AM with low level modulation and efficient RF amplification--they have been used in AM broadcast transmitters. For the cost of a good high-level modulator, how many kilowatt-hours can you buy? Cheers, Tom |
Modulation Inductor?
On 19 May 2006 10:32:56 -0700, "Telstar Electronics"
wrote: Yes... I think he should seriously consider low-level modulation instead of high-level modulation. www.telstar-electronics.com Looking at your web site I see you seem to manufacture CB amplifiers? I also see you fail to post the 3rd and 5th order intermodulation performance info. They must be pretty bad. Also no low pass filter to attenuate the harmonics. -30 db second and third harmonics are unacceptable. And those are at below power spec on the amplifier. How do you measure peak envelope power? Where do you come up with PEP at 3 times carrier power? What makes you think that PEP is going to be any greater than the CW power output capability of the amplifier? 73 Gary K4FMX |
Modulation Inductor?
On Sun, 21 May 2006 18:27:52 -0700, "Lynn Coffelt"
wrote: "Gary Schafer" wrote in message .. . It looks like AM is really becoming a "lost art". Of all the responses here, only one or two guy knows what you are talking about. The purpose of the choke inductor like you are describing is to keep the DC off the secondary of the modulation transformer. This allows the use of a modulation transformer with much less iron, which will give better low frequency response. If the finals DC current is run through the modulation transformer, as is commonly done, there needs to be an air gap in the transformer to keep the core from saturating from the DC current. The bigger the air gap the less inductance, the greater amount of DC current it can handle before saturation and the poorer low frequency response. To increase the inductance with a larger air gap requires more iron (larger core) to bring the inductance back up. By using a choke and capacitivly coupling the audio from the modulation transformer to the final plate there is no DC on the secondary of the modulation transformer to cause saturation. The modulation transformer then can be any audio transformer. It doesn't need an air gap. The transformer can have much less iron in it for the same amount of inductance as one with an air gap. Very good low frequency response can be had this way. By the way the screens of the final tubes need to be modulated along with the plates too. The easy way is to supply the screens through a dropping resistor from the top of the choke, the same point that feeds the plates. THIS IS NOT HEISING MODULATION. It is regular old plate modulation just like you would do with a conventional modulation transformer. Yes power transformers can be used quite successfully as a modulation transformer with this type of setup even though there is no air gap in the transformer. There is no worry about core saturation because there is no DC current on the transformer. A large enough choke for this application can be had by putting several chokes in series to obtain the required inductance. Regular power supply chokes work fine. Do not use a swinging choke! It has no air gap and will saturate. A TV transformer will not work well as a choke because it has no air gap and the core will saturate quickly with DC going through it. Cathode modulation is very similar to grid modulation in performance. After all audio is placed between the grid and cathode just the same as it is with conventional grid modulation. The amount of cathode to plate modulation is minimal. Screen modulation is similar in performance to grid modulation also. Efficiency is around 35% carrier efficiency. Very tricky to tune up properly. Low level modulation with a linear amplifier behind the modulated driver also has an efficiency of around 35%. Excellent AM can be generated this way as is done with some SSB transmitters in the AM mode. If properly set up one can not tell the difference between it and a high level plate modulated transmitter except that the plate modulated transmitter will probably have higher distortion. 73 Gary K4FMX Oh, my gosh, you're right, Gary! It is not Heising modulation. For 60 years now I've had the mistaken impression that "Heising" implied simply that a series choke was used in the plate/screen supply to allow ANY modulation to be applied, either by modulator tube directly, or through a suitable capacitor. I am embarrased. "Googling" brought up a whole lot of other aspects of high level modulation that I didn't know (or hopefully just forgot!) Does anyone remember negative peak clipping that was the rage about 1958 or so? (Amateur only, as far as I know)...... If the scope wasn't lying, one of my home brew "high level" AM rigs was capable of modulation of over 100%. There was an awful lot of controvery at the time, and our nearest FCC chief engineer said that he didn't care if it did, it was still illegal. Sigh. Old Chief Lynn, W7LTQ Hello Chief Lynn, You are a couple of years ahead of me. I didn't get started until around 1960. I built a small modulator a few years ago and played around with negative peak clipping. Never could get it to sound good though. It will allow higher positive peak modulation without over modulating in the negative direction but the clipping on the negative side created a lot of distortion. Broadcast stations regularly run around 120% positive peaks and near 100% negative but they usually do it with clipping at low levels and clean things up after the clipping. I remember hearing about the FCC being in a quandary about some of the super modulation schemes like the Taylor system that could create upwards of 150% or greater positive modulation. 73 Gary K4FMX |
Modulation Inductor?
K7ITM wrote:
Low level modulation is a huge waste of RF power?? Hey, _AM_ is a huge waste of RF power, and of spectrum to boot! Except with unusual modulating waveforms, 2/3 of the power is in a useless carrier. Now, compared with plate modulation of the PA, low level AM modulation followed by a linear amp may be somewhat less efficient (though a waste of DC power, not RF power), but remember, the modulator, presumably being a linear audio amplifier, isn't all that efficient either. In addition, there ARE ways to generate AM with low level modulation and efficient RF amplification--they have been used in AM broadcast transmitters. For the cost of a good high-level modulator, how many kilowatt-hours can you buy? Cheers, Tom The only modulation method that I've ever found to be of any real efficiency in generating AM is "outphasing" or "ampliphase". You can use the /most/ effective PA designs - optimised FET-based Class E stages can be over 90% efficient! This approach completely obviates all the problems with modulation transformers or big chokes. Haven't tried it with valves ("tubes" - US) - perhaps that should be a future project. Bob -- Everything gets easier with practice, except getting up in the morning! |
Modulation Inductor?
Gary Schafer wrote:
Cathode modulation is very similar to grid modulation in performance. After all audio is placed between the grid and cathode just the same as it is with conventional grid modulation. The amount of cathode to plate modulation is minimal. Correct, and the distortion can be pretty nasty, too. Screen modulation is similar in performance to grid modulation also. Efficiency is around 35% carrier efficiency. Very tricky to tune up properly. It was the /only/ type of AM I used at first, as I was unable to get big mod transformers. It worked well enough, though it was difficult to get close to 100% mod. Lots of envelope feedback cured the non-linearity distortions. Low level modulation with a linear amplifier behind the modulated driver also has an efficiency of around 35%. Excellent AM can be generated this way as is done with some SSB transmitters in the AM mode. If properly set up one can not tell the difference between it and a high level plate modulated transmitter except that the plate modulated transmitter will probably have higher distortion. Low-level mod with a linear is very inefficient (with respect to DC input), but is sometimes the only really practical way to proceed. Again, lots of envelope feedback can sort out the distortions caused by less than perfect PAs! Bob -- Everything gets easier with practice, except getting up in the morning! |
Modulation Inductor?
On Mon, 22 May 2006 19:47:14 GMT, biascomms
wrote: Gary Schafer wrote: Cathode modulation is very similar to grid modulation in performance. After all audio is placed between the grid and cathode just the same as it is with conventional grid modulation. The amount of cathode to plate modulation is minimal. Correct, and the distortion can be pretty nasty, too. Screen modulation is similar in performance to grid modulation also. Efficiency is around 35% carrier efficiency. Very tricky to tune up properly. It was the /only/ type of AM I used at first, as I was unable to get big mod transformers. It worked well enough, though it was difficult to get close to 100% mod. Lots of envelope feedback cured the non-linearity distortions. Low level modulation with a linear amplifier behind the modulated driver also has an efficiency of around 35%. Excellent AM can be generated this way as is done with some SSB transmitters in the AM mode. If properly set up one can not tell the difference between it and a high level plate modulated transmitter except that the plate modulated transmitter will probably have higher distortion. Low-level mod with a linear is very inefficient (with respect to DC input), but is sometimes the only really practical way to proceed. Again, lots of envelope feedback can sort out the distortions caused by less than perfect PAs! Bob Plate modulated transmitters are not all that low distortion in general. It is much easier to get a very clean low distortion AM signal by doing it at low levels and amplifying with a linear amp, provided the amplifier is run properly. At low levels modulation can be done with class A modulation stages as opposed to typical class B high level modulators and non linear modulation transformers. Even with 33% carrier efficiency of the linear amp the total amount of AC power required from the power company isn't much different. With the linear amp there are no large modulator tube filaments to light, screen, bias and plate supplies to run the modulator. With the new FCC rules limiting the peak envelope power to 1500 watts it makes more sense to use low level modulation and an amplifier than it did with the old rules. Now plate efficiency does not matter, you can still put the same amount of power into the antenna no matter what the efficiency of the transmitter is. In the past you wanted the highest plate efficiency that you could muster to get the most power into the antenna. I believe that the thinking of the past that "plate modulation is the only way to go" has more to do with not coming to grips with the fact that it is about how much power we can put into the antenna and not how much plate input we run as in the past. 73 Gary K4FMX |
Modulation Inductor?
I believe that the thinking of the past that "plate modulation is the
only way to go" has more to do with not coming to grips with the fact that it is about how much power we can put into the antenna and not how much plate input we run as in the past. ================================ Question : Do the new FCC rules, eg 1500 W PEP mean ,transmitter output power or power to the antenna , the latter meaning that any feeder losses may be compensated by a higher max. transmitter output ? The latter is the case pertaining to the UK amateur radio licences. With a maximum output of 26dBW = 400 W PEP (on most bands) , in the case of 3dB feeder loss ,the permitted transmitter power is 800 W PEP. Frank GM0CSZ / KN6WH (who has NOT read as yet the 'new' FCC max power rules) |
Modulation Inductor?
The FCC allows broadcast stations to compensate for losses before the
antenna, but not amateur stations. You are legally limited to 1500 WPEP output at the transmitter. Glenn AC7ZN |
Modulation Inductor?
MadEngineer wrote:
The FCC allows broadcast stations to compensate for losses before the antenna, but not amateur stations. You are legally limited to 1500 WPEP output at the transmitter. Glenn AC7ZN ====================== Tnx Glenn . Frank GM0CSZ / KN6WH |
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