THE HISTORY OF VACUUM TUBES

 

The main problem with the wireless receivers during the “Wireless Telegraph Era”, was the DETECTOR. All the detecting options that were used (Electromagnetic, Coherer, Galena Crystal, Electrolytic and others), were unstable and not sensitive.

 

In 1881, while looking for a way to prevent the carbon coating in his carbon filament light bulb, Thomas Edison added a metal plate inside the sealed bulb. He noticed that when the metal plate was connected to a positive voltage, current flows from the filament to the metal plate through the VACUUM inside the bulb. It was called Edison Effect and a patent was awarded to Edison in 1884.

 

There was no practical use for this Edison patent, until John Ambrose Fleming (who worked in the past with Edison) while working for Marconi in 1904, found out that the Edison Effect can be used for detecting wireless signals. He also found that the bulb with the metal plate, can RECTIFY ALTERNATING ELECTRIC CURRENT (AC) to DIRECT CURRENT (DC). Fleming was awarded the patents in England and the USA for this invention.

 

Thermionic Diodes based on Fleming’s patent have been produced and replaced the crystal detectors in the Wireless Receivers, with better and stable reception. In 1906, Lee de Forest tried to improve the diode tube performance by affecting the conductivity. Adding an external wire wrapped around the bulb and later on inside the bulb, resulted variations in the flowing current. De Forest have connected the wireless signal from the antenna tuning circuit to the wrapped wire. The headphones were connected to the metal plate circuit. The result was a more sensitive detector which Lee de Forest called AUDION.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Early experimental Diode           Early Experimental AUDION            AUDION Advertisement                  Early Triode

      

The sealed glass bulbs that de Forest experimented with, were partly filled with gas. He believed that the variable conducting effect with the wrapped wire, is due to the gas inside the bulb. At the same time that Lee de Forest experimented with the Audion, Robert von Lieben in Austria developed a vacuum bulb with three elements, but it was filled with Mercury vapor and acted like a THYRATRON .

 

Langnuir from Gottingen University in Germany worked on the research of the thermionic emission in vacuum bulbs. In 1909, he moved to GE USA to research the capabilities of Tungsten filaments in Audions. In 1913, he published the results (partly based on the research of Richardson of the UK regarding thermionic emission). His conclusion was that no gas is required in Audions, but high degree of vacuum.

 

The research of Arnold of AT&T on the efficiency and life of filaments in Audion bulbs included adding a layer of oxide over the filament coil. This improved both the electrons emission and the life of the Audion bulb.

 

 

AMPLIFICATION

 

The first use of the Audion as an amplifying device was in 1911 by Lowenstein in the USA. His purpose was to develop a Telephone Repeater for the Trans Atlantic Telephone Line. In 1913, Lee de Forest designed a three Audions amplifier with a total amplification of 120. Lowenstein and de Forest proposed to Bell Telephone a design of amplified repeater for the Trans Atlantic Telephone. Bell Telephone decided to develop and produce the amplified repeater, based on the Arnold amplifying bulb.