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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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