8.1.1 - THE VALVES USED IN THE TELEPHONY SYSTEMS

The first telephonic repeaters were of electromechanical type and in this way insufficient to follow the increasing expansion of the telephonic lines.
The engineers had soon noticed they needed a new type of telephonic repeater that besides acting as

Irwing Langmuir

signal amplifier, it should have also several technical operational and commercial capabilities such as: - to amplify all the frequency range without distorsion; durability and quick interchangeability with similar devices, as well as low manufacturing cost.
However, around 1910, the triode valve was still a simple thermionic device and in this way, it did not have the necessary conditions to meet such requirements.
Around 1916, the valve had already suffered enormous technological modifications either on its structural as well as its operational concepts. Amongst its main technological advances are:
-Filament or heater: the first type, was the one coated by oxides whose technique was invented by Wehnelt. Soon followed the tungsten-thoriated filament, an alloy made of tungsten and thorium. (a) However, those new types of filaments demanded the operation in low oxygen atmosphere for the proper electron emission.
The gettering technique: this deficiency was overcame by the vaporization or the burnning inside of the glass bulb of active chemical substances such as magnesium during the final process of evacuation of the air in such away to remove the small particles of atmospheric air.
The Getter could still continue active during all the life of the valve, absorbing small amounts of emitted gaseous particles from the structural elements of the valve, or even due to an incompleted rarefaction. (b)
-The structural elements: the first valves used as telephone repeaters were substantially improved by eliminating its operational fragility. New materials were used in the manufacturing processes; this included the base as well as for the valve inner elements originating an asembling technique as known as Iron Clad. Through the same the inners, elements were perfectly lined up and finally mounted inside a glass bulb. (c) -
-The glass bulb: as aforementioned the technology of the valve manufacturing had its origin in the electric lamp. (d) In the first valves, the air was removed through superior part of the bulb and then sealed by a small tip in a pearl shape. (e) Due to the improvement in the rarefaction processs the bulb suffered several modifications in its original shape as well as new type of glasses were used in its manufacturing. Among them were the lead -carbonates, and the borosilicates glasses.
In this way, instead of using the primitive blowing process, the bulb now was moulded in machines or lathes improving the glass wall thickness. Around 1928 the bulb shapes were already standardized.
During the First World War, 1914 - 1918, all the operational conditions of the valve were already known due to the experience acquired in the manufacturing of the ones used for telephony applications. Such improvements certainly were extended to those destined for equipments used for military applications.
Hence the military communication equipments should be built considering size, rigidity as well as operating in adverse enviroment conditions, ranging from a freezing radio tent in the Artic, to a hot destroyer warroom in a in the tropic, the technological evolution of the thermionics led to the development of new and reliable types devices such as the metal valve.

a) Power triode made in the USA using filament coated with Oxides.

(b) The valve type 201, with porcelain base showing the mirrored bulb due to the gettering processs;

(e) In the left: valve type 201 with brass base showing the glass bulb where on its top the air was removed; in the right: same valve but with a spherical mirrored bulb due to the gettering.