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THEORY AND WIRING OF AMPLIFIERS 521
The same matching requirements apply to full-wave rectifier tubes, Figs. 50, 51, fur example. Rectifiers of the type of VT-5 of Fig. 201 can be checked with a portable test meter, if one is available, for internal match of its two halves ; most such tubes do not have a common filament as shown in the drawing, but two separate filaments in one envelope.
Fig. 200 is typical of many amplifiers in that meters are not provided. Inserting new tubes and checking their effect on volume provides a very crude test. A better way is to use a portable test meter. The theatre can afford to buy this instrument; it will repay its cost many times over in case of trouble, and without trouble will pay for itself in a few years by economizing tube life and otherwise facilitating maintenance.
High space current is less common than the reverse condition, but more dangerous. It may cause the amplifier to overload, distorting the sound, and it endangers every part (for example, resistor R-9 in Figure 201) through which the excessive current must flow to complete its circuit. There are three general causes of high space current. One is high line voltage, which, connected across the primary of T-2 in Figure 201, or of T-l in Figure 51, will generate excessive voltage in the secondaries of those transformers. The remedy for this condition has been mentioned. A second cause is a "gassy" tube, which must be discarded. A third is low grid bias, which is the commonest cause in amplifiers that use "C" batteries.
We have seen in connection with rectifier tubes (Page 93) that inclusion of mercury vapor gas increases the emission. Argon or other gas is intentionally added to photo-electric cells for the same reason. But gas is highly undesirable in amplifier tubes for the reason that the positive ions are too large and heavy to respond readily to changes in the grid charge. The presence of gas in an amplifier tube therefore upsets the amplifying action, and renders the tube unsuitable for use. Amplifier tubes are carefully pumped to the highest possible vacuum, and when the pump has done all it can a charge of magnesium