SUMMARY

This chapter has introduced you to the four basic types of vacuum tubes. The following is a summary of the main points of the chapter.

THERMIONIC EMISSION is caused when metallic substances are heated to high temperatures. Electrons liberated by thermonic emission provide the conduction currents of vacuum tubes.

A DIODE VACUUM TUBE is composed of two elements: the cathode and the plate.

THE CATHODE is the electron-emitting element of a tube. Cathodes are usually composed of special materials that are heated either directly or indirectly.

DIODE OPERATION depends upon current flow through the tube. Because the cathode is the only electron-emitting element in the tube, current can only flow in one direction, from the cathode to the plate. For current to flow, the plate must be positive relative to the cathode. When the plate is negative relative to the cathode, current cannot flow within the tube.

THE CHARACTERISTIC CURVE for an electron tube is a graphic plot of plate current (I_p) versus plate voltage (E_p). From this, dc plate resistance can be computed by the formula:

FACTORS THAT LIMIT VACUUM TUBE OPERATION are plate dissipation, maximum average current, maximum peak-plate current, and peak-inverse voltage.

DIODE RECTIFIERS take advantage of the fact that diodes will conduct in only one direction. When ac voltages are applied to diodes, conduction occurs only on the alternation that makes the plate positive relative to the cathode. Because of this, the output current consists of one polarity. Because it flows in pulses rather than continuously, it is called pulsating dc.

DIODE CONSTRUCTION is the basic construction plan of most vacuum tubes. The tube is constructed of the following parts: filament and/or cathodes, plates, envelope, and base.

A TRIODE is basically a diode with a control grid mounted between the plate and the cathode. The control grid gives the triode the ability to amplify signals.

THE OPERATION OF A TRIODE depends on the ability of the control grid to either increase or decrease conduction through the tube in response to an ac input signal. The output voltage is developed across the tube between the cathode and plate because of the voltage drop across the plate-load resistor changing as the plate current responds to the input signal.

TUBE BIASING is the process of placing a dc voltage, usually negative, on the grid. Bias has several functions in circuit design. Biasing may be divided into two types: fixed and self. Tubes using fixed bias have a dc voltage applied to their control grids from an external source such as a battery. Self-biasing voltages, on the other hand, are derived from current conducting through the tube. The most common types of self-biasing are cathode biasing and grid-leak biasing.

THE CLASS OF OPERATION OF AN AMPLIFIER is determined by the bias applied to a triode. An amplifier operating as class A conducts continually through the duration of the input cycle. Class AB operation occurs when the amplifier conducts for more than half but less than the entire duration of the input cycle. A class B amplifier conducts for only 50% of the input cycle. The class C amplifier conducts for less than half of the input cycle.

TRANSIT TIME is the time required for electrons emitted by the cathode to reach the plate. Because transit time in a vacuum tube is considerably less than the speed of light, vacuum tube operation is affected at high frequencies.

INTERELECTRODE CAPACITANCE is created by the naturally occurring capacitance between elements in a vacuum tube. One effect of interelectrode capacitance is to feed back a portion of the output of a triode to the input. This effect is a prime-limiting factor in applying triodes. It is a major reason why triodes are seldom used - especially at the higher frequencies.

MU AND TRANSCONDUCTANCE are measures of tube efficiency. Mu (μ), or amplification factor, is a measure of the amount that plate voltage varies in relation to variation of the input voltage. Mathematically, mu (μ) is expressed as

TRANSCONDUCTANCE, on the other hand, is a measure of the amount of variation of plate current caused by a variation of the input signal. Mathematically, it is expressed as:

TETRODES were developed to compensate for the effects of interelectrode capacitance. Placing a positively charged screen grid between the control grid and plate has the effect of adding a capacitor in series with the capacitance that exists between the control grid and plate. This reduces total capacitance below the value of either capacitor as shown by applying the formula:

SECONDARY EMISSION of electrons from the plate is caused by the acceleration of electrons by the screen grid. This causes the performance of a tetrode to be degraded. In addition to reduced amplitude, the output signals become noisy.

PENTODES do not suffer from the effects of secondary emission. This is because a negatively charged suppression grid placed between the screen grid and plate forces any electrons emitted back to the plate.