Tube Theory Revisited Thomas Edison, famous (at least in America), for inventing the light bulb, made many discoveries before he completed his task of lighting the path of the world. Along the way, he incidentally noted that if a filament were energised within a vacuum, that after time, a "shadow" would be left on the inside of the glass, which resembled the shape of the filament. He surmised from this, that within a vacuum, particles (we now call them electrons) were emitted around the wire, forming a cloud, or SPACE CHARGE. This effect became known as the EDISON EFFECT, which is the basic operating theory behind all vacuum tubes. Later, J. Ambrose Fleming invented the FLEMING VALVE, when he noticed that a second ELEMENT, or ELECTRODE within the vacuum along with the filament, but not touching it, electricity would flow through the vacuum and be collected on the second element. The second element was called a PLATE. He further noted that electricity would flow from the filament to the plate, but not in the opposite direction. The Fleming valve was later dubbed the DIODE, because it has 2 elements inside the vacuum - the filament and the plate. The AUDION came about when Lee DeForest, In 1906, added a 3rd element between the two. This third element, a control grid, allowed one to electronically control the output of the tube based directly upon the input. This was the birth of Amplification. The term AUDION was later replaced by the term TRIODE, as the tube has 3 elements within the vacuum. Later improvements included the adding of 2 more elements, the supressor and accelerator grid, which allowed higher frequency operation, increased stability, and eliminated unwanted oscillation. The 4 element tube was called a TETRODE and the 5 element tube was called a PENTODE. The biggest problem in tube design came when trying to reach higher power levels, at higher frequencies. The higher the frequency, the tighter the tolerances became. In an effort to overcome this problem, the BEAM POWER TUBE was developed. This tube was special, in that it FOCUSED a BEAM of electrons, rather than simply creating a cloud of electrons boiled off the cathode. The beam is focused by applying a sufficiently high negative potential to repel the electrons being boiled off the cathode. At the same time the highly positive plate is attracting the negatively charged electrons. This focused beam of electrons places more energy directly on the plate, eliminating losses, and allowing for better heat distribution. Designed for graphical display, CRT's (Cathode Ray Tubes), are commonly called picture tubes. They are used for radar screens, TV and Video monitors, and test equipment, such as the oscilloscope. The face of a CRT (Cathode Ray Tube) is coated with phosphor, which gives off light when struck by electrons. But how exactly do the electrons get to the face of a CRT? The filiment heats the cathode to the point of thermonic emission. The cathode emits electrons, which flow toward the anodes. The control grid partially surrounds the cathode, and only allows the electrons flow toward the anode in a straight line. The anodes, which operate at very high potentials, so the electrons are drawn toward them at a very great speed. The anodes are constructed in such a way, that the high speed stream of electrons doesn't hit the anodes, but rather go straight through the middle of them. The first anode is a focusing anode, which forms a sharp, thin beam. The second is an accellerating anode, which thrusts the beam onward to the phosphor screen, where we see a glowing dot on the face of the tube. The horizontal and vertical deflection plates are used to deflect the beam left and right, up and down across the screen. By controling the amount of voltage aplied to the plates, the beam is bent. This deflection can be accomplished by electrostatic or electromagnetic means. The speed of the movement of the beam gives the impression of a line, or even a picture, instead of a dot on the screen. The intensity of the light at any given moment is controlled by the potential on the cathode.

This Course was written by Ray Dall © All Rights Reserved.
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And for what it's worth... this page was last updated HexDate 01-11--7D1

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