|[AMRadio] AM Transmitter Advice??|
k4kyv at hotmail.com
Wed Feb 15 17:41:44 EST 2006
> > I have heard from many AM ops that with the correct setup, a
> > linear amp works great.
Linear amps pre-date high level class-B plate modulators by many years. In
the early days, the only kind of plate modulation that was known used
class-A audio amplifiers. This usually was in the form of Heising "constant
current" modulation, although "series modulation" was also sometimes used.
Since the class A modulator worked at such low efficiency, better overall
efficiency was attainable with linear rf amplification. The original
class-B ratings for tubes was intended for rf linears.
Sometime in the late 1920's, it occurred to someone that class-B linear
amplification would work just as well for audio as for rf, and the class-B
audio amplifier was developed. The unusual thing about this setup was the
large audio transformer required. Many rf linears were single-ended, and
depended on the fiywheel effect of the rf tank circuit to supply the missing
half of the sinewave output. With class-B audio amplification, the tubes
have to be in pushpull in order to reproduce both halves of the sinewave.
Thus the well-known "class B modulator" came into being.
Some of the earliest AM broadcast and ham transmitters, beyond the simple
modulated oscillator, used linear amplifiers for the final. High level
class B modulators didn't come into common use until about 1932 or 1933.
There is an article in an early 30's QST describing how to build your own
modulation transformer to use in a "new" class B modulator.
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