6. Vacuum Tubes. While vacuum tubes are not used to the extent that
they were in the past, you may still encounter electronic circuits
that use these devices. As with electronic components, vacuum tubes
come in a variety of configurations.
They may range from a simple
two element diode to a multielement cathode-ray tube.
Each type of vacuum tube has a symbol to represent it in a circuit
schematic.
Vacuum tubes are indicated by a circle representing the
envelope of the tube, with the elements of the device shown within.
Figure 1-24 illustrates a triode vacuum tube. As shown, the element
labeled 1 is the cathode heater.
Element 2 is the cathode.
(Sometimes a directly heated cathode will be represented by the
symbol of the cathode heater alone.) The element labeled 3 is the
plate or anode.
Element number 4 is the control grid.
Additional
elements may be added depending on the circuit function of the vacuum
tube.
Figure 1-25 shows several symbols for various designs of
vacuum tubes.
The general operating principle of a vacuum tube is control of the
current flow through the tube. This is done by heating the cathode,
directly or indirectly, and applying a bias voltage between the
cathode and the plate. The amount of current that flows through the
tube is controlled by adjusting the bias or through the use of a
variety of grids. Figure 1-26 shows the triode shown in figure 1-24
hooked up in a circuit to provide voltage amplification.
Vacuum
tubes are labeled with the letter "V. "
Figure 1-24.
Triode Vacuum Tube.
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