Power-Limiting Factors

Author: N.H. Crowhurst

The discussion thus far has been about getting audio voltage from tubes. To be able to hear sound from the loudspeaker, however, we need power. The loudspeaker needs watts to drive it, not just volts. For every watt audio of output that we get, there must be some power dissipated in the output tube, because the audio current in the plate circuit of the tube that goes to the voice-coil circuit also has to go through the tube.

This means that some power must be used up in the tube, which, in turn, makes the tube get hot. Thus there is a power limit on the tube before it will start to destroy itself. This limit is ietermined by the size of the internal components of the tube and the arrangements made by the tube designer to dissipate the heat. The manufacturer always specifies the maximum power dissipation of a tube intended for power-output use.

The tube is always in series with the load or impedance into which the power is fed because the tube is basically a current controlling device. This means the same current fluctuations flow in the tube as in the output circuit. By using a higher supply voltage, it is possible to drop a smaller proportion of the output power in the tube and get a larger proportion into the output circuit. It is thus advantageous to use a higher supply voltage because it gives better efficiency. This unfortunately brings us against another limitation. For any particular tube, there is a limit to the working voltage that may be used before some kind of breakdown might occur. This limit also is usually specified by the tube manufacturer.

Power to drive loudspeaker has to flow through output tube as well.