■ FIGURE 5. Mag-amp Audio Amplifier (push-pull).
would send energy back into the
control circuit. This effect is cleverly
cancelled by running the AC through
a pair of parallel loading coils which
are wound in opposite directions.
Figure 4 is your basic functional
mag amp represented by the appropriate schematic symbols. The control
coil symbol is a single sharp angle-line, but the control coil actually has
many more turns than the loading coil.
How many turns? The rule of thumb
is control-coil ampere-turns equals
loading-coil ampere-turns plus sufficient extra turns to saturate the core.
(Much of the how-it-works above
is from Magnetic Amplifiers by the US
Navy, 1951, recently republished.)
■ FIGURE 6. GE Mag-amp Modulator by Alexanderson.
The mag amp still has industrial
uses in the
control and regulation of power
utilities and big electric motors, as in
locomotives, but its most fascinating
applications — mostly forgotten —
are in electronics.
The mag amp can modulate, switch,
invert, convert, multivibrate, audio-amplify, radio-amplify, frequency-shift,
phase-shift, and multiply. Stages can be
cascaded. Simple feedback techniques
enable gains in the millions.
The mag amp can even compute.
Trouble-proof magnetic binaries
replaced the less reliable vacuum tubes
used in some early digital computers.
Figure 5 shows the incredibly simple circuit for a mag amp audio amplifi-er.Mag-amp audio would be a challenging pursuit for some adventurous
audiophile. But the mag-amp electronics which engaged this writer is in radio.
■ FIGURE 7. Frequency Multiplier.
Mag Amps in Radio
The first patent for a mag amp was
in 1903, but little attention was paid
until 1916 when radio pioneer E.F.W.
Alexanderson seized on the idea as a
means of controlling the giant rotary
alternators he was using for high-power
radio transmitting (at 10,000 to 100,000
cycles). The Magnetic Amplifier
Bibliography (by the US Navy, 1951)
lists three Alexanderson patents in 1916
and three more in 1920, the last titled
“Transoceanic Radio Communication.”
The mag amp can turn the
alternator on and off for telegraphy
and vary the signal for speech
modulation (see Figure 6).
The frequency limits of an alternator are low, so the mag-amp was reinvented in that era as a frequency multiplier (doubler, tripler), as seen in Figure
7. The Bibliography cites many radio-transmitter frequency-multiplier patents
up through the 1920s. These are simple
circuits compared to those of vacuum-tube frequency changers that came later.
Early mag amps with solid iron
cores never got above a few hundred
kilocycles. Powdered-iron cores, the
known as ferrite, and later the ultra-thin magnetic tapes liberated the mag
amp, so by the 1950s the limit was up
to a megacycle and switching rates
were in microseconds, suitable
then for computer applications.
Techniques for the modulation
even of microwave frequencies
were also developed in the
1950s (see Figure 8).
■ FIGURE 8. Microwave Mag Amp.
I wanted to see if a mag