The Basics Editions
The Basics Edition 1
Where did it all start…..?
In short. Thomas Edison experimented in 1884 with vacuum tube and found that emission could take place. This discovery was patented by him and is known as the “Edison Effect”.
Only in the 20th century John Ambrose Fleming who was employed at the Marconi Company used vacuum bulbs imported from the USA, discovered that a heated Cathode was capable of thermionic emission of electrons which flowed to the Anode, with a higher potential than the Cathode. It was also found that the reversal of current were not possible as the opposite Anode is not heated.
This became known as the Fleming valve, which developed in the rectifier to be used as well as detectors in radios.
The Cathode ray tube/Triode
Robert Von Lieben patented the Cathode ray tube in 1906. Magnetic deflection methods were included.
Lee De Forest created the first triode in 1907. He also patented it. The first transmitters and receivers had the tendency to oscillate if the stage Voltage gain were high. This was due to parasitic capacitance between the Anode and the Control grid. This phenomenon is known as the Miller capacitance.
This Miller effect caused the development of a valve which would increase the stage Voltage gain. Walter Hermann Schottky developed the Tetrode in 1919. This added a second grid to the thermionic valve. The Tetrode had its own problems with secondary emissions, which lead to less current at the Anode, thus less amplification. At worst case the secondary electrons could be larger than the primary electrons. Destruction of the screen grid could occur if the current is high enough.
This led to the introduction of the suppressor grid, creating the Pentode. The Pentode were introduced in 1928 by Bernard DH Tellegen. An improved version of the Tetrode and Pentode is the Beam Tetrode and Pentode.
Taxation of the number of valves caused engineers to develop the multi-section types. The complexity of new circuits also let to this development. Two and three section valves became more and more common.
Envelope of Valves
Glass and various metals are used to contain the various valve structures in the vacuum area. A typical example would be the 6L6 which were made in a metal can and is still produced in a glass envelope.