Commercial AM transmitters have changed a *heck* of a lot since I was chief
engineer of WWHG AM-FM. I am not familiar with what they use now other than
most of them are using switch mode amplifiers (which can run 90%
efficiency). Even though they are either on (saturation) or off (open),
there are means available to effectively "modulate" them. I sure wish I had
saved that link two years ago which described a switch mode amplifier. A
kit was available from a university in California for ham operators. It was
not, however, linear and had means to control the leading and trailing edges
of the signal when turned on or off. It was only good for cw or fm, but ran
around 92% efficiency. Due to this, there is a lot less heat generated than
what used to be generated. (the amps in the commercial stations *are*
linear)

As to power, 50,000 watts means a 50,000 watt rms carrier. At 100%
modulation, if you consider that under high level modulation, one has to
supply an audio signal that will exactly reach cutoff, that audio signal
will have to be equal (and exactly cancel the B+ to the final at the
negative peak of audio) in voltage to the power supply. This means that the
plate voltage is effectively running (under modulation) between zero and
twice power supply voltage. This implies that the peak envelope power will
be reached at the peak positive swing of audio modulation, at which time
plate voltage will be twice normal. Since power equals voltage squared
divided by resistance (and the resistance hasn't changed), the power at
twice voltage will be four times normal (unmodulated carrier). Thus, 50,000
watt rms carrier will be modulated to a 200,000 watt peak envelope power.
Radio stations will not reach 100% modulation; there are various limiters in
the lines to keep it a few percent below 100% max for obvious reasons. I've
never worked on one of those blow torches, but I've managed to get a 5,000
watt rf burn. Not recommended at all. Takes a long time to heal and
burning flesh does not smell nice at all BTW, it tore a nice hunk of
skin out of my finger and cauterized it instantly. No bleeding. Man, that
hurt like heck, stunk like heck, and was most embarrassing!

73 from Rochester, NY
Jim


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