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THEORY AND WIRING OF AMPLIFIERS 547
Five-Element Tubes
(18) Although the amplification of a tube can be increased by addition of a screen grid, even this improvement does not enable the tube to work at its maximum capacity. There still remains another limiting factor which is called "secondary emission." This is the emission of negatrons from the plate, caused partly by the direct impact of negatrons coming from the cathode, and partly by the fact that the plate is heated by the multitude of such impacts. The presence of a positively charged screen grid close to the plate encourages such emission. The "plate space charge," that is to say, the cloud of negatrons that surrounds an emitting plate, repels negatrons coming from the cathode and limits the "space current" across the vacuum.
SUPPRESSOR cm
CONTROL GMD CATHODE
Fig. 136. — Five-electrode tube, or pentode.
A fifth element may be introduced into the tube to prevent secondary emission. That addition creates a fiveelectrode, or pentode tube. The fifth element is the "suppressor" grid, which is located between the screen grid and the plate, and is negatively charged. The suppressor grid discourages secondary emission and drives any negatrons that may escape into the vacuum back to the plate again. It thus destroys the space charge that surrounds the plate in a four-element tube, and permits still further amplification per stage.
Suppressor grids commonly receive their negative charge through direct connection with the cathode. In some pentodes this connection is made inside the tube.