The Audio-frequency Power Amplifier

Introduction to Audio Power Amplifier

In this tutorial we shall study vacuum-tube amplifier circuits which give a large output voltage of substantially the same waveform as that of the input voltage, accompanied by a large curr ent of like waveform, so that there is considerable power delivered to the load, rfhe operation really is a power-conversion process, as in most cases all of the output power derives from the plate d-c source, under the control of the signal voltage applied to the grid. In general the treatment can often be made similar to that used for voltage amplifiers, except that usually the operation is not linear and therefore graphical or experimental procedures give most accurate results.

Except for details, a power tube resembles a voltage-amplifier tube. Power tubes vary in size depending upon the amount of output power desired and the efficiency of operation. For a small amount of output power a voltage tube may be used. The tubes with the higher power ratings need larger anodes to dissipate by radiation the heat derived from the energy of the impinging electrons, without making too high a temp erature necessary. For tubes of very high power rating the heat may be carried away by convection by an air or a water stream. A second requirement for a power tube is a sufficiently large cathode to furnish the necessary electron emission. This also means that a relatively large amount of heating power will be needed. Small power tubes use oxide- coated cathodes, while those of medium power employ thoriated tungsten, which will withstand somewhat more ionic bombardment. High-power tubes with their very high voltage drops use pure-tungsten filamentary cathodes. Tubes of small and moderate ratings may be triodes, tetrodes, or pentodes, but for the very high power ratings there is a problem of cooli ng the screen grid of the latter two kinds, which makes the triode usually preferred.
In making a choice of tube, the class of operation, which is related to the efficiency, must be taken into account. If the very best quality of output is required, as in some measurements circuits, it is well to use class A1 operation as this renders the output most free of distortion. In this connection it may also be advisable to choose a tube of higher power rating than actually required, so that the grid drive may be made small compared with the maximum allowable and thus decrease the distortion. 

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