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Vacuum Electron Tubes Electronics Triode

OddMix.com - Technology Note - TN1204 - Karl Nagy

Figure 1. Lee De Forest External Grid Vacuum Triode [4 KB]
Figure 1. Lee De Forest External Grid Vacuum Triode

Figure 2. Triode [2 KB]
Figure 2. Triode

The first commercially useable vacuum amplifier device was a three electrode electronic tube. Thus given its the name triode. It was invented and patented by Lee De Forest in 1906. The first patent disclosed with an external wire spiral wound around an Edison incandescent lamp. Only his second patent had the control grid - G1 internally were it belongs - Figure 1.

Since that time the tube undervent a fast revolutionary then a slow evolutionary changes. This fundamental device is continually perfected to its present form. The triode was initially the most often used vacuum tube in the world until its popularity exceeded by the newer pentode. Often used triode symbol on schematics and drawings is on Figure 2.

Triodes and other electronic tubes are useful for the restoration of vintage collectible radios and tube type electronics instruments. Many times, electronic circuits are simpler to design and build and more reliable when made with vacuum tubes then with semiconductors.

Initially the production of every single working triode was a major achievement thus early devices were almost always a single triode within one glass envelope. Later when manufacturers had the process under control, they realized that two triodes fit into a single glass encloser nicely and allowed the escalation or at least the holding of the high prices. The majority of the present day supply of triodes are almost always dual or twin devices.

When the term "vacuum tube" is used - contrary to the vague Lee De Forest audion patent explanation - it always means high-vacuum. The harder is this high-vacuum, the better is the electron tube. It was found out in the early part of the 1900's when the same type tubes were made by several different manufacturers that the best tube was always the one that was evacuated the best and hardest.

The three triode electrodes - in order of increasing potential - are the cathode, the control grid - G1, and the anode or plate - A. The cathode is located at the center of a cylindrical triode assembly and is usually is at zero or reference potential. The G1 control grid electrode operates at a small negative potential with respect to the cathode. The space-charge electrons are repelled by this negative potential and they avoid the G1 thus no grid current flows, allowing excellent high input impedance for the tube.


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