Trinkaus.qxd 1/11/2006 1:04 PM Page 68 The Magnetic Amplifier A Lost Technology of the 1950s Anyone can build it! ■ by George Trinkaus ost folks believe that first came the vacuum tube and right on its heels came its successor, the transistor — an historical fact, correct? Not really. Another competitive control technology developed by US and Nazi engineers came in between. It was the magnetic amplifier. Rugged, dependable, EMP- proof, and capable of handling greater electrical powers than either transistor or tube, the magnetic amplifier is a simple device that can be built by anyone. By the 1950s, the magnetic amplifier M was not just an experimental dream languishing in some inventor’s notebook. Nor was this ingenious technology sitting unexploited in patent archives. The mag amp was in manufac- ture in a number of versions and had a clique of boosters, including many electronics engineers, especially within the US Navy. ■ Home-built Mag Amp. 68 February 2006 Trinkaus.qxd 1/11/2006 1:05 PM Page 69 ■ FIGURE 1. Principle of Operation. ■ FIGURE 2. Saturable Reactor. The mag amp is an American that’s the definition of an “amplifier.” core’s permeability (its receptivity to invention and has been used in heavy A mag amp can be put in series magnetism) can be varied by degrees, electrical machinery regulators since with any circuit carrying an alternat- thus controlling a larger AC flow. 1900. In the 1940s, the Germans took ing current and control that flow. No Fully energized, the control coil the American’s relatively crude device, external power supply is required to can reduce the permeability of the assigned their best scientists, invested run the device. The simple mag amp core to zero, in which case the core is millions, and developed it into a faster, is just a core of iron or ferrite with said to be saturated. Then it becomes lighter electronic tool competitive with some coils of wire wound around it. so magnetically unresponsive it’s like the vacuum tube in performance but One other basic component is the core has been removed. more simple and dependable. It’s also the rectifier. Today, rectifying diodes Figure 1 is a way of showing the much more rugged. A mag amp can be are compact, easily available, and principle. With the core completely with- made to be nearly indestructible. cheap. The old selenium rectifiers in the coil, the impedance to the flow is The Germans used the mag amp used back in the 1950s were large, high, permitting perhaps only a fraction in electric brakes for trucks, street- cumbersome, and expensive. of a volt to appear across the load. cars, and locomotives. They used it A variety of ferrite core materials Pulling the core out causes the load volt- for high-voltage utility-power controls are also available to today’s builders. age to rise progressively to 115. Since it and even for early computers. With some spools of wire, a ferrite took only a few watts of muscular energy Appreciating its indestructibility, rod, and a couple of diodes, you can to move the iron core within the coil, the Nazi military used it in gun stabi- throw together a little high-frequency which may, in turn, control several lizers, in automatic pilots, and in mis- mag amp on a Sunday afternoon. horsepower, the device is an amplifier. sile-guidance, including the rocket sta- Compare the construction chal- Figure 2 is another demonstration. bilizer and steering systems of the V-2. lenge of a vacuum tube or transistor. This qualifies as a saturable reactor. After the war — like German rock- And the mag amp can handle voltages This circuit could be for a dimmer for etry itself — the mag amp emigrated to and currents that you would never put theater stage lighting. Add a diode, and the US, where it got further development into the average transistor or tube. you have a basic mag amp (see Figure by enthusiastic American engineers. By 3). The larger coil is the control coil. 1951, a Navy engineer could write, How it Works The smaller is called the loading coil. “Electronics engineers are now forced to The diode rectifier makes the load concede recognition of the magnetic The mag amp is a sort of variable current unidirectional, which assists amplifier, as it has demonstrated its choke. It controls the impedance the control winding in saturation. value beyond question in many fields (opposition) to alternating current in Considerably less power is now required, dominated by the electron tube.” a coil by controlling the magnetic making it a more potent amplifier. condition of the core on which the coil This mag amp, however, will Simplicity is wound. This is done by energizing function as a step-up transformer, another winding on the core called a which would be undesirable since it The mag amp, like the vacuum control coil. tube and transistor, is an electrical Depending on the ■ FIGURE 4. Functional Mag Amp. control valve. When a smaller circuit energy in the controls another circuit’s larger flow, control coil, the ■ FIGURE 3. Mag Amp. February 2006 69 Trinkaus.qxd 1/11/2006 1:06 PM Page 70 ■ FIGURE 5. Mag-amp Audio Amplifier (push-pull). would send energy back into the uses in the control circuit. This effect is cleverly control and regu- ■ FIGURE 6. GE Mag-amp Modulator by Alexanderson. cancelled by running the AC through lation of power a pair of parallel loading coils which utilities and big electric motors, as in alternators he was using for high-power are wound in opposite directions. locomotives, but its most fascinating radio transmitting (at 10,000 to 100,000 Figure 4 is your basic functional applications — mostly forgotten — cycles). The Magnetic Amplifier mag amp represented by the appropri- are in electronics. Bibliography (by the US Navy, 1951) ate schematic symbols. The control The mag amp can modulate, switch, lists three Alexanderson patents in 1916 coil symbol is a single sharp angle- invert, convert, multivibrate, audio- and three more in 1920, the last titled line, but the control coil actually has amplify, radio-amplify, frequency-shift, “Transoceanic Radio Communication.” many more turns than the loading coil. phase-shift, and multiply. Stages can be The mag amp can turn the How many turns? The rule of thumb cascaded. Simple feedback techniques alternator on and off for telegraphy is control-coil ampere-turns equals enable gains in the millions. and vary the signal for speech loading-coil ampere-turns plus suffi- The mag amp can even compute. modulation (see Figure 6). cient extra turns to saturate the core. Trouble-proof magnetic binaries The frequency limits of an alterna- (Much of the how-it-works above replaced the less reliable vacuum tubes tor are low, so the mag-amp was rein- is from Magnetic Amplifiers by the US used in some early digital computers. vented in that era as a frequency multi- Navy, 1951, recently republished.) Figure 5 shows the incredibly sim- plier (doubler, tripler), as seen in Figure ple circuit for a mag amp audio amplifi- 7. The Bibliography cites many radio- Uses er.Mag-amp audio would be a challeng- transmitter frequency-multiplier patents ing pursuit for some adventurous up through the 1920s. These are simple The mag amp still has industrial audiophile. But the mag-amp electron- circuits compared to those of vacuum- ics which engaged this writer is in radio. tube frequency changers that came later. ■ FIGURE 7. Frequency Multiplier. Early mag amps with solid iron Mag Amps in Radio cores never got above a few hundred kilocycles. Powdered-iron cores, the The first patent for a mag amp was ceramic-iron-oxide composition in 1903, but little attention was paid known as ferrite, and later the ultra- until 1916 when radio pioneer E.F.W. thin magnetic tapes liberated the mag Alexanderson seized on the idea as a amp, so by the 1950s the limit was up means of controlling the giant rotary to a megacycle and switching rates were in microseconds, suitable ■ FIGURE 8. Microwave Mag Amp. then for computer applications. Techniques for the modulation even of microwave frequencies were also developed in the 1950s (see Figure 8). My Home-built Mag Amp I wanted to see if a mag 70 February 2006 Trinkaus.qxd 1/12/2006 12:51 PM Page 71 Amazing Devices www.amazing1.com amp could modulate a Tesla coil (see George Trinkaus’ High Voltage Figure 9), as Alexanderson modulated Press is at teslapress.com Laser Modules his big alternator-transmitter. I used All laser modules operate from 3 volts and include built in optics providing a parallel beam of 1mr or less. Includes instructions on the schematic in Figure 4. The Navy the 1N4008s with two low-power sig- safety requirements for FDA full compliance Red - Class IIIa booklet, Magnetic Amplifiers served nal diodes. In series with the control LM650P3 - 3mw 650 nm12 x 45 mm ........ $14.95 as a reference. coil, I put a 12-volt battery and a tele- LM650P5 - 5mw 650 nm 12 x 45 mm ....... $24.95 LM630P3 - 3mw 630 nm 10.5 x 45 mm .... $34.95 I first obtained a ferrite rod graph key, as a convenient switch. The Red - Class IIIb LM650P10 - 10mw 650 nm 12 x 51 mm ... $99.95 (material #33), six inches by just mag amp is frequency-specific; you LM650P30 - 30mw 650 nm 12 x 51 mm . $249.95 under 1/2-inch diameter. I got it design it for a particular range. Keyed Green - Class IIIa LM532P5 - 5mw 532 nm 12X45 mm .......