de Forest Audion Vacuum Tube

The history discovery of the de Forest Audion vacuum tube and the way it was used.

History of the Valve / Tube Includes:
History overview     Early discoveries     Fleming's oscillation valve     de Forest's Audion     Development of the basic idea     R-type valve     6L6 valve    

Although Fleming had taken the first step in introducing thermionic technology, it took others to be able to develop it further.

The Fleming valve had limited applications and the way in which Marconi's company held the patents meant that few others could use the concept and this significantly limited its uptake.

The next major step in the development of this technology was taken by an American named Lee de Forest with the introduction of his Audion.

The Audion development was of major significance, although this was not realised fully at the time.

Lee de Forest's early ideas

Lee de Forest had developed his interest in wireless technology while he was studying at Yale University - his Ph.D. thesis was concerned with the reflection of radio frequency signals at the ends of what would be called transmission lines today.

He also became interested in the detection wireless signals and like Fleming de Forest saw the limitations of existing means of signal detection using coherers, magnetic detectors and the like.

Initially de Forest developed a detector, with a colleague, called a responder, although the idea did not come to anything. It nevertheless set in place an interest in detection for later.

His idea was to investigate the response to electrical vibrations in a gas flame, even applying for a patent for several devices based on Bunsen burners.

de Forest Audion work starts

It was towards the end of 1905 that de Forest first came across the possibility of using thermionic technology. One of his assistants at asked a local incandescent lamp maker to make a copy of a Fleming valve he had. Having completed this order, a number of other variants were also made.

de Forest looked at a variety of ways of using thermionic technology. He even applied for a patent for using the two electrode diode. In this he used separate batteries for the filament and anode circuits.

The device that de Forest used in this experiment, he referred to as his two electrode Audion.

Enter the de Forest Audion triode

de Forest continued to work on the idea of developing better detectors using thermionic or vacuum tube technology.

He experimented with various forms and configurations for the two electrode Audion, and came up with a device that had the possibility of amplifying 'feeble electric currents.' He applied for a patent for the device (Patent 841 387)

In this device he used an incandescent lamp with a carbon or metal filament that had two metal wings either side of the filament and parallel it.

Although this was still a two electrode device the patent and it could possibly not have provided amplification, there was mention of a third electrode placed between the filament and the anode. de Forest thought this would impede the flow of electrons, but later added a single wire shaped in the form of what he described as a gridiron. This term then gave rise to the term 'grid' that was later adopted for this electrode.

Audion potential recognised

The full potential of the Audion, though, was not recognised for some while as it was only used a detector. It was not until 1911 that the vacuum tube was used properly as an amplifier - even though the original patents had described the possibility of amplification. After this discovery people were quick to try to exploit it. De Forest built an amplifier using three Audions and demonstrated it to the telephone company A.T & T. Although the performance was poor they saw its potential and soon started to build repeaters using vacuum tubes which they had improved.

Naturally as soon as the tube was used as an amplifier, people were quickly able to use it as an oscillator. Indeed, one of the problems soon encountered was difficulties in preventing oscillations in view of the high values of grid anode capacitance.

With the first valves or tubes now available, at least in the laboratory, the way was set to further develop the technology and improve their performance.

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