When a three electrode tube is used as an amplifier, the grid is kept at a negative potential with respect to the filament. This negative grid potential is called the "grid bias" of the tube. If an alternating voltage is added to the steady negative potential of the grid, the relative negative potential between the grid and the filament will vary in accordance with the alternating voltage. The variations of the grid voltage will cause variations in the plate current. As the grid becomes more negative the plate current decreases and as the grid potential becomes less negative the plate current increases. If an electrical impedance, such as resistance, is placed in the plate circuit, between the plate and the battery, a voltage will be produced across it in proportion to the plate current. When an alternating current is impressed on the grid, a pulsating voltage will be produced across the resistor in the plate circuit. If the resistance in the plate circuit is sufficiently high, the pulsations of the voltage produced in the circuit, will be greater than the alternating voltage impressed on the grid. Therefore, three electrode vacuum tubes can be used to amplify (increase) variations of voltage. Various types of tubes are used for various purposes. Tubes designed primarily to amplify alternating voltages, where the energy output is not important, are called "voltage amplifier tubes." Tubes designed primarily to give a large energy output are called "power amplifier tubes."

120. Description of the UX-281 Radiotron.-Figure 83 shows the construction of the UX-281 Radiotron, which is a two element tube and is made for use as a rectifier. The action of this type of rectifying tube is similar to that of the Tungar discussed in section 56, but the UX-281 tube is capable of rectifying higher voltages than the Tungar bulb and its output is more readily filtered to give a constant direct current.

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