Dave Platt wrote:

It doesn't take all that much of a drop in voltage to result in a
visible change in the brightness of an incandescent-filament light.
According to one site I've found, light output is roughly proportional
to the voltage raised to a power between 3.1 and 3.4. A 5% drop in
voltage would thus result in a decrease in light level of more than
15%, which would be pretty easy to notice.

If you were transmitting 100 watts CW key-down, I'd guess that your
transmitter was probably drawing at least 300 watts from the
mains, and perhaps rather more depending on its design.

Add a few more likely or possible elements e.g. a linear power supply
which tends to draw most of its current from the mains during the
voltage peaks (and thus causes a disproportionate amount of voltage
sag on the lines during those peaks), insufficiently-thick house
wiring in the walls, and perhaps a slightly loose or corroded
connection to one of the outlets or switches or breakers, and I can
well believe that keying a 100-watt CW transmitter could cause enough
voltage drop to result in visible dimming of the lamp bulb.

A friend of mine back in the 70's had me come over to listen to his new
stereo. It was a Phase Linear system as I recall. The speaker cabinets
had 9 components each in them. We put "Hall of the Mountain King" on
the turntable and turned up the level to the point just before it began
to distort. The power amp was pretty substantial and I noticed the
lights were dimming to the music. I went outside and could see the
power meter change speed in time to the music. And this was in an
industrial building. That was the loudest home stereo system I ever
heard. Probably the best sounding as well.

ac6xg