Vacuum Tube - Description

Description

A vacuum tube consists of two or more electrodes in a vacuum inside an airtight enclosure. Most tubes have glass envelopes, though ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads, in the form of pins, plug into a tube socket for easy replacement of the tube (tubes were by far the most common cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves). Some tubes had an electrode terminating at a top cap which reduced interelectrode capacitance to improve high-frequency performance, kept a possibly very high plate voltage away from lower voltages, and could accommodate one more electrode than allowed by the base.

The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermionic emission. A second electrode, the anode or plate, will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament (cathode) has a dual function: it emits electrons when heated; and, together with the plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode, and is used for rectification. Since current can only pass in one direction, such a diode (or rectifier) will convert alternating current (AC) to pulsating DC. This can therefore be used in a DC power supply, and is also used as a demodulator of amplitude modulated (AM) radio signals and similar functions.

While early tubes used the directly heated filament as the cathode, most (but not all) more modern tubes employed indirect heating. A separate element was used for the cathode. Inside the cathode, and electrically insulated from it, was the filament or heater. Thus the heater did not function as an electrode, but simply served to heat the cathode sufficiently for it to emit electrons by thermionic emission. This allowed all the tubes to be heated through a common circuit (which can as well be AC) while allowing each cathode to arrive at a voltage independently of the others, removing an unwelcome constraint on circuit design.

During operation, vacuum tubes require constant heating of the filament, thus requiring considerable power even when amplifying signals at the microwatt level. In most amplifiers, further power is consumed due to the quiescent current between the cathode and the anode (plate), resulting in heating of the plate. In a power amplifier, heating of the plate can be quite considerable; the tube can be destroyed if driven beyond its safe limits. Since the tube requires a vacuum to operate, convection cooling of the plate is not generally possible (except in special applications where the anode forms a part of the vacuum envelope; this is generally avoided due to the shock hazard from the anode voltage). Thus anode cooling occurs mainly through black-body radiation.

Except for diodes, additional electrodes are positioned between the cathode and the plate (anode). These electrodes are referred to as grids as they are not solid electrodes but sparse elements through which electrons can pass on their way to the plate. The vacuum tube is then known as a triode, tetrode, pentode, etc., depending on the number of grids. A triode has three electrodes: the anode, cathode, and one grid, and so on. The first grid, known as the control grid, (and sometimes other grids) transforms the diode into a voltage-controlled device: the voltage applied to the control grid affects the current between the cathode and the plate. When held negative with respect to the cathode, the control grid creates an electric field which repels electrons emitted by the cathode, thus reducing or even stopping the current between cathode and anode. As long as the control grid is negative relative to the cathode, essentially no current flows into it, yet a change of several volts on the control grid is sufficient to make a large difference in the plate current, possibly changing the output by hundreds of volts (depending on the circuit). The solid-state device which operates most like the pentode tube is the junction field-effect transistor (JFET), although vacuum tubes typically operate at over a hundred volts, unlike most semiconductors in most applications.