A vacuum tube

Today many of us know the vacuum tube only as a TV screen, or the screen of a computer or an oscilloscope. But there used to be times when any electric machine couldn’t exist without a vacuum tube. TV, radio, tape recorder, amplifier, transmitter – all of them had to have a vacuum tube. Let’s have a look at the historical development of this invention – the greatest one in the history of radio-engineering. . .

In 1873 an English physicist and chemist Frederic Guthrie griped a bar with a glowing metal ball on its end and put it near a live electroscope. In that moment he could see, that the split gilding on its end have drooped: the electroscope was discharged! This accidental invention was the first step, at the other end there were the vacuum tubes and the photocells.

This piece of knowledge was later utilised by two German scientist – Julius Elster and Hans Friedrich Geitel. 10 years later they were doing several experiments and came to the following conclusion: Gas between a glowing body and another (even cold) body conveys current, and this current is simplex. At that time Edison tried to solve the mystery, why some parts of the carbon fibres in his bulbs shine more bright and why there are black blurs on the bulb just opposite these parts. Most distinctive was the blur when a fibre was burnt in two. The blur was always nearer to one of the fibre-ends. Edison had a wire sealed in the bulb, on the end of the wire – in the space between the bow of the filament – there was a small plate. He connected this bulb with the sucked out air to the simplex current. Then he alternately connected the outlet of the plate with the positive and negative ends of the outlets of the fibres. First nothing happened, but in the second case the indicator on the galvanometer has deflected: There was current in the vacuum space between the live fibre and the metal plate!

Edison couldn’t explain this strange phenomenon. He didn’t know, that he invented a golden treasure. He didn’t suspect, that he constructed something more ingenious then the bulb. And we mustn’t be surprise. Edison was not a scientist. He hated science and most of all mathematics. His inventions arose just thanks many experiments and many errors. He was “just” an ingenious deviser.

But he was also a typical American, whose only luck was a commercial success. As many other people he couldn’t understand, that someone would create and use something just like that, just for his of her own satisfaction, for joy. And so as to the luck could not escape, he had this invention patented, although he didn’t know what it could be good for. He receive the patent no 307 031 on 21st October 1884. He didn’t know, that he actually had the vacuum tube patented.

The origin of the phenomenon patented by Edison was clarified in 1897 by the later Nobel. prize winner – an Englishman Joseph John Thomson and a German Philipp Eduard Anton (originally a Slovak): The current carriers were the elements called by the Englishman Johnston Stoney the electrons.

Professor at the London university John Ambrose Fleming was a very talented scientist. For the realisation of the project First transatlantic flakes´ telegraph station it was necessary to increase the effort of the sender and mainly the sensibility of the detector. The mechanic detector could not work any longer and so Fleming remember the Edison’s phenomenon. So he used Edison’s bulb as a detector: the high-frequency telegraph current was under control. The first real vacuum tube – diode was born. Originally it was called “oscillation receiving vacuum tube” and on 16th February 1904 it got a British patent no. 24805.

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Dual diode

The American Lee de Forest made something similar as Fleming. In contrast to him he found out, that there is much more energy running out through the headphones and the battery to the earth than through the detector. After different experiments he disposed this fault – he sealed another outlet into the lamp, this outlet created a spiral between the fibre and the plate. He named his vacuum tube audion – according to the Latin word audio – "I can hear". He proved his audion in 1906 in a very unusual way: he made a marriage proposal. The radio-waves connected their homes and both said their “yes” into the microphones. In contrast to the audion this marriage hadn’t last more than a year …

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The first vacuum tubes were quite insecure. Mainly because they contained a great amount of residual gas. The first highly drawn vacuum tube was not manufactured until 1915.

But the development was continuing. Later the vacuum tube had more functional parts with outlets. Because of better arrangement these were called the electrodes and every electrode got its name – the fibre is called a cathode, the plate opposite to it is an anode and the spiral between them is a grating. According to the number of electrodes the vacuum tubes were later divided into diodes (two electrodes), triodes (three electrodes), tetrodes (four electrodes – invented in 1916 by a German Walter Schottky), pentodes (five electrodes – invented in 1923).

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Measuring of the vacuum tubes (30's).

Later on the vacuum tubes were being improved: the carbon fibre was replaced with a wolfram one. But even this was not enough. For increasing the effort of the emitting electrons the fibres started to be covered with different coats.

Today, in the epoch of semiconductors it can seem to us, that the vacuum tubes were rang out. However, they will survive for some time: furthermore the connoisseurs do like the sound of the electronic amplifiers more…

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Nowaday high-powered vacuum tube stereo amplifier
(Power Output 70W/channel, Frequency Response 10 Hz - 80 kHz ,
Power  420 W maximum, price 1 800 $  . . .

(taken from http://www.vacuumtube.com/model270.htm)

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