page_title: Radio receiver
section_title: The first vacuum tube receivers
hierarchical_section_title: Radio receiver / History / Vacuum tube era / The first vacuum tube receivers
caption_attribution_description: English: One of the first amplifying radio receivers, the De Forest Type RJ6 receiver made by the De Forest Radio Telephone and Telegraph Co., from an advertisement in a 1915 trade journal. It was a one-tube radio that used the Audion (triode), the first amplifiying vacuum tube, invented by Lee De Forest in 1906, both as a detector to rectify the radio signal, and as an audio amplifier. The Audion is mounted on the front panel so the operator can see if the filament is lit, and adjust the filament current by its brightness. The Audion is mounted upside down, with the delicate filament hanging down so it won't sag and touch the grid. The large dial on the right is the tuning, the multiposition switch in upper center adjusts the plate voltage, and the multiposition switch in lower center adjusts the filament current. The single Audion did not produce enough audio power to drive a loudspeaker, so the headphones at left are used for listening. The advertising copy surrounding the picture read: "AUDION RECEIVING CABINET TYPE RJ6 Incorporating the Famous De Forest Audion Detector, The Most Efficient Receiving Set For Experimental And General Purposes Ever Offered For Less Than $500"
context_page_description: In radio communications, a radio receiver, also known as a receiver, a wireless or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
  Radio receivers are essential components of all systems that use radio. The information produced by the receiver may be in the form of sound, moving images, or digital data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. The most familiar type of radio receiver for most people is a broadcast radio receiver, which reproduces sound transmitted by radio broadcasting stations, historically the first mass-market radio application.
context_section_description: The first amplifying vacuum tube, the Audion, a crude triode, was invented in 1906 by Lee De Forest as a more sensitive detector for radio receivers, by adding a third electrode to the thermionic diode detector, the Fleming valve.  It was not widely used until its amplifying ability was recognized around 1912.  The first tube receivers, invented by De Forest and built by hobbyists until the mid 1920s, used a single Audion which functioned as a grid-leak detector which both rectified and amplified the radio signal. There was uncertainty about the operating principle of the Audion until Edwin Armstrong explained both its amplifying and demodulating functions in a 1914 paper.  The grid-leak detector circuit was also used in regenerative, TRF, and early superheterodyne receivers (below) until the 1930s.
  To give enough output power to drive a loudspeaker, 2 or 3 additional Audion stages were needed for audio amplification.  Many early hobbyists could only afford a single tube receiver, and listened to the radio with earphones, so early tube amplifiers and speakers were sold as add-ons.
  In addition to very low gain of about 5 and a short lifetime of about 30 – 100 hours, the primitive Audion had erratic characteristics because it was  incompletely evacuated. De Forest believed that ionization of residual air was key to Audion operation.  This made it a more sensitive detector but also caused its electrical characteristics to vary during use. As the tube heated up, gas released from the metal elements would change the pressure in the tube, changing the plate current and other characteristics, so it required periodic bias adjustments to keep it at the correct operating point.   Each Audion stage usually had a rheostat to adjust the filament current, and often a potentiometer or multiposition switch to control the plate voltage.  The filament rheostat was also used as a volume control.  The many controls made multitube Audion receivers complicated to operate.
  By 1914, Harold Arnold at Western Electric and Irving Langmuir at GE realized that the residual gas was not necessary; the Audion could operate on electron conduction alone.    They evacuated tubes to a lower pressure of 10⁻⁹ atm, producing the first "hard vacuum" triodes.  These more stable tubes did not require bias adjustments, so radios had fewer controls and were easier to operate.   During World War I civilian radio use was prohibited, but by 1920 large-scale production of vacuum tube radios began. The "soft" incompletely evacuated tubes were used as detectors through the 1920s then became obsolete.
caption: De Forest's first commercial Audion receiver, the RJ6 which came out in 1914.  The Audion tube was always mounted upside down, with its delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.
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