[QUOTE The fact that this happens on all three transmitters makes me think it's an inherent property of AM plate modulation of vacuum tube amplifiers. I need to brush up on vacuum tube physics, but I think the varying B+ changes the distribution and density of electrons within the tube, varying the plate-to-grid capacitance and thus the phase shift of the amplifier.[/QUOTE] Bingo! Indeed the phase shift of a tank circuit changes with the resistive loading, (and reactance) which is constantly changing in a plate modulated stage. The FCC never specified incident phase modulation on an AM transmitter, since nobody had any means to measure it until quite recently. And it has no audible effect on a conventional demodulator. (The FCC, on the other hand DOES have severe limits on the incidental AM on an FM transmitter!) Where this REALLY has a nasty effect is on NTSC video transmitters. The chroma phase can depend a great deal on the underlying luminence...and the effect is called differential phase. Incredibly complicated and convoluted circuitry is generally used to compensate for differential phase. Or at least WAS, when analog TV was still around.