Don, Well, I don't care about modulator efficiency at all. I like to have at least the input power of the rf deck available as modulator power if possible. On the 813 rig, I run 2000 volts at .4 amps, 800 watts input. I only have 600 watts out of the 4x150a mod deck, but the ppp 100th deck could do more I guess, 900 watts at 3000 volts.
On the 812 rig, I have 811a modulators at 1500 volts, supposed to put out over 300 watts, so I am good there. I limit my quick peaks in the mike preamp, and include negative cycle loading in all RF decks, but don't push things much. I should turn the plate voltage on the 813 rig down, run lower output power, and see what the peaks get up to. As it is now, I cant measure them, they are off the 2000 watt scale, but suspect they run 4 to 1- 600 watts out would give 2400 pep. But it all depends on how loud I talk, when the FCC truck pulls up, speak softly and carry a big stick.... There are people on the band who run/can run excessive positive modulation, but you need a special detector I guess, because they sound bad in my shack when they do. The best sounding signals to me sound quite natural and mellow. WA3VJB and another guy were on 80 a few days ago, good conditions, good subject matter, wonderful to listen to while doing odds and ends in the shack. STRONG carriers and mellow modulation. Brett N2DTS Brett N2DTS > > I agree wholeheartedly. Also, careful examination of the > wording of Part 97 > reveals that the regs define the pep figure as "under normal > conditions". > Most of the time the voice remains at a fairly low value of > voice peaks > (about 30% modulation), with a few reaching 100% while an > occasional peak > attempts to go sky high. These exceptional peaks add next to > nothing to the > total sideband power or the interference potential of the > signal. It's > where the majority of peaks go, along with the modulation > density of the > audio that determines how loud a signal is. That is what could be > interpreted as "under normal conditions". > > One advantage of modulating down to just below 100% negative > and letting the > positive peaks go where they may is a cleaner signal. With > limited positive > peaks (most transmitters are not capable of modulating much > more than 100% > if even that much), the temptation is to modulate to 100% in > the negative > direction, and let the occasional high positive peaks clip off at the > maximum positive modulation capability of the transmitter. > It is very > difficult to even see the positive peak clipping on the > envelope pattern of > a scope unless you drive the audio so high that a large > percentage of the > dominant peaks are flat-topped. If a transmitter is capable > of 150 or 200% > positive peaks to accomodate the occasional wayward peak > (comprising less > that 1% of the total number of positive peaks), there will be > no splatter > resulting from the hard clipping of these miniscule peaks. > When these peaks > are clipped, as in the case of 99% of the AM transmitters on > the air, both > amateur and broadcast, they add to the spurious sideband > components of the > signal, even though the clipping effect may otherwise be > inaudible and > invisible on the envelope pattern. > > A disadvantage of high positive peak capability is low > modulator efficiency. > A class-B amplifier's efficiency is directly proportional to signal > amplitude, reaching a maximum at peak output capability. If > the modulator > is capable of delivering peaks at 200% modulation, but you > run it at about > 100% most of the time, the average efficiency will be much > less than that of > an amplifier capable of delivering only 100% at its peak > output. However, > the duty cycle of human speech is so low that this will make > very little > difference in the ratio of overall 60~ a.c. power input to > r.f. output > delivered to the antenna. > > One of the early double-sideband schemes just after WWII > described in QST by > Villard employed the "upside-down tube" circuit. Although this was > exploited later to make the signal louder by transmitting DSB reduced > carrier, keeping the DC input within the 1 kw limit while > generating several > kilowatts of sideband power, the original intent of the > scheme was to take > care of occasional negative overmodulation peaks without > generating suprious > sideband products. The article was entitiled "overmodulation without > splatter". The sole purpose of the original proposal was to > clean up the > signal back in the days when few hams had monitor scopes or > fully understood > the principles of modulation, and the phone bands were full > of trash from > overmodulated signals. > > I think some AM'ers are overly paranoid about p.e.p. I > wonder if these same > folks are as cautious about going even 1 mph over the posted > speed limit > while driving their automobiles; certainly driving a 4000-lb. > machine beyond > the legal speed limit on a public highway causes greater > endangerment to the > public than hitting a few positive peaks beyond 1500 watts > on a ham rig . > > The fault really lies with the FCC (Johnny Johnston and his > cronies) for > coming up with such a foolish power limit rule for amateur > radio in the > first place. Common sense would have dictated AVERAGE (mean) > power output. > > Don K4KYV > > _________________________________________________________________ > Has one of the new viruses infected your computer? Find out > with a FREE > online computer virus scan from McAfee. Take the FreeScan now! > http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
