Hey Geoff: Since there's already a screen at hand, why not use it to your advantage in this case by making it variable to allow clean adjustment of output power? It just seems like what the doctor ordered for driving a linear, that's all.
As for your second question, I should have clarified that I was not thinking of a simple phase inversion aimed at impressing the highest voice peaks to the negative modulation direction, (the primary idea of the article) but taking it to an extreme of radically reducing the positive peaks by some means (such as very agressive positive peak clipping, or unplugging the positive tube in your push pull modulator!). Of course, simply inverting the phase in and of itself will not hurt quality in any way. But while aggressive positive limiting of some sort would allow higher carrier power before reaching the PEP limit of 1500watts, it will also increase distortion. How bad or tolerable it might be depends on a lot of variables and the limiting techniques employed. If you just want the most intense 'communications quality' result possible from the rig at hand, then it might make some sense. But in the real world, how much of a potential benefit is at stake here? Even wildly asymmetrical voices aren't going to buy more than several dB relative difference between positive and negative peak voltages, an amount that can easily be made up for with modest audio limiting. Since some sort of negative peak limiting should be used anyway to protect from carrier pinch off, some amount of that asymmetry is going to be given up right there. Finally, if your voice doesn't happen to be wildly asymmetrical, you're out of luck anyway. A lot of AM hams don't seem to use any negative peak control other than the mic gain pot, and many don't even have a scope to monitor for carrier pinch off, so a lot of this is like counting pixies on the head of a pin. FWIW my prejudice is looking at this as a broadcast engineer, which may be a bit different than some AM ops. Not better, just different. That prejudice steers me toward high audio quality, consistently very high average modulation levels (loudness) being almost always more useful than modest increases in carrier power, and a paranoid fear of negatively overmodulating. I admit to impatience with low power stations that do not agressively modulate to make up for it, which is common. Sorry. My object is to rattle the speaker on the other end, and make the station easy to listen to no matter what power level is in use. But, clearly hams can operate successfully without concerning themselves with any of this and still have a ripping good time. We're all looking for our own buzz. g Geoff wrote: > > >...I'll be the first one to admit that I'm 'weak' when it comes to > pentode/tetrode design/operation. I like triodes. Their easier > to work with, and require fewer power supplies. Less can go wrong. > > > ...Why wouldn't it sound as good? You've just reversed the 'phase' > of the audio if you, say, switch the grid caps on the modulators, > or switched the plate caps on the modulators, even reversing the > polarity of the microphone would have *basically* the same > effect. Yes, your positive peaks would reduce, and you can run > the carrier level back up. > > At 1500wPEP output (as John so eloquently described in his > article) with his rig and voice, he would have to keep his rig at > 220w input (around 160w of carrier out) to keep within the 1500w > limit. Inverting the audio phase, he could probably run 1000w of > carrier, with PEP audio to 1500w, still have the same QUALITY of > audio, -and- probably be heard better, due to the lack of > interference from the 160w carrier, to the 1000w carrier. > It just wouldn't sound -as loud-.
