123. Screen Grid Vacuum Tubes. - Figure 86 is an illustration of the UY-224 Radiotron. This screen grid tube (sometimes called a four electrode tube) is of the heater type, and is different f rom the three electrode tube in that it has another grid called the screen grid, which very nearly surrounds the plate. The use of the screen grid permits a much greater amplification of an alternating voltage than is possible with a three electrode tube. In spite of its great amplification ability it has not come into general use as an audio frequency amplifier.

124. Vacuum Tube Symbols. - It is customary in circuit diagrams to use symbols to indicate tubes. Figure 87 shows four of the symbols ordinarily used to indicate the different types of tubes. The symbol for a rectifier tube of the UX-281 type is shown in (a). The symbol for the ordinary type of three electrode tube, in which the filament emits electrons, is shown in (b). The symbol for the heater type three electrode tube is shown in (c), and (d) is used to indicate the heater type of four electrode (screen grid) tube. Other symbols are sometimes used, but except for slight variations, those shown here are the most common.

125. Vacuum Tube Amplifier Circuits. - Some means must be provided for coupling (making a connection between) vacuum tubes, if their amplifying property is to be utilized. There are three methods of coupling in general use, known as "resistance coupling," "impedance coupling," and "transformer coupling." These different methods are often used in combination so that a great variety of circuits are possible.

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