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550 RICHARDSON'S BLUEBOOK OF PROJECTION
tubes. We are now ready to inquire in detail into the power requirements of amplifying tubes.
Plate Power Supply
(22) The plate of the amplifying tube uses d. c. only. In this the amplifying tube again differs from the rectifying tube. The function of the rectifying tube is to convert a. c. to d. c, and a. c. is applied to its plate for that purpose. But the function of the amplifying tube is to superimpose an a. c. ripple on steady d. c. Alternating current cannot be used for the plate supply of amplifiers, and the d. c. that is used must be extremely steady and pure. Any ripple it might contain would be heard in the loud speakers as sound.
The plate voltage runs from about 90 volts for small tubes up to about 1,000 volts. The maximum current per tube is something under 100 milliamperes, while the smallest tubes have a plate current of about 1 milliampere.
(23) Plate power for amplifiers is provided from any of four sources. Dry batteries are used for small tubes, particularly for those of photo-electric cell amplifiers mounted on or near the projector. Storage batteries with a total of 350 volts have been used in some systems, although this method is substantially obsolete. Highvoltage generators enjoyed a certain vogue, but are also obsolete or rapidly becoming so. The fourth method, now universally favored, is that of the built-in rectifier, using rectifying tubes as a rule, although occasionally "stack" or "disc" rectifiers (see Page 287) are encountered. Line a. c. is supplied to a power transformer built into the amplifier. A. c. drawn from the high-voltage secondary of this transformer is rectified, filtered, and supplied to terminals "B" of Figure 128. These terminals may actually exist in the amplifier, or wires may run solid from the rectifier filter to the amplifying tube sockets, since there is no real necessity for terminals when the power supply is built into the same cabinet or panel. If the panel carries amplifier tubes of more than one type,