page_title: Radio receiver
section_title: Vacuum tube era
hierarchical_section_title: Radio receiver / History / Vacuum tube era
caption_attribution_description: Vintage Zenith Console Radio, Model 12S-568, With the Zenith Robot (or Shutter) Dial, Circa 1941. It is a 12 tube superheterodyne, with pushbutton tuning and a 12 inch cone speaker.
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 Audion (triode) vacuum tube  invented by Lee De Forest in 1906 was the first practical amplifying device and revolutionized radio.  Vacuum tube transmitters replaced spark transmitters and made possible four new types of modulation: continuous wave (CW) radiotelegraphy, amplitude modulation (AM) around 1915 which could carry audio (sound), frequency modulation (FM) around 1938 which had much improved audio quality, and single sideband (SSB).
  The amplifying vacuum tube used energy from a battery or electrical outlet to increase the power of the radio signal, so vacuum tube receivers could be more sensitive and have a greater reception range than the previous unamplified receivers. The increased audio output power also allowed them to drive loudspeakers instead of earphones, permitting more than one person to listen.  The first loudspeakers were produced around 1915.  These changes caused radio listening to evolve explosively from a solitary hobby to a popular social and family pastime.  The development of amplitude modulation (AM) and vacuum tube transmitters during World War I, and the availability of cheap receiving tubes after the war, set the stage for the start of AM broadcasting, which sprang up spontaneously around 1920.
  The advent of radio broadcasting increased the market for radio receivers greatly, and transformed them into a consumer product. At the beginning of the 1920s the radio receiver was a forbidding high-tech device, with many cryptic knobs and controls requiring technical skill to operate, housed in an unattractive black metal box, with a tinny-sounding horn loudspeaker.   By the 1930s, the broadcast receiver had become a piece of furniture, housed in an attractive wooden case, with standardized controls anyone could use, which occupied a respected place in the home living room.  In the early radios the multiple tuned circuits required multiple knobs to be adjusted to tune in a new station.  One of the most important ease-of-use innovations was "single knob tuning", achieved by linking the tuning capacitors together mechanically.   The dynamic cone loudspeaker invented in 1924 greatly improved audio frequency response over the previous horn speakers, allowing music to be  reproduced with good fidelity. Convenience features like large lighted dials, tone controls, pushbutton tuning, tuning indicators and automatic gain control (AGC) were added.  The receiver market was divided into the above broadcast receivers and communications receivers, which were used for two-way radio communications such as shortwave radio.
  A vacuum tube receiver required several power supplies at different voltages, which in early radios were supplied by separate batteries.  By 1930 adequate rectifier tubes were developed, and the expensive batteries were replaced by a transformer power supply that worked off the house current.
  Vacuum tubes were bulky, expensive, had a limited lifetime, consumed a large amount of power and produced a lot of waste heat, so the number of tubes a receiver could economically have was a limiting factor.  Therefore, a goal of tube receiver design was to get the most performance out of a limited number of tubes.  The major radio receiver designs, listed below,  were invented during the vacuum tube era.
  A defect in many early vacuum tube receivers was that the amplifying stages could oscillate, act as an oscillator, producing unwanted radio frequency alternating currents.  These parasitic oscillations mixed with the carrier of the radio signal in the detector tube, producing audible beat notes (heterodynes); annoying whistles, moans, and howls in the speaker.  The oscillations were caused by feedback in the amplifiers; one major feedback path was the capacitance between the plate and grid in early triodes.  This was solved by the Neutrodyne circuit, and later the development of the tetrode and pentode around 1930.
  Edwin Armstrong is one of the most important figures in radio receiver history, and during this period invented technology which continues to dominate radio communication.  He was the firs
caption: During the "Golden Age of Radio" (1920 to 1950), families gathered to listen to the home radio in the evening, such as this Zenith console model 12-S-568 from 1938, a 12 tube superheterodyne with pushbutton tuning and 12 inch cone speaker.
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