Last month we looked at radio at the start of the Great Depression and how RCA’s President David Sarnoff made the decision to license the heterodyne circuit to any manufacturer willing to pay royalties to use it. The heterodyne circuit, combined with the new developments in vacuum tube technology, allowed manufacturers to build affordable radios that performed far better than most of the high end sets from just a few years prior.
Pictured here is arguably one of the most iconic vintage radios of all time. The Philco Model 70 in the cathedral cabinet is often seen as a movie or TV prop when an antique radio is called for. The model 70 chassis was offered in several different cabinet styles including consoles and even a full size grandfather clock outfitted as a radio. The radio has a label on the back stating that it is "licensed for broadcast reception and amateur use under Radio Corporation of America (RCA) patents," as were all heterodyne receivers manufactured at the time. Philco was not considered a "high end" radio company, but none the less, the new heterodyne radio would have put some stress on the finances of a depression period worker. It sold for $49.95. Not bad though, for a large table radio in an elegant cabinet that would have outperformed the $179 RCA Radiola Model 46 featured in my May 2014 article. The radio would have been a good choice for a middle class wage earner lucky enough to still have a job.
The first thing that the owner of a new Philco Model 70 would have noticed was the vast improvement in selectivity(note 1) over the typical three stageTRF (note2)receivers of the 1920s. Now it was easy to separate stations as the band became crowded with many new stations. The set used the same type tubes as the Radiola Model 46, but now configured as a heterodyne receiver. One exception being the audio tube, where Philco replaced the type 45 triode with the new type 47, a power amplifier tetrode, having about twice the output power. Philco still recommended an outdoor long wire antenna for best reception, but the set will perform well when refitted with a modern loop antenna concealed in the cabinet. Soon almost all new AM broadcast receivers would come with built in antennas.
I consider the vacuum tube to be the most substantial invention of the early electronic age. As far as radio was concerned, the heterodyne circuit was the most substantial development that used the vacuum tube.
So how did this new circuit work? Bear with me if I get a bit technical, I promise to keep it short.
The principle behind a heterodyne circuit is relatively simple. Viewing the chart above, it uses a vacuum tube to amplify the weak radio frequency (RF) signal from the antenna. Next a tube is used to mix the amplified RF signal with a slightly different frequency created by an oscillator (note 3) circuit in the radio. Four distinct signals will appear at the mixer tube’s output, each of the input signals, plus the sum and difference of the signals. For example, let’s say that you want to listen to a station on 800 kHz. You point your dial to 800, but what you have actually done is to tune your radio’s oscillator to 1060 kHz. This signal is mixed with the 800 kHz signal from the station and the output produces four signals, 800, 1060, 260 (the difference) and 1860 kHz (the sum). Next these signals are fed into a filter that will only pass the 260 kHz signal. (note 4) The 260 kHz signal, known as the intermediate frequency, or I.F. is then greatly amplified and sent to the detector which extracts the audio content. So what does this accomplish? It makes it very easy for the receiver to reject adjacent stations. The heterodyne circuit offers vastly improved selectivity over the TRF or the Regenerative circuits.
This schematic includes a detailed explanation of how the model 70 works.
Not every radio manufacturer took advantage of licensing the RCA heterodyne patent. The Grigsby Grunow Company, a major player at the time, countered with somewhat massive eight tube, 5 stage tuned radio frequency (TRF) set, the Majestic Model 90. It performed just about as well as the $49 Philco, but it cost $137.50. Not the best approach in a depression. They went bankrupt in 1933, a result of both their attempts to market expensive radios and being heavily invested in the stock market. (*1)
Atwater Kent, then the largest US radio manufacturer, was a privately held and suffered little at first when the market crashed. The owner, Mr. Kent, released a letter shortly after the crash to assure suppliers and dealers that the company was not invested in the stock market and was on firm ground. By now his 15 acre radio factory had grown to more than 30 acres. He had disdain for the concept of paying royalties to use a competitor’s (RCA) circuit. He also refused to compromise and produce what he referred to as "cheap, low quality radios." He continued to try and sell "Cadillac" radios in a "Volkswagen" market. His plant closed in 1936, putting thousands out of work in Philadelphia. I’ll spend more time on Atwater Kent in the future. (*2)
The price of the heterodyne receivers continued to drop. By 1939, Emerson was selling small table radios for as little as $9.95. The heterodyne circuit spurred the development of new tube designs that further simplified and improved performance. Tubes that would make radios small and portable, the beginnings of being truly "wireless."
Emerson, Zenith, Silvertone and countless smaller companies adopted the heterodyne technology, allowing them to produce cost effective radios and survive the Great Depression.
Next month we will look at radio journalism with the coverage of the Hindenburg disaster, and the rising popularity of international short wave broadcasts.
Note 1. Selectivity-The ability to separate one nearby station from another.
Note 2. TRF- The Tuned Radio Frequency circuit was the circuit of choice for most radios during the 1920s. Several of my past articles including A Rhode Island Radio explain the circuit in more detail.
Note 3. Oscillator circuit. A circuit which creates a low power signal at a specific frequency. In a radio, the oscillator is tunable to a desired frequency.
Note 4 . The model 70 uses 260 kHz filters and intermediate frequency amplifiers. Most modern AM radios use 455 kHz for this purpose. FM radios usually use 10.7 mHz for this purpose.
(*1) You can read quite a bit about the Grigsby Grunow company’s radios and their stock market antics which are still studied as a case of "how not to do it" by the financial community. Two good articles written by Paul Turney appeared in Antique Radio Classified and are archived on line: http://www.antiqueradio.com/SepOct10_Turney_Grigsby-Grunow_Part1.html and http://www.antiqueradio.com/SepOct10_Turney_Grigsby-Grunow_Part2.html
(*2) Atwater Kent Company: http://www.atwaterkent.info/akHistory.html
See part one of this article for additional references.