Vacuum Tubes
Vacuum Tubes.—A vacuum radio tube in its simplest form is somewhat similar to an electric light bulb, but contains a number of metallic elements for various purposes for use in conjunction with radio transmission and reception. The parts of a typical vacuum tube is shown below.
- Fig. 1.—Parts and assembly of a typical glass radio tube. The filament, grid screen and plate are sealed in the tube, from which the air has been removed. (Courtesy R.C.A. Mfg. Co.) '
Fig. 2.—Internal structure of a glass radio tube. There is at present a great number of tubes utilized for various requirements. In radio applications they are used for amplifying radio waves; for converting radio waves to sound waves; for converting alternating current to direct current, etc. (Courtesy R.C.A. Mfg. Co.)
Fig. 2.—Internal structure of a glass radio tube. There is at present a great number of tubes utilized for various requirements. In radio applications they are used for amplifying radio waves; for converting radio waves to sound waves; for converting alternating current to direct current, etc. (Courtesy R.C.A. Mfg. Co.)
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Fig. 3.—Inside view of a metal radio tube. The inside parts are similar to those of a glass radio tube. The housing of the parts consists of metal, which has certain advantages to that of the glass in required electrical shielding. These types of tubes generally are smaller and have a central guide pin projecting from the base for easy insertion in the sockets, and are therefore not as a rule interchangeable with glass tubes. (Courtesy R.C.A. Mfg. Co.)
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Fig. 3.—Inside view of a metal radio tube. The inside parts are similar to those of a glass radio tube. The housing of the parts consists of metal, which has certain advantages to that of the glass in required electrical shielding. These types of tubes generally are smaller and have a central guide pin projecting from the base for easy insertion in the sockets, and are therefore not as a rule interchangeable with glass tubes. (Courtesy R.C.A. Mfg. Co.)
Purpose of Vacuum Tubes.—The general purpose of a vacuum tube is to detect and amplify radio waves; for changing alternating current into direct current; for producing oscilla-
tions or rapid electrical pulsations; for changing an electric current of one degree of pulsation to those of another, and for innumerable other purposes.
Materials Used in Vacuum Tubes.—The materials used for housing the elements of a vacuum tube may be glass or metal— sometimes a combination of the two. The essential difference between metal and glass tubes is that the metal tubes as a rule are smaller, and have a central guide pin for insertion in the sockets'and hence they are not readily interchangeable with one another.
DIRECTION OF FLOW
FILAMENT OR CATHODE
DIRECTION OF FLOW
FILAMENT OR CATHODE
- PLATE OR ANODE
Fig. 4.—Illustrating the flow of electrons in a vacuum tube.
Principles of Operation of Vacuum Tubes.—In any electronic tube, thinny electrical charges called electrons jump from a heated metallic surface, in a vacuum, to another metallic surface and cause current to flow between the two wheri connected together as shown in fig. 4. This current flow is always one-directional in that it will flow in one direction only—never in the reverse.
To produce such a flow of electrons, which constitutes an electric current the following fundamental requirements must be obtained:
1. There must be a continuous source of supply for the cathode which produces the current flow. ,
2. It must be maintained at the high temperature necessary for the dissipation of electrons from it.
3. To produce this continuous flow of electrons a force must be supplied to transfer them through space.
Now, as the electrons consist of infinitely small negative charges of electricity, it is evident that they are attracted to a positively charged body and repulsed by a similarly or negatively charged substance. (See page 27.)
Hence if a second element (anode) be added within the vacuum enclosure and in addition be maintained at a positive potential with respect to the cathode, it will in accordance with the above reasoning, attract the negatively charged electrons to it and at a rate which is dependent upon the rate at which they are supplied by the cathode.
In its simplest form therefore a .vacuum tube consists of two electrodes—a cathode and an anode (sometimes referred to as filament and plate), the former emitting or discharging the electrons and the latter acting as a collector of electrons. When this condition exists the vacuum tube is called a diode or two-electrode vacuum tube.
- Fio. 5.—Shows how a current will be registered by the galvanometer when the switch is connected to make the insulated plate positive. The current flow, however, will cease when the plate is made negative—the switch connects the insulated plate with the negative terminal of the filament.
Electron Emission.—The phenomenon that electrons can be made to leave a conductor when properly stimulated to do so as in the case of a radio vacuum tube, is called thermonic electron emission, sometimes called only electron emission.
The electron emission also known as the Edison effect was found by the famous inventor in his early experiments with the incandescent lamp sometime before 1890.
Edison observed that when a metal plate was sealed inside a lamp bulb so that it was between and separated the two sides of the carbon filament, but was entirely insulated electrically from the filament, that an electric current flowed through a galvanometer when connected between the outside terminal of the metal plate and the positive terminal of the filament.
When on the other hand, the connection was reversed, and the galvanometer connected between the negative terminal of the filament and the outside terminal of the plate, the current flow stopped.
Although this phenomena was known at this early date, its availability could not be utilized, due to the absence of the vacuum tube. It was only after discovery of the vacuum tube by Prof. J. A. Fleming and Dr. De Forest that this perhaps greatest invention in the twentieth century could be made serviceable.
The flow of current can be amplified by small voltage changes, the control of all changes is very marked and instantaneous in action—there being no lagging, also the electricity utilized or generated can be almost limitless in their number of pulsations.
This current flow or movement of electrons may be accelerated by increasing the temperature of the conductor. Once free, most of the emitted electrons, in a vacuum tube make their way to the plate, but others return to the cathode, repelled by the cloud of negative electrons immediately surrounding the cathode. This cloud of electrons surrounding the emitting cathode is known as the space charge.
A few of the electrons that reach the plate may have sufficient velocity to dislodge one or more electrons already on the plate. The dislodging of those electrons from the plate by other fast moving electrons are called secondary emission.
When this occurs there is actually a simultaneous electron flow in two directions.
DIODE
(2 ELEMENT TUBE)
TRIODE (3 ELEMENT TUBE)
PLATE-
FILAMENT
- PLATE
Not Connected
PLATE
FILAMENT-
- FILAMENT
Fig. 6.—Views of two and three element vacuum tubes showing arrangement of prongs and wiring symbols.
TETRODE
(4-ELEMENT TUBE)
PENTODE
(5-ELEMENT TUBE)
PLATE
GRID
GRID
PLATE
- GRID
GRID (Connection)
SCREEN
SCREEN GRID
CATHODE
HEATER
GRID (Connection)
SCREEN
PUTE CATHODE (Inside of Grid)
HEATER
SUPPRESSOR GRID
IATHODE (Inside of Control Grid)
(Heater not Shown Located Inside of Cathode)
GRID
SCREEN GRID
CATHODE
GRID
HEATER
Fig. 7.—Views of four and five element vacuum tubes, showing arrangement of prongs and wiring symbols.
HEATER
HEATER
Fig. 7.—Views of four and five element vacuum tubes, showing arrangement of prongs and wiring symbols.
Vacuum Tube Fundamentals.—By definition a vacuum tube consists of a cathode, which supplies electrons and one or more
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