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Audion The First Lee De Forest Vacuum Tube Triode

OddMix.com - Technology Note - TN1008 - Karl Nagy

Figure 1. Lee De Forest Triode
Figure 1. Lee De Forest Triode "Audion" with outside control grid

At the beginning of the radio era, around 1880 -1900, RF (Radio Frequency) signal amplification was unavailable. All RF signal generation and detection had to rely on the capability of a high wire named aerial or antenna and good luck. To compound this communication problem, selecting signals were not possible either, as all then used signal sources was wide band.

Spark inductor transmitters relied on sparks and arcs as their radio frequency signal generator. Any time a transmitter worked; it covered the entire RF spectrum. The nearest transmitter usually covered up all others resulting in incredible receiving difficulties. To compound this problem multitudes of stations operated, some licensed others not. Even Government stations were interfered with so much that at times the maritime authorities had to beg to let them get in touch of ships at sea.

Figure 2. Lee De Forest Triode
Figure 2. Lee De Forest Triode "Audion" with inside control grid

The best available method to make oneself audible was a good antenna. Much experimentation went on to find the most perfect aerial device. On the receiving side the antenna was the only real hope as all receivers used passive methods for picking up the RF signal. Initially, receivers circuits were simplicity itself as almost all receivers used barretters or the Branly type coherer wave detector. The newly invented crystal detector was a major improvement. Interestingly Thomas Edison discovered the real diode, but the device was unused, and dormant for quite a few years.

The "Audion" first the device, then the circuit came by sheer desperation. Lee de Forest tried hardest and by a happy accident, eventually he discovered the triode. At first his crude device - patented in 1906 November 13 by Lee De Forest, U.S. Patent 841,386 - had a few turns of wire wound on the outside of an Edison type vacuum diode - Figure 1. By 1908, U.S. Patent 879,532 - the third (control) electrode was placed inside between the filament wires (cathode) and anode and because of it's shape, received the name of grid. - Figure 2.

The triode by far was the most important electronic component ever invented to date. Although it was patented in 1906, even the overly proud inventor had not a clue as how and why the device or the amplifier circuit worked. And worked it did! In a short decade, the triode has taken over signal generation and detection, but its most important action was amplification.

In 1911, after the initial triode rush was over, the PHILIPS Company in the Netherlands invented a dual grid electron tube named the tetrode. The pentode was shortly followed and become the most successful and most often made vacuum tube in technical history.


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