[AMRadio] AM Transmitter Advice??

Donald Chester 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.

Don k4kyv

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