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Alnico permanent magnets are made from an alloy containing varying percentages of iron, aluminum, nickel, cobalt, copper and sometimes titanium. It is produced in different grades, which offer increased magnetic strength (maximum energy product) and resistance to demagnetizing forces (coercivity). Alnico offers the best temperature stability of any standard magnet material produced, but it is also the most susceptible to demagnetizing forces. It is often selected for modern products that must operate at extremely high temperatures. In guitar speakers, alnico is usually selected to help recreate the sound of ‘50s and ‘60s blues, jazz and rock and roll music.
Electrodynamic Loudspeaker History
One of the first modern-style (electrodynamic or “moving coil”) loudspeakers, the Magnavox, was demonstrated in 1915 by Edwin S. Pridham and Peter L. Jensen. Their 1920 patent application (US Patent 1448279) describes “...an annular coil rigidly connected to a diaphragm. This coil is disposed, so as to be freely movable, in a strong concentric magnetic field produced either by a permanent or an electromagnet.” Today, the vast majority of speakers use permanent magnets; however, in the 1920s the permanent magnets available were relatively weak, and those capable of producing a strong magnetic force were costly and difficult to make. Most early electrodynamic loudspeakers used an electromagnet (also known as the field coil).
Electromagnet vs. Permanent Magnet
An electromagnet is composed of a coil wrapped around an iron core. When DC current passes through the coil it generates a steady magnetic field. Electromagnets may be thought of as temporary magnets that lose their magnetism when power is shut off, while permanent magnets do not require an electrical power source to remain magnetized.
Alnico Magnet Development
In the early 1930s, most of the world was in the depths of the economic downturn known in the US as the Great Depression. Early patents relating to the development of alnico express their objective to provide a permanent magnet composed of relatively low-cost materials, which could be fabricated inexpensively and have superior magnetic characteristics.
In US Patent 2027994 (Applied For: 1/20/1932; In Japan 3/09/1931), inventor Tokushichi Mishima of Japan explains how a strong permanent magnet comprised of nickel, aluminum and iron could be produced economically and with superior magnetic characteristics to the “magnet steels” available at the time, including tungsten, chrome, and chrome manganese. Months later, he added to this the discovery that the addition of cobalt could further improve the magnetic characteristics of the alloy, as well as improving its tenacity and ductility (US Patent 2027996).
An advertisement from QST amateur radio magazine (April 1945, p. 75)
In US Patent 2295082 (Applied For: 6/29/1939; In Germany 12/06/1938), inventor Gottfried Bruno Jonas of the Netherlands explains that an alnico alloy in the anisotropic form can yield a permanent magnet with a 50% to 200% higher maximum energy product than the isotropic version. This is the discovery behind what would become known as Alnico 5. This application was patented on Sept. 8, 1942 in the United States.
Alnico had been developed into a robust permanent magnet by 1939. In that same year, Charlie Christian joined Benny Goodman’s band and is credited with popularizing the electric guitar as a lead solo instrument on par with the trumpet and saxophone, but his Gibson EH-150 amplifier used a field-coil speaker. It was not until about 1947 when Gibson and Fender began stocking their guitar amps with alnico speakers. What could account for such a delay?
Don’t you know there’s a war on?
The Nazis invaded Poland in 1939 and WWII began in Europe. In January of 1942, a month after entering the war, the United States government established the War Production Board to regulate the production and allocation of raw materials. Basic metals including steel, copper, and aluminum were heavily regulated to supply military demand, and only essential civilian products would be allocated a percentage of these metals. In order to stay profitable, many American companies had to shift their production to supply parts and equipment the military needed. For example, Gibson made radar assemblies, glider skids for airplanes and precision rods for use in submachine guns. It would not be until after the war ended in 1945 that many peacetime products could make full use of alnico, the miracle metal.
Kurt Prange (BSEE) is the Sales Engineer for CE Distribution, LLC, in Tempe, AZ (jensentone.com). Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.