That was Fred, W3PHL. He built a high level balanced modulator (called the upside down tube circuit), and biased it to 600 watts carrier input. Then he applied a 2400 watt modulator to it. He used heavy clipping, like about 30 dB of clipping, and a low-level low-pass filter. At that time, plate power input to an AM transmitter was measured by the reaings of DC meters with approximately 0.25 second time constant. That time constant would make the meters read average voltage and current, which smoothed out SSB and controlled carrier signals. But on plate-modulated AM it did not read true power during modulation, because it was looking at average voltage and average current separately, while in AM the voltage and current are not separate, but they increase and decrease together, resulting in higher power input during modulation.
Fred's X volts and Y milliamps held steady with or without modulation, looking like 600 watts input, even though easily 3 kilowatts of power were going to that final amp during modulation. The way plate input power was measured at that time, this was legal... so the FCC tried to nail him for overmodulation. Fred countered that overmodulation was a function of nonlinear distortion, and since his modulator correctly handled modulation beyond 100% negative, and it was not saturating at any point, it could not be said to be overmodulating. And if the FCC was going to consider this to be overmodulation, then what about all of theose guys on SSB and DSB... they would have to nail all of them for overmodulation, too! So the FCC resorted to nailing Fred on splatter out of band. It was a cheap trick, because Fred's signal was pretty clean. But they railroaded it through, and Fred lost his license for a year. When he came back on, he was using some fantastic Marconi high-level multi-pole filter, and there was no way they could accuse him of splattering out of the band. The FCC had lost interest by that time anyway, and they left Fred alone after that. But that high-level balanced modulator loophole was the reason that the FCC went for the PEP measurement technique in 1990. Theoretically, one could have used a modulator from a 500,000 watt broadcast station on a 1KW upside-down tube final, and it would have been legal. They couldn't have that! Darn it... Bacon, WA3WDR ----- Original Message ----- From: "John E. Coleman (ARS WA5BXO)" <[EMAIL PROTECTED]> To: "'Discussion of AM Radio'" <[email protected]> Sent: Saturday, February 18, 2006 6:39 PM Subject: RE: [AMRadio] AM Transmitter Advice?? Don, I have heard rumors of a citation or two but only in extreme cases such as 10 or 20 KW PEP and I don't know if the rumors are true. As I understand it the trick, that the FCC was to prevent, and someone was trying to get away with, was to run a 304TL with about 100 Volts on the plate in a GG configuration. Forward bias it to a high plate current like 1 Amp, so that it acts like a switch that is on. Then drive it with 5 KW PEP. It may have been some other some scenario as this but I think you get the picture. An Now, For Something Completely Different. Then there was the trick that a gentleman up in 3 land, I think, was going to run the high level double sideband reduced carrier generator type rig but he was not reducing the carrier just increasing the SBs via an extra upside down tube, as it was commonly called. The sideband power would continue to go up without distortion (if copied on a proper synchronized product detector) after the first tube was over modulated in the negative direction. The voltage and power would be diverted to the upside down tube where sideband power would continue. There was trouble with the specific rule interpretation at the time in the FCC. Of course any of us today, would be able to see that the upside down tube's audio plate current and audio voltage must also be counted as part of the input power. But the FCC was having trouble deciding, at least as I understand it. At any rate, I think they got him for being outside the 40 meter band limits. You may remember more of the specifics on this Don. John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Donald Chester Sent: Saturday, February 18, 2006 4:41 PM To: [email protected] Subject: RE: [AMRadio] AM Transmitter Advice?? >My understanding was. >Under the old rules stage or stages, that provide output to the antenna, >total power should not exceed 1000 watts DC input. This includes the sum >of >the driver and final in the case of GG output circuit. >I think that rule was tested. I recall that was the rule. But it seems to me it was much ado about nothing. At most, the feedthrough power would be 10% of the total output power. How much signal strength gain could you get from increasing your power 10%? Of course, back in those days the FCC was very nitpicky about the ham rules, as they still are with broadcast stations. But they have shifted to the opposite approach with ham radio. Riley's efforts have rooted out the rottenest of the apples, but I suspect ham radio enforcement is still pretty low on the FCC's priority list, as long as the violations don't cause interference to other radio services. Since they changed the power rule, I have never heard of a SINGLE case of a ham receiving a citation for running too much p.e.p. Don K4KYV _______________________________________________________ ________ This message was typed using the DVORAK keyboard layout. Try it - you'll like it. http://www.mwbrooks.com/dvorak/ http://gigliwood.com/abcd/ _______________________________________________________ _______ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb _______________________________________________________ _______ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
