Stone Vintage Radio Museum - Antique Radios, Wireless, Crystal Sets, Tubes, and Valves

A Brief Chronology of Development in the Manufacture of Tubes for Radio Receivers

The electron tube, also known as vacuum tube or valve, is a glass or metal enclosure in which electrons move through a vacuum or gas from one electrode to another. The vacuum tube primarily is used as a detector of radio waves, an amplifier of weak currents, or as a rectifier of electric current.

Before the invention of the transistor and solid state electronics, the vacuum tube was the basis of most electronic equipment. The first vacuum tube was a diode invented in 1904 by John Fleming. The first diodes were modified light bulbs with an added element, a metal plate, inserted inside the glass envelope. The filament of the light bulb emitted electrons and an anode or metal plate, mounted near the filament and connected to the outside of the tube, captured those electrons. In these first vacuum tubes electrons only move in one direction to the cathode. The diode served as a detector of radio signals replacing crystal detectors, magnetic detectors, and other early radio detectors.

A few years later Lee De Forest in 1906/7 developed the Audion, the first triode vacuum tube. “In a triode, current can be all-the-way-on, all-the-way-off, or anything in between—like water coming out of a faucet. In a triode the strength of the current, like the flow of the water, can vary. The triode is able to do this by using a tiny wire grid placed between the cathode and the anode to form the third electrode, simply called the grid. A voltage sent to the grid—such as a radio wave, or the weak signal from a microphone—controls electron flow. If the grid is positively charged, it helps accelerate the electrons across the gap between the other two electrodes. If the grid is negatively charged, then it repels electrons and greatly reduces or stops the flow across the gap.” (http://www.ieeeghn.org/wiki/index.php/Electron_(or_Vacuum)_Tubes). Key to the value of the triode was its ability to amplify small currents. A diode tube can detect the presence of radio waves, waves that are still very weak. A triode vacuum tube can not only can detect radio waves, but also amplify them.

The following is a very brief chronology of a few of the key developments in the design of radio tubes from the beginning of the broadcast era (1920) until the start of the transistor/solid state era (1940s). It mentions only a small number of the many developments in radio receiving tubes in the United States; hopefully it will help the reader a gain a glimpse of why these advances meant so much for development of radio receivers. A major reason that the quality of broadcast radio receivers advanced so fast for the two decades after the beginning of general radio broadcasting in 1920 is the rapid progress that vacuum tubes made.

1920: RCA introduced two tubes: the UV-200 (detector) and the UV-201 (amplifier). The UV-200 and the UV-201 were the first radio tubes manufactured for the home radio mass market. They had a 5 volt, 1 amp filament and were lighted by a car six volt storage battery.

1922: Westinghouse introduced the WD-ll in 1922. It had a 1.1 volt filament and could be operated by a single dry cell battery. The base pin design was different from any other tube in the US with one pin fatter that the others.

1923: The following year three new tube types were introduced: the UV- 20IA, a superior version of the UV-201 requiring only one-quarter of an amp of filament current; the UV-199 which required only three volts for its filament and a mere .06 amps. The thoriated tungsten filaments in the UV-20IA and UV-199 tubes required the use of a “getter” to remove any oxygen left inside the tube. The getter, typically an internal magnesium pellet, was fired off as the tube was being evacuated; the result was a silvery mirror-like deposit seen on the inside of the glass of these tubes.

1923/4: The glass tip (pip) on the top of tubes started disappearing from the top of the UV200, UV-201 and UV-201A vacuum tubes moving to the bottom inside the base. Also brass based tubes were now replaced by Bakelite bases.

1925: The long-pin UX-type base designed for push-in sockets replaced the short pin UV-type base made for bayonet-mount sockets. In addition several new tubes were launched: the UX-112, 120, 210, all power amplifiers and the UX-213 a full-wave rectifier.

1927: The year 1927 was important in tube development with the introduction of tetrode (4 element) tubes; an additional grid was now inserted in some of the new vacuum tubes: The UX-222 was a screen-grid amplifier; the UX-226 and UY-227 were triode amplifiers with AC filaments; the UX-112A and UX-171A were improved power amplifier tubes; and the UX-280 full-wave rectifier which was used for many years in radio receivers.

1930: The locking pins started vanishing from tube bases. A quicker-heating version of the 24 tube, the 24A, was introduced.

1931: The first pentodes (5 elements in a tube) were introduced. They were such tubes as the 33, 38, and the 47.

1932 : The globe or pear-shaped bulb used on most tubes up to 1932 began to be replaced by the type “ST” or Coke bottle shaped tubes.

1933: Two digit vacuum tube numbers were replaced with a new letter/numbering system. The new system of giving tube types was based on tube function and characteristics. The 6A7 was an early example of the new labeling system.

1935: The first metal tubes and the first glass octal (8 pin) tubes were introduced.

1938: In 1938 tubular glass octal or GT tubes were introduced.

1939: Sylvania introduced Loktal tubes with wire pins and a center locking mechanism that holds the tube in the socket.

1940 Miniature tubes were introduced.

This History of early broadcast tubes is based on the work of Marc F. Ellis, “The Receiving Tube Story, Part 4,” The AWA Gateway, vol. 1, No. 4, December 2011. In addition The IEEE Global History Network article “Electron (Or Vacuum) Tubes,” http://www.ieeeghn.org/wiki/index.php/Electron_(or_Vacuum)_Tubes.
Howard Stone StoneVintageRadio.com

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