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THEORY AND WIRING OF AMPLIFIERS 519
long as the power line voltage does not fluctuate.
In many houses, however, the line potential may vary from 85 to 135 in the course of one evening. This will happen wherever the local power house is underequipped, cannot maintain full output during the peak load of early evening, and does not cut down its output quickly enough when the peak load has passed. In such cases, which are common, the remedy is not to install a rheostat in the a. c. filament circuits, but to install a voltage control rheostat or transformer in the power line. If tubes are subjected to prolonged high voltage, even the heavy filament of a large tube is likely to burn out.
Much more common than the burning out of a filament is loss of the power of emission. This happens sooner or later to all tubes. While "rejuvenation" is possible, the method employed for that purpose varies greatly according to the type of filament involved, moreover, a mistake in rejuvenating is likely to result in stopping a show when the rejuvenated tube is put back into use. The common practice is to discard tubes when once they have lost emitting power. Tubes are no longer so expensive as to justify the time, effort and risk involved in attempting to restore them after they have worn out.
Loss of emitting power is revealed by a decline in plate current reading, plate voltage and grid bias remaining unchanged. It is usual to assume that those voltages have not altered, and merely to install a new tube whenever one in use shows low space current. Only when a series of new tubes all show low space current is it worth while to check the supply voltages.
In Fig. 201, meter M-l can be used to reveal loss of emitting power, since it indicates space current values. As stated, the meter can be switched to read either the combined space currents of the two left-hand tubes or the total space current of the two right-hand tubes. If both readings show low, there must be some common cause, not loss of emission in a single tube. Loss of emission in VT-5, the rectifier tube, may be suspected, or low line voltage. However, if only one of the readings shows low, while the other is normal, one of the two sets of