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THEORY AND WIRING OF AMPLIFIERS 551
requiring more than one voltage for plate power, the rectifier supplies the highest voltage necessary, and power for the smaller tubes is obtained by means of voltage drop through resistors included in the rectifier output circuit for that purpose. Screen grid voltage, which is commonly lower than the plate voltage of the same tube, is secured in a similar way. Sample plate supply circuits are traced in detail on Pages 556 and 564.
Filament and Heater Supply
(24) Filaments may be heated by either a. c. or d. c, depending on the type of tube, while indirect heaters are usually supplied with a. c. Direct current is compulsory for the smaller filaments. Filament and heater voltages are usually low, not more than ten volts. Current requirements range from a quarter of an ampere to about three amperes. One Western Electric rectifier tube, however, uses six amperes at fourteen volts. D.-c. applied to the filaments of small tubes may be filtered to remove ripple.
(25) Four methods of providing heater or filament power are in common use. One is the external storage battery. Another is the external motor-generator, and a third the external rectifier. All these provide d. c. The commonest method provides a. c, and consists of a transformer with a low-voltage secondary and a 1 10volt primary, built into the amplifier.
Very often this transformer, and the plate supply transformer mentioned above, are the same instrument. The same transformer can have more than one secondary, and it is common practice to provide a high-voltage secondary, consisting of many turns of wire, and one or more low-voltage secondaries, with comparatively few turns of wire, all on the same core. Such secondaries operate independently of each other. The voltage delivered by each secondary will be governed entirely by the ratio between its turns and the number of turns on the primary.