TUBE RADIOS CLASSIFICATION
II. THE MODERN RADIO – The Super Heterodyne
In this circuit, a frequency is generated in the radio itself, which mixed with the tuned station wave, results in a third one. This frequency fusion process is called heterodyne. This third wave is called Intermediate Frequency, or IF, which was usually fixed between 455 and 465 kHz. As it is lower, it is an easier frequency to be operated by the radio.
A circuit used currently, the super heterodyne is much more selective concerning station tuning, more sensitive and less vulnerable to interferences. It is called, in a technical context, super-het.
This system first patent was in 1917, by Frenchman Lucien Levy. In 1918, Edwin Armstrong patented the circuit with refinements. In 1920, Armstrong sold super heterodyne rights to RCA, what resulted in a fabulous monopoly: all the producers had to pay rights to Radio Corporation of America in order to produce receivers with the new system, considered the ideal one.
The first super heterodyne with commercial purpose was made in 1924, with a small selling rate. At that time, RCA sealed the chassis to avoid “copies”, reason why they were called “vaults”.
In 1929, Philco – Philadelphia Storage Battery Co., launches the first radio with automatic volume control, known as AVC, reducing volume differences between stations with different power. Those were the Series 95 receivers.
Therefore, in the 30s, most of the receivers started to have a super heterodyne circuit. In the mid 30s, the number of devices commercialized in the United States was over 20 million of units.
In Europe, brands such as Philips (Netherlands) and Telefunken (Germany), crossed boarders towards other continents.
Battery Powered
Primitive receivers, with any kind of circuit, were powered by batteries. Later, even with the electricity power supply, some devices were still made to be used with batteries.
They were known as Farm, farm radios, made since 1930 until the end of the 1940s, to places with no electric energy. Some had a vibrator, a set that converted tension in discharge current of the battery to alternating current.
On the side picture, an Erla receiver, Electrical Research Labs, from the author’s collection, made in USA in 1935. It’s functioning depends on discharge current, working perfectly with a rectified source of 32 volts and 2 amperes. |
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AC/DC Radios
As the alternating current power supply advanced, many radios were built to tension in alternating and discharge current (AC/DC). Their tube filament circuit is serial, with no need for the heavy input transformer.
 Some were called in Portuguese "rabo quente" (hot tail), due to the fact that they had a resistance on the electricity plug power cable. This cable heating originated the nickname. Other came with big internal resistances or with a tube to switch from 110/220 volts, called Ballast.
The picture on the left shows an AC/DC Philco receiver, model 40-710, made in 1940, from the author’s collection. Called Midget, these radios were made until 1950, becoming popular for its cheap prices, lightness and small dimensions, what made it possible to place them anywhere in the house.
 Other receivers were made for multiple usages, accepting different tension values in alternating or discharge current. On the side picture, a Pilot T133, from 1942, that works with 5 to 40 volts batteries, and 100 to 260 volts with alternating or discharge current. Author’s collection.
Alternating current power supply
The end of the 1920s witnessed the consolidation of Westinghouse alternating current electric power supply over Thomas Edison discharge current. As the domestic alternating current electric expanded, receivers changed from battery power (DC) to urban power supplies (AC).
On the radio golden age (1930s), the production really increased: the radiophonic communication phenomenon gets the homes worldwide, making the radio a common domestic appliance, cheap and indispensable.
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In the picture on the right, a Deso-Dewald radio, model 453-D, made in Switzerland in 1944/45. It has the popular “magic eye”, which is a tube that indicates tuning, in this case an EM4 working, in the detail on the left. |
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In the picture below, a receiver brand name Zenith, model 6S321, made in 1937, from the author’s collection. It’s a standard from that time, called table, a name that became common for medium dimension radios th at were placed over furniture.
These sets, with super heterodyne circuits and alternating current, became a pattern. In general, they have input transformer, parallel tube filament connection, speakers with permanent magnet and short wave, this last with stronger usage after World War II.
In the right picture, a receiver made by Guglielmo Marconi in England. It’s T23-BT model, from 1950. It has an audio amplification with two output tubes, the push-pull. Author’s collection.
In 1933, a new type of transmission appeared, called Frequency Modulation, or FM as it’s commonly known. Also created by Edwin Armstrong, the frequency modulation was a new circuit to eliminate the static phenomenon found on the Amplitude Modulation AM. Not considered at first, FM consolidated from the 1950s. The picture on the left shows a Eumig, model 382W, made in Austria around 1956, with medium wave broadcasting and FM. Author’s collection.
Two decades after the beginning of radio set large scale production, dimensions and weight started to be reduced. New materials and artistic tendencies influenced, in the 1950s, the production of plastic and plane angle cabinets. The picture on the right shows Philco radio, model 50-922, from 1950, from the author’s collection.
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