The Thermionic Valve or Vacuum Tube has a rich and interesting history steeped in lore and traditional values. It all started with the invention of the incandescent light bulb by Thomas Edison. The first lights were very short lived and were prone to darken. This darkening was termed the Edison effect. It was found that this darkening was the deposition of carbon onto the glass, except in the plane of the filament; where there was little deposition. Edison noticed that, with an extra electrode connected to the positive end of the filament, a small but measurable electric current flowed between them. He termed this device an Electrical Indicator.
From there, Sir John Ambrose Fleming worked to develop the first rectifier and in 1904, while working for the Marconi Company, he was faced with the problem of detecting weak wireless signals. He was inspired by his work with Edison’s lamps back in 1889 and decided to try inserting one of the lamps in an oscillatory circuit containing a galvanometer. He had found the solution to the problem of rectifying high frequency wireless circuits. Below is a Picture of a Fleming Valve.
Fleming made many adjustments to his valve over the next few years. Some of these adjustments included tungsten filaments and the addition of a shield within the tube to eliminate electrically charged bodies from affecting the valve. He special ordered several types of specially designed lamps from Edison’s factory and continued his perfection of the valve. Fleming applied for a patent on January 25, 1908. The institution of the valve was almost immediate; being employed in several electrical devices soon after the development. One of the first receivers to use the valves was the Marconi-Fleming valve receiver. This was the start of the wireless revolution.
The next big step in the development of the vacuum tube came along with Lee de Forest. In 1905 de Forest was working with lamps as well and developed what would become one of the most important developments in vacuum tube history; the Audion. In all great developments, there must come some controversy. Lee de Forest held steadfast in his assertion that he developed the Audion independently from Fleming’s research. Lee de Forest applied for and was granted a patent for the Audion on November 13, 1906.
The Audion went through many electrical design changes; mostly due to the manufacturer’s wanting to cut costs. There were Audions with single filaments, double filaments, single wings, double wings and so on. Physical appearance of the Audion had many morphological changes as well. At the time, the most economical way to produce Audions was to use spherical glass. The addition of a grid between the plate and filament was the most profound. This modification allowed for the amplification of an incoming weak signal to a relatively strong signal. This is the basis for audio amplification.
The possibilities seemed endless. The Audion was brought to AT&T with the idea of using them in telephone repeaters for longer distance signal transmission. AT&T was so impressed, they purchased some of Lee de Forest’s patents for $50,000 and assigned their electrical unit, Western Electric, to develop and improve the Audion. Western Electric was the critical step in the evolution of the vacuum tube. WECo took a crude, unreliable device and converted it into a high standard of reliability and longevity. The advancement of the WECo tubes started with a 101A. There were many refinements and adjustments applied to the WECo line. These refinements gave us such beauties as the 101D, the 205D, the 252A, and the venerable 300B.
All of this set the ball rolling and the rest is history. Many different companies and many different countries aided in the development of the vacuum tube. Germany’s Telefunken produced some of the most reliable, best sounding tubes availiable. Brittain’s Mullard manufactured one of the finest EL34s in the world. United States RCA, GE, Sylvania, and many others all had qualities to add to the vacuum tube as well. I feel the real history and lore lies in the origins. It is for this purpose we will stop here.
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