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546 RICHARDSON'S BLUEBOOK OF PROJECTION
secure excessive amplification. When that is done the three-element tube will distort sound by amplifying some frequencies more strongly than others. If the amplification, and therefore the feed-back, is pushed still further the tube will "howl" at the frequency it has amplified most strongly, and nothing will come from the speakers but a continuous whistle. (15) The tube is in a condition of "oscillation," that is, acting as a converter that uses its d. c. plate power supply to generate an alternating current of audible frequency. It is no longer acting as an amplifier, but as an oscillator.
A good theatre amplifier is designed with sufficient "loss" to make sure that the maximum volume control setting cannot possibly cause any tube to oscillate, or even to distort sound seriously, although some theatre amplifiers of excellent manufacture do distort slightly at extreme high volume.
Increasing Amplification — Screen Grid Tubes
(16) To obtain more amplification per stage without distortion, and thus use fewer tubes, the design of the tube is modified to include a fourth element. This fourth element is the so-called "screen grid," and tubes containing it are "screen grid tubes."
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COATTROL CMD W
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HEATER. Fig. 135. — Screen-grid four electrode tube, or tetrode.
(17) The screen grid completely surrounds the plate. When this element is built into the tube there is no longer any effective inter-electrode capacitance between control grid and plate. The inter-electrode capacitances are between control grid and screen grid, and between screen grid and plate. The screen grid is maintained at a positive voltage that is slightly lower than the plate voltage.