> The first rule was, and is, "thou shalt not overmodulate."
So fuzz-boxes are immoral?
Dave, All.... this has been bugging me all day. I can't think of any techniques other than the ones I mentioned, and for quick clean GR it really comes down to grid-control.
There is one other: there is a way to wire a pentode as a poor multiplier. And I think it is to put some negative voltage on the suppressor grid (normally tied to cathode, in many tubes it IS tied to cathode internally). The Gm is nearly linear with G3 voltage. Tying G1 and G2 together gives nearly "squared" plate current. Never seen this done in audio. And frankly, I would call it just another grid-volt technique.
Same with Pentagrids, used as frequency changers in radios. Some of them let you put signal in one grid and control gain with another grid, which sure simplies push-pull drive (don't have to handle GR voltage at the signal grids). Still just grid-volt control.
There is yet another trick, though it seems to have arisen very late 1950s. There is a special beam tube that can be used to demodulate FM by switching current from one plate to another. The idea could be used to steer audio to the output plate or a dummy plate. But for best FM detection it needs to switch very abruptly, and the available parts don't seem to be useful.
One wacky way I never saw: set up a CRT, with two metal areas on the faceplate. Modulate the cathode current with audio. Use electrostatic deflection plates to steer the beam to one plate or the other. Use a defocused beam to get a soft transition. But the bean current in a CRT is very low, so the available output power may be too small to use. (Similar things were tried as computer memory, and generally frustrating because of instability.)
Given film-sound shutters (which can move pretty fast), it should be possible to modulate light with audio, modulate again with AGC, and detect that in a phototube. But phototube sound is generally bad enough even before you play games to reduce it. And most such modulators acted very badly in overload.
> Diode clamping today. These were not meant to sound good, just save the transmitter and get the message across.
In low-quality message systems, GROSS distortion is not a problem. Over 100% THD sounds funny, but after a minute you can understand speech just fine. In fact for a given power limit, grossly clipped speech with some spectrum shaping filters can be understood at much greater range than clean speech. In classic AM transmitter design, overmodulation (especially negative) isn't just clipping, but throws sidebands all over adjacent and distant channels. (It could also cook the transmitter, though that's not so easy on commercial gear.) The B-town cops hear monkey-chatter when A-town's cops splatt their transmitters. That's illegal. Clipping in the audio path then low-passing at 3KHz gives "clean clipping" that gets-out without crapping-up other channels.
