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The General Electric Phasitron FM Transmitter

In the early 1950s, many saw the emergence of FM on a new band (88–108 MHz) as a waste of time and money. Only a few visionaries, people who were ready to try different paths, create new audiences and serve new needs, succeeded with FM.

Is the glass half full or half empty? This test divides the optimists and the pessimists among us.

Those rare few who see adventure and opportunity in every choice, question or direction view the glass as mostly full. Right now, many view the H2 channels in the new digital FM signal as akin to a few drops of water in the bottom of the glass, academically interesting at best, an annoying distraction at worst.

In the early 1950s, many saw the emergence of FM on a new band (88–108 MHz) as a waste of time and money. Only a few visionaries, people who were ready to try different paths, create new audiences and serve new needs, succeeded with FM.

Transmitter shown with door open, WBJC. Edwin Howard Armstrong developed wide-band FM technology — literally the whole concept, from mic input to speaker on the receiver — but World War II and a move from one band to another almost put the kibosh on this fledging medium.

Finally, with the adoption of FM sound for TV and the availability of receivers (sound familiar?), it looked like an open field run for FM.

Armstrong, like Allen B. Dumont in television, viewed his role as more of an entrepreneur than a technocrat. He realized that the bigger the audience, the bigger would be the returns on his patents and the greater the positive use of his work.

To further this universality, Armstrong made many bold moves. He established a network of stations utilizing FM’s superior fidelity, chaining them together via off-air pickup and garnering a large potential audience.

He also allowed “educational stations” the use of all his FM patents for $1 a year. A new concept in broadcasting, these stations had been given the “reserve” portion at the bottom of the FM band (88–92 MHz).

Package deal

This generosity created another opportunity, this time for broadcast equipment manufacturers.

Suppliers moved quickly to sell “station packages” to educational institutions nationwide. Such licenses ranged from the flea-powered classic Class D allocation, a 10 watt authorization, to parity with the biggest mega-power commercial broadcaster at 400,000 watts ERP.

The block diagram from the GE instruction book. Notice the diode demodulator (discriminator) using a sample from the multiplier chain that is used as part of a negative feedback concept to reduce audio distortion. The ‘complete loop’ notion was found even in AM transmitters of the time such as the RCA tube version of the Ampliphase. At the simplest end was a combination mixer board and 10 watt transmitter along with two turntables and a few mics. This package would allow you to create great radio for your school’s campus with the bonus of adjacent neighborhood coverage.

Somewhere in the middle was a popular sort of mid-power package such as we had at WBJC(FM) in Baltimore. GE supplied a 250 watt transmitter, combination mono modulation and frequency monitor, one-bay halo antenna, 200 feet or so of coax, a mixing console, two turntables and a mic. When it was over and done, the 125 watts of ERP provided Baltimore-area coverage from the tower parapet of City College across from the old Memorial Stadium.


GE was confident that it was supplying a superior transmitter product, as it had beaten the frequency stability problem of FM by using direct modulation.

The heart of this solution was its proprietary, legendary Phasitron FM modulator tube, GL-5593.

The Phasitron proportionately frequency modulates an ultra-stable, crystal-generated, very low subharmonic of the output signal, then multiplies this up 432 times to the FM channel of interest. With this high multiplication, at the output, the signal becomes frequency modulated +/- 75 kHz.

This modulation is introduced using essentially TV deflection concepts, electromagnetically influencing the phase/frequency of the frequency source inside the Phasitron tube using a surrounding magnetic coil. That influence of the source is the audio of interest varying the magnetic field.

The GE exciter — essentially a complete 10 watt transmitter — did not need any of the complex, elaborate automatic frequency control (AFC) extant in other products that used a reactance (phase) modulator.

By 1962, GE had made about 300 BT-1-B 250 watt transmitters, such as we had at WBJC, and 260 of these were still operating as mains, according to a letter from GE at the time. The mechanical designer was Ross A. Lash and the electrical designer was W.F. Goetter. H.B. Thomas was the designer of the Phasitron circuit used in the exciter.

Late 1940s WFBL(FM) in Syracuse sports a GE Phasitron FM transmitter, with 250 watt ‘exciter’ at left and 3 kW amplifier at right. Stations were manned by First Phone operators, hence the console. WFBL was in a Quonset hut; notice the curved wall. Steve Auyer, from whose collection this image is taken, said the station went on in 1947 but failed to make money and the owner returned the license three years later. It is now WNTQ. Photo courtesy Steve Auyer via Mark Humphrey. GE viewed itself as a source of both transmitters and tubes, so it also offered the Phasitron to other manufacturers including Collins, AEL, REL and Federal, according to the John F. Rider publication “FM: An Introduction to Frequency Modulation.”

Overall the GE exciter was elegant in simplicity, a reliable performer with superior fidelity. Unfortunately the Phasitron’s electromagnetic concept was not equally effective above 20 kHz of audio input. With stereo and the introduction of the cost-effective varicap diode, the Phasitron went the way of the dodo.

The last Phasitron transmitter I encountered was a 250 watt unit that was serving as a standby for a Los Angeles radio station in 1970; it was still type accepted and licensable at the time.

FM’s dominance of the radio marketplace today makes it difficult for many to realize that it took a 50-year meandering march from near-oblivion — with many owners turning in their licenses as worthless — to the present, when some licenses are worth hundreds of millions.

The Phasitron and these GE rigs gave FM broadcasters with vision a tool to make reliable, hi-fi radio a success.

These rigs were not the only ones with personality and a story. Please share with us your tales of early FM. E-mail the author c/o Radio World at [email protected]. Past Milestone columns are archived at

Sources used in this story include the following. For information concerning the Phasitron, visit About Armstrong: Also see Lawrence Lessing’s book “Man of High Fidelity: Edwin Howard Armstrong” and “FM Atlas” by Bruce F. Elving.