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THEORY AND WIRING OF AMPLIFIERS 513
"shadowed" by the negative control wires. Since the control wires, being negative, repel electrons, very few reach the positively charged screen, but instead pass between the turns of the screen to the plate. Screen current therefore is unusually low. This feature, combined with the suppressor action already described, results in a tube of high-power output, high-power sensitivity, and extraordinarily high efficiency.
Phase Inversion
VT-2 is not an amplifier in the sense of adding to the volume of sound, although it does amplify. It is not intended to increase sound volume, but to act as a phase inverter.
To understand this function, refer to Fig. 195. In that diagram an input transformer with a center-tapped secondary makes it possible for speech current which is pulsating d.c. to be applied to push-pull grids as a.c. The accompanying description indicates that when the lower grid becomes more positive the upper grid becomes more negative, and vice versa, which is the indispensable condition for push-pull amplification.
Now in Fig. 201 it is desired to couple the pulsating d.c. output of VT-1 to the push-pull stage composed of VT-3 and VT-4. A transformer could readily be used, as in Fig. 195, but a tube is not only less costly, but lighter in weight, and can be made to serve the same purpose.
The plate output from VT-1 can be traced through several paths. One leads through C-5 right to the grid of VT-3 in the ordinary way of condenser coupling. Another leads through R-6 to the grid of VT-2. The amplified output of VT-2 is coupled through C-6 to the grid of VT-4. Since a tube's output speech voltage is always opposite in phase to its input speech voltage, VT-4 grid is 180 degrees out of phase with VT-3 grid — the same condition produced by the transformer in Fig. 195.
However, push-pull grid input signals must be equal in volume as well as opposite in phase, and the amplification added by VT-2 would upset the balance if not counter