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THEORY AND WIRING OF AMPLIFIERS 537
direct and indirect heaters. The direct heater may be lit by either d. c. or a. c, since its temperature cannot change quickly enough to fluctuate with the a. c. cycle, and its emission will not vary at line frequency. If it is small, and quick to heat and cool, the cathode must be heated by d. c. Otherwise its emission will fluctuate at the line frequency (25, 50 or 60 cycles) and a corresponding hum will be heard in the sound. Indirect heaters have already been explained. (Page 452.) They make it possible to heat the cathodes of small tubes with a. c, without corresponding variation in emission. In such tubes the heating circuit and the cathode circuit are electrically insulated from each other.
Anode of the Amplifier Tube
The anode of an amplifier tube is commonly a flattened ring of metal, but in large tubes it is sometimes made of graphite or carbon. The choice of these latter materials arises from a desire to keep the m&mBtA'c\s cool as possible. In large-size theatre tubes, metal anodes become heated, sometimes to the point of glowing red or even white-hot, because they are subjected to incessant bombardment of negatrons emitted from the cathode. But an incandescent anode is in itself an emitter, like any other incandescent metal. And while negations emitted from the anode are drawn back into it again by its powerful positive charge and do not take any direct part in the tube action, the cloud of negatrons that have left its surface and not yet returned to it constitutes a space charge surrounding the anode which tends to repel negatrons approaching from the cathode. Carbon or graphite anodes, being thick and massy, are not easily heated to the point of incandescence. A graphite-anode tube is therefore capable of somewhat greater current flow from cathode to anode (and hence of greater volume without distortion) than a metal-anode tube of the same size. There are other advantages, however, in favor of metalplate tubes. Both types are widely used at the present day, but the latter are more common.