More on PEP, AM, average power, etc.
Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. |
More on PEP, AM, average power, etc.
"Straydog" wrote in message .com... Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Terman explains how high-level Class C modulation works in Radio Engineering (1932 edition) on page 370. |
More on PEP, AM, average power, etc.
Are you as dumb as you seem??
-- Clif "Straydog" wrote in message .com... Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. |
More on PEP, AM, average power, etc.
On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote:
Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". 73 Gary K4FMX |
More on PEP, AM, average power, etc.
On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote:
Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. Methinks you are way too hung up on the abtract theory of how linear tubes are. In practice the majority of AM transmitters rated at 100 watts of carrier output are indeed putting out 400 watts PEP with 100% modulation. As another poster pointed out, this is easily proved by using an oscilloscope or with SOME Peak reading wattmeters. The FCC certainly agrees with the 4:1 ratio. That's why when the Amateur power levels permitted by the FCC in the USA were raised to 1500 watts output PEP, the net result was that users of A.M had to REDUCE carrier power to approximately 375 watts output. Many diehard AM'ers and even the ARRL vigorously protested this net reduction of power for AM use. As I remember the FCC grandfathered the old power limit of 1000 watts DC input to the final amplifier for Am'ers but only did so for a couple of years. Back in the good old days, I used to run a Technical Materiel Corp GPT-750 AM transmitter on 3885KHZ. I ran 1KW DC input on the plate with 100% modualtion. That required a 500 watt modulator in the transmitter. The pair of 4-400A's in the rig easily achieved a power output of 800 watts under class C high level modulation. Thus I was legally running 3200 watts PEP output power. The power supply exceeded 3000 volts and was rated at 1.5 amps CCS, easily achieving the peak power demands.. You are sort of beating a dead horse. this was extensively discussed everywhere in Amateur circles about a decade ago. Doug/WA1TUT |
More on PEP, AM, average power, etc.
On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. This was discussed in the RCA transmitting tube manual, but it also referenced the technical references which go into this in much more detail. However, if you want to say "you can't just look at static curves" then you also can't just say "doubling plate voltage also doubles plate current" either. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". I will decline to check this but words and phrases like "almost exactly" and "considerably less" are unquantitative. 73 Gary K4FMX |
More on PEP, AM, average power, etc.
On Thu, 26 Jan 2006, Doug wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. Methinks you are way too hung up on the abtract theory of how linear tubes are. In practice the majority of AM transmitters rated at 100 watts of carrier output are indeed putting out 400 watts PEP with 100% modulation. As another poster pointed out, this is easily proved by using an oscilloscope or with SOME Peak reading wattmeters. Well, I'll buy into the scope measurement. I'll also buy into my prior incomplete understanding of Ip vs Vp relationships when the tube curves are looked at without realizing that screens have to be modulated as well as plates. I don't like wattmeters (with mechanical needles) that claim to indicate peak watts or have peak watt scales. But, that is my prejudice. The FCC certainly agrees with the 4:1 ratio. That's why when the Amateur power levels permitted by the FCC in the USA were raised to 1500 watts output PEP, the net result was that users of A.M had to REDUCE carrier power to approximately 375 watts output. I remember some of that. But at that time I was an SSB user only. Many diehard AM'ers and even the ARRL vigorously protested this net reduction of power for AM use. As I remember the FCC grandfathered the old power limit of 1000 watts DC input to the final amplifier for Am'ers but only did so for a couple of years. Back in the good old days, I used to run a Technical Materiel Corp GPT-750 AM transmitter on 3885KHZ. I ran 1KW DC input on the plate with 100% modualtion. That required a 500 watt modulator in the transmitter. The pair of 4-400A's in the rig easily achieved a power output of 800 watts under class C high level modulation. Thus I was legally running 3200 watts PEP output power. The power supply exceeded 3000 volts and was rated at 1.5 amps CCS, easily achieving the peak power demands.. And what do you do now? If you don't mind me taking a little more of your time. And, what about all these guys I hear on AM (160 & 75) who say they are running Johnson "Desks" at 1 kW input, and modified former AM broadcast transmitters with 833s in the final? You are sort of beating a dead horse. this was extensively discussed everywhere in Amateur circles about a decade ago. A decade ago I was not interested in anything AM. A decade ago I had nothing that would even come close to putting out any power that would have even made me have to think about this FCC specification. Recently, out of pure nostalgia, I partly restored a very chopped up Johnson Ranger and got it back on the air, on AM, to my considerable delight. Otherwise it was too chopped up for a full restoration (to include CW). Similarly, lots of things I was interested in a decade ago, I'm not interested in any more. And, I spend much more time at the bench now than at the microphone. Art, W4PON Doug/WA1TUT |
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On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote:
On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. This was discussed in the RCA transmitting tube manual, but it also referenced the technical references which go into this in much more detail. However, if you want to say "you can't just look at static curves" then you also can't just say "doubling plate voltage also doubles plate current" either. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". I will decline to check this but words and phrases like "almost exactly" and "considerably less" are unquantitative. Do you even know who Terman is? I would doubt that you do or you would not make statements like that. As a matter of fact if you had read any of his work you would not be making most of the statements that you are in these threads. At first I thought that you were interested in learning but I see you would rather argue for the sake of arguing. 73 Gary K4FMX 73 Gary K4FMX |
More on PEP, AM, average power, etc.
"Straydog" wrote in message .com... looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. In some cases it is a lot easier to accept what is technically correct, and work backwards to correct erroneous conclusions. First, a Class C amplifier is driven into grid conduction, almost to the point of plate saturation. High Level AM modulation is applied to the SCREEN and PLATE, only doubling the plate voltage as in your 6F6 example to show a non linear relationship isn't a valid argument. What is the operating Class of the tube, and did you account for the modulating voltage also being applied to the screen grid? To quote Henny: "A linear relation must exist between plate voltage and tank circuit current for good operation... In such a modulated amplifier, the output peak will be four times the unmodulated carrier and the continuous power output with complete modulation is 1.5 times the power at zero modulation." Note that is only true for a true Class C power amplifier stage, and not for Class A or B. I doubt that Henny or Tenny based their texts on conjecture or misguided realities. Pete |
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" Uncle Peter" wrote in message news:cufCf.16380$bF.7979@dukeread07... I doubt that Henny or Tenny based their texts on conjecture or misguided realities. Pete Henny or Terman!! Arggh! Typo. |
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On Thu, 26 Jan 2006, Gary Schafer wrote: On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote: On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. This was discussed in the RCA transmitting tube manual, but it also referenced the technical references which go into this in much more detail. However, if you want to say "you can't just look at static curves" then you also can't just say "doubling plate voltage also doubles plate current" either. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". I will decline to check this but words and phrases like "almost exactly" and "considerably less" are unquantitative. Do you even know who Terman is? Yep, and I've even looked in his books. But its more than I want to go into. I would doubt that you do or you would not make statements like that. As a matter of fact if you had read any of his work you would not be making most of the statements that you are in these threads. I don't have the benefit of reading his works, I'm presuming that you have, is that correct? At first I thought that you were interested in learning but I see you would rather argue for the sake of arguing. Very early in my comments I brought up the issue of Ip being independend of Vp in all of the curves (these are facts) for tetrode and pentode transmitting tubes and receiving tubes and nobody but nobody called attention to the possibility that this conflict with claims of plate current doubling with plate voltage doubling could be resolved by including changes in screen voltage proportional, in some relationship, to changes in plate voltage. A few of your statements were a little bit helpful but even the comments in the RCA transmitting tube manual were weak in dealing with this issue. What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output measurement the new criterion by which transmitter power is to be measured. The only thing I can think of is that there were, in the far past, some AM amateurs who were running some form of ultra modulation or super modulation and putting KWs of audio on a 1 KW DC input to the final signal and the FCC didn't like that. Maybe if any of you have some background on this, you could mention it. |
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On Thu, 26 Jan 2006, Uncle Peter wrote: " Uncle Peter" wrote in message news:cufCf.16380$bF.7979@dukeread07... I doubt that Henny or Tenny based their texts on conjecture or misguided realities. Pete Henny or Terman!! Arggh! Typo. I knew what you meant. We all make typos from time to time. |
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On Thu, 26 Jan 2006, Uncle Peter wrote: "Straydog" wrote in message .com... looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. In some cases it is a lot easier to accept what is technically correct, and work backwards to correct erroneous conclusions. Well, I didn't have trouble seeing that if a scope shows modulation RF peaks that are double the unmodulated carrier, then the instantaneous power at the peak is 4X. What I had trouble with was from the curves that show plate current in tetrodes, pentodes independent of plate voltage. What I did not consider is how screen voltge would affect plate current if the screen voltage were modulated along with plate voltage. Previously, I thought power input could only double but that would leave a deficit since peak output voltage shows on the scope that power at the peak has to be 4X the unmodulated power. First, a Class C amplifier is driven into grid conduction, almost to the point of plate saturation. High Level AM modulation is applied to the SCREEN and PLATE, only doubling the plate voltage as in your 6F6 example to show a non linear relationship isn't a valid argument. Well, it was a 6FG6 (not a 6F6) and what we are trying to do is find out how power _input_ at the peak of a modulation cycle becomes 4X the power input when an unmodulated carrier is being put out. Part of the answer comes from modulating the screen, and one of the guys was talking about some modulation of the control grid through a "grid leak" resistor which was also mentioned in the RCA transmitting tube manual in the front which gives some rudimentary explanations for all of this (and was helpful for me to re-read). What is the operating Class of the tube, and did you account for the modulating voltage also being applied to the screen grid? Well, if you look at all of the curves showing Ip vs Vp, they usually give curves for a fairly large range in control grid voltages (but at only one screen grid, if it is present, voltage) so you can look at how Ip changes for any range in changes in control grid so you can know about what class of amplifier you are running by looking at highest control grid voltage and lowest control grid voltage you want to use and whether you get close to cutoff (where Ip goes to zero or, maybe, close to zero). To quote Henny: "A linear relation must exist between plate voltage and tank circuit current for good operation... In such a modulated amplifier, the output peak will be four times the unmodulated carrier and the continuous power output with complete modulation is 1.5 times the power at zero modulation." I understand this, now, and know where it comes from. Fine. Note that is only true for a true Class C power amplifier stage, and not for Class A or B. I can accept this, too. I doubt that Henny or Tenny based their texts on conjecture or misguided realities. I can appreciate that in the more advanced treatises on the subject that the guys know more about what is going on. The ARRL handbooks gloss over a lot of this and I always wondered why the FCC changed the rule from measuring simple DC power input (plate volts X plate current), even on a linear for SSB, with a simple D'arsonval movement meter (or a digital bar graph meter that could be made to mimic a mechanical meter), to the rule that PEP output not exceed 1500 Watts. One would have to surely use a scope and I'd prefer not to have to trust these so-called PEP reading meters that are all over the place now. But, thanks for your contribution. Pete |
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On Thu, 26 Jan 2006 22:32:41 -0500, Straydog wrote:
On Thu, 26 Jan 2006, Gary Schafer wrote: On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote: On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. This was discussed in the RCA transmitting tube manual, but it also referenced the technical references which go into this in much more detail. However, if you want to say "you can't just look at static curves" then you also can't just say "doubling plate voltage also doubles plate current" either. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". I will decline to check this but words and phrases like "almost exactly" and "considerably less" are unquantitative. Do you even know who Terman is? Yep, and I've even looked in his books. But its more than I want to go into. His writings are very easy to understand compared to many engineering books. He leans less on the math and more on practical explanations. He was one of the most highly thought of professors in the radio field. Although his books were written in the 30's and 40's, they do not include some of today's newer discoveries, they are very well written to explain circuit theory and things like modulation. I would doubt that you do or you would not make statements like that. As a matter of fact if you had read any of his work you would not be making most of the statements that you are in these threads. I don't have the benefit of reading his works, I'm presuming that you have, is that correct? I am not an expert by any means but I often refer to a few of his books. At first I thought that you were interested in learning but I see you would rather argue for the sake of arguing. Very early in my comments I brought up the issue of Ip being independend of Vp in all of the curves (these are facts) for tetrode and pentode transmitting tubes and receiving tubes and nobody but nobody called attention to the possibility that this conflict with claims of plate current doubling with plate voltage doubling could be resolved by including changes in screen voltage proportional, in some relationship, to changes in plate voltage. A few of your statements were a little bit helpful but even the comments in the RCA transmitting tube manual were weak in dealing with this issue. That is exactly what I told you in my very first post to you. That was the one that had several different topic headings. The one that you deleted most of the headings as "incorrect information". This was under AM TRANSMITTERS. "Screen grid tubes are not linear in this respect. Plate current is somewhat independent of plate voltage. That is why you must also partly modulate the screen along with the plate when using a screen grid tube in the final. You want to have a linear plate voltage to plate current relationship." What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output measurement the new criterion by which transmitter power is to be measured. The only thing I can think of is that there were, in the far past, some AM amateurs who were running some form of ultra modulation or super modulation and putting KWs of audio on a 1 KW DC input to the final signal and the FCC didn't like that. Maybe if any of you have some background on this, you could mention it. That could have been part of it. It is difficult to tell exactly how much power is really going out with different types of modulation. Probably the biggest reason for the change was SSB. Watching the plate meter kicking up and down was not a very accurate means of measuring power but in the old day's access to a PEP watt meter was almost non existent. Now with a PEP wattmeter it is much easier to read power output than it is input power. Another part of the change was to reduce the maximum power that hams were able to use. As discussed the 1 kw input AM transmitter could easily have in excess of 3000 watts PEP output and lots more with some modulation schemes as you refer to. SSB also could have well in excess of 3000 watts PEP output as well. The old means of measuring SSB power was the plate current meter on the final not kicking over the 1 kw DC input level. The meter had to have a time constant of less than .025 seconds. No sluggish meters allowed. But the average power in speech is only around 10 to 20% and that is what was measured. 73 Gary K4FMX |
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The harder the modulated tube is driven into class-C conditions and
saturation, the more linear does the plate modulation become. Operation of the tube becomes independent of curvature in the tube's characteristic. The plate current operating angle is small. It behaves more like an on/off switch. The more non-linear it is, the smaller the operating angle, the more linear is the modulation. ---- Reg, G4FGQ. |
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The more non-linear it is, the smaller the operating angle, the more
linear is the modulation. ---- Reg, G4FGQ. Hello Reg: Does this mean that you are going to have a software program for us soon? You have written us programs for almost everything else useful in ham radio. Actually, what I need is a program that designs a maximum legal limit AM rig using the parts I have, and that then tells me where to get the parts I do not have with the least work and expense. 73, Colin K7FM |
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"Reg Edwards" wrote in message ... The harder the modulated tube is driven into class-C conditions and saturation, the more linear does the plate modulation become. Operation of the tube becomes independent of curvature in the tube's characteristic. The plate current operating angle is small. It behaves more like an on/off switch. The more non-linear it is, the smaller the operating angle, the more linear is the modulation. ====================================== PLATE MODULATION. It's really all very simple. Imagine a class-C triode amplifier with very small operating angle and running nearly into saturation. The plate load is a tuned tank circuit having a high impedance at resonance. Or it can be a Pi-tank circuit. Makes no difference! With the high impedance load, conditions are such that whatever is the DC plate voltage, the RF plate volts swing down to a very low plate-to-cathode voltage. Ideally it should be zero volts. But in practice it cannot fall below the positive, peak, instantaneous, RF grid volts. This corresponds to the instant of peak plate current. The RF voltage across the tank is then very nearly EQUAL to the DC plate voltage regardless of the tubes characteristic curves. Curvature doesn't matter. It is obscured by the small operating angle. The tube is conducting only for a small fraction of the time. Modulate the DC plate voltage at an audio frequency. With 100 percent modulation the DC plate voltage swings between a very low voltage and twice the DC supply volts. And so do the RF volts across the tank. The job is done. You have an almost perfect linearly modulated amplifier. It is necessary only to ensure grid drive is just sufficient to drive the tube into saturation when the DC plate volts is twice the DC supply volts. It will then remain saturated at all lower voltages. With a triode, saturation occurs when the RF plate voltage swings down to not much more than the peak RF grid volts. With a beam tetrode, saturation occurs when the RF plate voltage swings down only to something less than the DC screen-grid volts and 100 percent linear modulation cannot be achieved. But 100 percent modulation is always undesirable because of the risk of over-modulation. With a bipolar transistor, modulation can be even more linear because, with the high impedance tuned tank, the device saturates or 'bottoms' at very nearly zero RF collector volts. About 0.7 volts. ---- Reg, G4FGQ. |
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"COLIN LAMB" wrote in message ink.net... The more non-linear it is, the smaller the operating angle, the more linear is the modulation. ---- Reg, G4FGQ. Hello Reg: Does this mean that you are going to have a software program for us soon? You have written us programs for almost everything else useful in ham radio. Actually, what I need is a program that designs a maximum legal limit AM rig using the parts I have, and that then tells me where to get the parts I do not have with the least work and expense. 73, Colin K7FM ========================================== Colin, A real amateur would visit hamfests, look around the junk, take a selection of the junk home, then sit and think about what he could do with it. If you stick out your tonge and pull a funny face, the stall holders might throw something at you - for free. The bits and pieces left over can be put towards the next project. ;o) ---- Reg. |
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On Thu, 26 Jan 2006, Gary Schafer wrote: On Thu, 26 Jan 2006 22:32:41 -0500, Straydog wrote: On Thu, 26 Jan 2006, Gary Schafer wrote: On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote: On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between plate current and plate voltage (meaning zero current at zero voltage, and a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. You can't just look at static curves. Consider that with AM modulation there is usually grid leak bias on the final tube being modulated. This allows the grid voltage to somewhat follow the modulation and helps smooth out the non-linearity in the plate. This was discussed in the RCA transmitting tube manual, but it also referenced the technical references which go into this in much more detail. However, if you want to say "you can't just look at static curves" then you also can't just say "doubling plate voltage also doubles plate current" either. If you have access to any of Termans books, as peter said, there is an excellent section on how modulation works. He in fact shows that "plate current follows plate voltage almost exactly with modulation". His words. He also says that "triodes have considerably less distortion than screen grid tubes". I will decline to check this but words and phrases like "almost exactly" and "considerably less" are unquantitative. Do you even know who Terman is? Yep, and I've even looked in his books. But its more than I want to go into. His writings are very easy to understand compared to many engineering books. He leans less on the math and more on practical explanations. He was one of the most highly thought of professors in the radio field. Although his books were written in the 30's and 40's, they do not include some of today's newer discoveries, they are very well written to explain circuit theory and things like modulation. Fine. Maybe next time I see some of his works at a hamfest, I'll take a look and see if I might want to delve more deeply. I might add that I've looked at and own one of the RSGB ham handbooks which are sometimes more detailed than the ARRL handbook. However, I also want to keep ham radio a hobby for me rather than a vocation (as, say, an EE) I would doubt that you do or you would not make statements like that. As a matter of fact if you had read any of his work you would not be making most of the statements that you are in these threads. I don't have the benefit of reading his works, I'm presuming that you have, is that correct? I am not an expert by any means but I often refer to a few of his books. Well, there are a lot of gaps in my knowledge, too, and, yes, I know there are books out there that go very deeply into theory, math, etc. My other favorite books are the bil Orr (I think W6SAI?) "Radio Handbooks" which I also think are very nice and cover things differently. At first I thought that you were interested in learning but I see you would rather argue for the sake of arguing. Very early in my comments I brought up the issue of Ip being independend of Vp in all of the curves (these are facts) for tetrode and pentode transmitting tubes and receiving tubes and nobody but nobody called attention to the possibility that this conflict with claims of plate current doubling with plate voltage doubling could be resolved by including changes in screen voltage proportional, in some relationship, to changes in plate voltage. A few of your statements were a little bit helpful but even the comments in the RCA transmitting tube manual were weak in dealing with this issue. That is exactly what I told you in my very first post to you. That was the one that had several different topic headings. The one that you deleted most of the headings as "incorrect information". This was under AM TRANSMITTERS. "Screen grid tubes are not linear in this respect. Plate current is somewhat independent of plate voltage. That is why you must also partly modulate the screen along with the plate when using a screen grid tube in the final. You want to have a linear plate voltage to plate current relationship." Well, you also deleted my response to this, too. What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output measurement the new criterion by which transmitter power is to be measured. The only thing I can think of is that there were, in the far past, some AM amateurs who were running some form of ultra modulation or super modulation and putting KWs of audio on a 1 KW DC input to the final signal and the FCC didn't like that. Maybe if any of you have some background on this, you could mention it. That could have been part of it. It is difficult to tell exactly how much power is really going out with different types of modulation. Yes, One other thing I was thinking about way back then as to why they could come up with this way of measuring power was that someone told someone else in the FCC somethin glike this: since the books say that in grounded-grid amplifiers, a part of the input drive power gets fed through to the antenna, maybe someone could build some kind of weird grounded-grid amplifier where the final has a DC imput of 1 kW and the final is driven with 5 kW of input drive power and the final puts out, say 0.5 kW and 4.5 kW of drive power feeds through the final and adds to the 0.5 kW from the final to give 5 kW of output with just 1 kW to the final and goes into the antenna and its legal. I don't know, just my wild speculation. Probably the biggest reason for the change was SSB. Watching the plate meter kicking up and down was not a very accurate means of measuring power but in the old day's access to a PEP watt meter was almost non existent. And, scopes were big and expensive, too. Personally I always just looked at plate current while saying "ahhhhhhhh" and multiplying by plate voltage and telling people VxI=watts and that is my "average" power DC INPUT. And, I've heard, on the air, all manner of misunderstandings of power. I actually heard one guy say "And, I'm getting 700 watts DC output out of my linear" and I was wondering how you get DC out of an RF output SO-239 connector off a commercial linear amplifier. :-\ Now with a PEP wattmeter it is much easier to read power output than it is input power. Well, I'd rather not trust the needle meters. At least I'd want to check it against a scope with bandwidth high enough to measure those voltage peaks under voice modulation (as I've done with my Ranger). Another part of the change was to reduce the maximum power that hams were able to use. As discussed the 1 kw input AM transmitter could easily have in excess of 3000 watts PEP output and lots more with some modulation schemes as you refer to. Yes, and it all seems so silly to me. I always had the feeling that talking (in the old days) about 2 kW PEP when 1 kW input was the max was more of a ego hype ploy to make people feel they had something when, from a practical point of view, the S-meter was going to be responding to the average power which was easier to measure anyway. But from this one can argue in lots of branching directions. SSB also could have well in excess of 3000 watts PEP output as well. The old means of measuring SSB power was the plate current meter on the final not kicking over the 1 kw DC input level. The meter had to have a time constant of less than .025 seconds. No sluggish meters allowed. But the average power in speech is only around 10 to 20% and that is what was measured. And, you could have some weird speech waveform with funny transients in it that spiked up, too. ' Anyway, 73 73 Gary K4FMX |
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Actually, when the tube(or any other active device) is delivering
power into a load, in a class C amp, you have either high Vp and Ip cut off, or LOW Vp and heavy Ip flowing. When the tube is turned on, the voltage should only be the tube "bottoming" voltage. for the 6146 I know best this will be between 75 and 100V, depending on Ip(peak). The part of the tube curves that determines modulating waveforms, efficiency, and maximum useable loading voltage is the part of the curve where full(peak) Ip is flowign at only a LOW voltage. For the 6146, for instance, that woudl be currents of 400-800ma flowing with only 75-125V. If you were drawing full tube current with all your Vp across the tube, your efficiency and power output would be zero! Where does the rest of the Vp go? it appears across the load. If the tube was a hypothetical perfect switch, bottoming voltage would be zero, and in Class C would be either all the way on or all the way off at all times. The 120 degree conduction angle common in Class C is used to minimize switching voltage and therefore overlap of Vp and Ip. That overlap and bottoming voltage are the source of the heat dissipated by the tube. Class E and F amps use different tuned circuits to get zero switching votage and efficiency as high as 90%! If the tube were a perfect switch, and the conduction angle were constant with respect to Vp, the resulting amp would give perfect linear plate modulation! This is becuase the load should be a pure ohmic resistance at resonance. The reason it does actually work this way is that bottoming voltrage runs up fast as Ip is increased, cutting the efficiency of the tube and cutting the percentage of Vp that appears across the load. If doubling the plate voltage only raises the voltage applied to the load by 90%, it will only increase current by 90% at most, meaning the power increase cannot be more than about 81%! This is why a little grid modulation added to plate modulation(ESPECIALLY WITH TETRODES/PENTODES!) does so much for the modulation characteristic of the amp. This is also true to a very severe extent of MOSFETS, BTW. Solid state AM is utter hell to modulate and I suspect will never sound as good as tubes. you know, just like transistor guitar amps sound like garbage(but for different reasons). To design an AM final for plate modulation, begin by figuring desired PEP, which is four times desired carrier power. Now select a tube(s) and operating point that will make this power at a reasonable efficiency. If the amp can run key-down at this level, great, but this is not necessary in any way. In teh real world, this would always be over its thermal dissipation rating. In other words, if you ran it without modulation at double plate modulation, it should run with good class C efficiency unit it overheats from being too small in dissipation rating! It must NOT be overloaded to the point that efficiency is dropping. This sets the loading used on the amp as built, tuned, and run. When operated at carrier, it will be at a rather light loading compared to the CW ratign for the same device. It could make more power, but you'd never modulate it. Do NOT increase the loading. At carrier, it will still make somewhat over 1/4 the PEP power, due to greater efficiency at the lighter current. Becuase your efficiency waqs good to begin with, this difference will not be severe. A small amount of grid modulation is now introduced. On a triode, the use of grid leak bias has this effect, as grid current falls with rising plate voltage durign the switchin time of a real tube. On a screen grid tube,you put a choke or tertiary winding in the screen supply. If you just rran the screen all trhe time at enough voltage for the PEP condition, you would little too much carrier power. The point of the grid modulation is this: it reduces efficiency just enough at carrier to be the same as the efficiency at PEP. This, in turn, is one of the reasons the PEP efficiency must be good-otherwise you now always have poor efficiency. Suppose you are designing a final for the amateur limit of 1500 wats PEP output. You are NOT going to design a 325 watt(the carrier power) amp! Thermal loading will be on a sine wave signal about that of a 437 watt class C amp, but the amp must not clip or round off the current pulses at 1500 watts output and double plate voltage. Essentially, your tubes must be able to efficiently MAKE 1500 watts, even if they can only get rid of 200 watts of heat in the process. I learned about this the ahrd way, on MOSFETS for a MW application. Two IRF 510's could make 50W carrier but wouldn't modulate worth garbage. To get acceptable modulation requred backing power off all the way to 27 watts and then still needed an assymetrical modulating voltage. Viewing the modulating voltage on the scope proved the modulator to be adequate when wound for a normal syymetrical output waveform, leavign the Class C amp as the culprit. To fix it right too four of those little MOSFETS, and 2V or gate modulation from a tertiary winding. Just forward biasing to handle the current didn't cut it. This gave 45W carrier, with still a shortfall in PEP, but now only the shortage predicted by the power supply voltage sagging about 3/4V out of 16V. The heatsink runs very cool at carrier, and doesn't get too hot even with the modulator running flat-out. A pair of 6146B's of course, with a pair of 6550 audio tube as a modulator, would have been able to pound out several times that much power! Still, they would never, ever be able to make four times the CW rating from the tube manual simply by applying twice the CW plate voltage. Similarily, to run the same current at carrier as in a CW rig, they would need a lot more drive, as at PEP the plate current would be higher, as high as the current drawn in a low-impedance VHF rig. Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma looking in my RCA receiving tube manual (RC-20) I found for a 6FG6 a sharp cutoff tetrode that only at zero grid volts was there a near linear relationship between d a straight line [which actually deviated slightly from a straight line] with some slope. But at 100 v on plate, current was 14 milliamps, at 200 v on the plate, plate current was 34 miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode, and at any of a wide range of grid voltages, plate current could be doubled with only a 15-20% increase in plate voltage. My thinking on all of this leads me to claim that anyone who can start with a 100 watt carrier from an AM transmitter and make a few assumptions about 100% modulation and come up with a _calculation_ of something like 400 watts of peak power and represent that as having something to do with reality is pure conjecture. If anyone wants to put an appropriate oscilloscope on the transmitter output and measure the RF voltage of unmodulated carrier into an appropriate load and then measure the peak RF voltage when the carrier is modulated, then and only then do they have a reasonable _basis_ for making a claim about peak (instantaneous) output power. |
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Jumping into the middle of a thread with a single, but useful, comment, Steve, K9DCI says: "Gary Schafer" wrote in message ... On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote: On Thu, 26 Jan 2006, Gary Schafer wrote: On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote: Since my earlier post (dealing with the question of what is peak evelope power output in an AM transmitter), I've been doing more scrutinizing of tube Ip/Vp characteristic curves. They are much more non-linear than the impression you get from just looking at the curves. Also, it is rare or almost non-existant to find Ip vs screen voltage! Probably because the screen and supressor grids are not the main controlling grids, but are there only to reduce Ip vs. Vp and control secondary emission off the plate respectively. Lets look at the venerable 833 (from my RCA TT-3 transmitting tube manual). This is a KW input class C triode. From the curve: at zero grid volts, 1 kV on the plate gives 175 ma plate current 2 kV 500 ma That's more than a doubling of Ip for a doubling of Vp at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current 4 kV 750 ma .... OK OK TWO comments... I think someone else pointed out that this is a static situation and in a circuit, the supply voltage and the plate voltage are not the same thing when you have a tank circuit in there. 73, Steve, K9DCI |
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"Straydog" wrote in message x.com... ,... What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output measurement the new criterion by which transmitter power is to be measured. The only thing I can think of is that there were, in the far past, some AM amateurs who were running some form of ultra modulation or super modulation and putting KWs of audio on a 1 KW DC input to the final signal and the FCC didn't like that. Maybe if any of you have some background on this, you could mention it. While possibly (probably) true... DC was easy to measure (by hams) and RF power difficult to measure in the early days. As time progressed, RF power became easier to measure and, after all, was the thing which would cause trouble as far as interference was concerned. 73, Steve, K9DCi |
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On 1/30/06 2:33 PM, in article , "Steve
Nosko" wrote: "Straydog" wrote in message x.com... ,... What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output measurement the new criterion by which transmitter power is to be measured. The only thing I can think of is that there were, in the far past, some AM amateurs who were running some form of ultra modulation or super modulation and putting KWs of audio on a 1 KW DC input to the final signal and the FCC didn't like that. Maybe if any of you have some background on this, you could mention it. While possibly (probably) true... DC was easy to measure (by hams) and RF power difficult to measure in the early days. As time progressed, RF power became easier to measure and, after all, was the thing which would cause trouble as far as interference was concerned. 73, Steve, K9DCi Even us guys with a commercial ticket used DC measurements to determine input power; it wasn't a ham thing. Watt meters were used on lower powered equipment, but it wasn't a requirement. Your "after all" comment is silly. Don |
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"Don Bowey" wrote in message ... On 1/30/06 2:33 PM, in article , "Steve Nosko" wrote: "Straydog" wrote in message x.com... ,... What is a further issue is why the FCC decided to drop steady DC input (easily measured with a plate current meter) in favor of making PEP output... While possibly (probably) true... DC was easy to measure (by hams) and RF power difficult to measure in the early days. As time progressed, RF power became easier to measure and, after all, was the thing which would cause trouble as far as interference was concerned. 73, Steve, K9DCi Even us guys with a commercial ticket used DC measurements to determine input power; it wasn't a ham thing. Watt meters were used on lower powered equipment, but it wasn't a requirement. Your "after all" comment is silly. Don Perhaps a poor expression of intent, but my intent was that it is the output power which the FCC (or equivalent) is concerned about, not input. As long as it bacame more feasable to measure output power, why not use that. Perhaps explicitly stating it was stating the obvious, but it was an attempt at being complete--sort of a flame insurance. 73, Steve, K9DCI |
More on PEP, AM, average power, etc.
I understood your post as if you would like to gain some understanding (without having to consult Terman) on why the voltage & current figures you read on your RCA TT-3 manual for the 833 tube do not apparently match the 400-W peak-envelope-power figure commonly quoted for a 100%-modulated 100-W AM carrier.
In my understanding, there are two main issues that you did not consider in your reasoning: 1) the voltage / current pairs you quoted for the 833 tube correspond to a fixed grid voltage (either 0V or -50V). In practice the grid voltage is sinusoidal (at the carrier frequency) and, for a class-C amplifer, it only causes plate current to flow when getting above the interdiction threshold. So, plate current only circulates for a fraction of the RF cycle (180 degrees). Therefore plate current is not sinusoidal and its average value (along half a cycle), i.e. what you read on a DC meter, is closer to the peak value compared to a purely sinusoidal waveform. When plate voltage varies due to modulation, the grid interdiction threshold varies and so the circulation angle does. This turns into a change of the average-to-peak current relationship. That said, it immediately follows that simply taking voltage & current figures at a fixed grid voltage would not make much sense. All what said does not take into account the presence of the plate tank circuit which, as someone else has noted, yields a remarkable effect on actual plate voltage & current figures 2) the modulator is designed to feed a given load resistance (depending on the modulation transformer winding ratio). If the modulator load (i.e. the class-C final stage) does not show a linear current / voltage relationship, the plate voltage will, partially, self-adapt itself along the audio-frequency cycle, thus smoothing the tube non-linear behavior. In addition to that, there are other effects (like the grid leak bias causing grid voltage to somewhat follow the modulation) that someone else has already pointed out. The above arguments do not provide a justification of why a class-C tube shows, in practice, a reasonably linear voltage / current behavior, but they should at least give you sufficient evidence why your reasoning is way too simplified to credibly deny the 1:4 PEP ratio of AM-modulated signals. 73 Tony I0JX |
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