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544 RICHARDSON'S BLUEBOOK OF PROJECTION
is the commonest method of controlling amplifier volume.
The volume-control resistor of Figure 133 is usually
built in a circle or a semi-circle, rather than as a straight
Fig. 133. — Illustrating use of coupling resistor for volume control.
line, and the adjustable contact is set by turning a knob, not by sliding it up and down as that drawing indicates. One manufacturer uses a motor to turn the knob, thus making remote control of volume a practical procedure. The motor can be operated from any point of the projection room where a suitable switch is installed, or from the auditorium. Mounted on the same shaft is a separate set of contacts (insulated entirely from the amplifier circuits) that operate a group of signal lamps. These lamps act as volume indicators, advertising to what point the sliding contact of Figure 133 has been set. Such motoroperated volume control is the exception. In most amplifiers the slider is adjusted manually. In some there is no slider, but only a switch that gives choice of either of two positions along the resistor, low volume and high volume. Sometimes there is a switch of that kind between tubes 1 and 2, and a sliding contact as in Figure 133 between tubes 2 and 3, or vice versa.
Push-Pull Amplification
(10) Figure 134 represents a very common variation of Figure 128, in which two tubes are used in the same stage of amplification. (11) This arrangement offers some improvement in both volume and quality (giving somewhat more than twice the volume of a single tube) and is especially favored in the later, or power, stages of an amplifier. The grid bias (12) of Figure 134 is shared by both grids, but when speech a. c. is induced in the secondary of the input transformer the upper grid will grow more negative while the lower grid becomes less