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INTERNATIONAL PROJECTIONIST
VOLUME VIII
NUMBER 4
APRIL 1935
STEP-BY-STEP ANALYSIS OF SOUND REPRODUCING EQUIPMENT
Aaron Nadell
IX. RCA Modern Power Amplifier
FIGURE 1 is the circuit drawing of a single-stage power amplifier, used in modern sound systems to provide the final step of amplification between a voltage amplifier and the loud-speakers.
Power input is shown at the lower right-hand edge of the drawing, just below and to the right of the long terminal board, by the two arrowheads labelled "60-cycle A.C. supply." Below and just to the left of these arrowheads is a rectangle drawn in dashes, which contains the time-delay starting relay.
Beginning at the lower arrowhead we may trace downward, left, up through a coil, right, and up to the upper arrowhead. That coil is the solenoid, or magnet coil, of the starter relay, and there is no switch in its line. As long as the external switch to the amplifier is closed, this coil remains energized. Its work is
to pull and hold down the plunger arm that is drawn through it, thereby closing the contacts seen above the coil and within the rectangle of dashes.
The purpose of this relay is to apply plate current to the tubes automatically after the filaments have had time to heat up, which in practice is about thirty seconds. The delay action in the type of relay shown in Figure 1 is created by motion of the plunger in a sealed chamber containing gas. The slowly-moving plunger presses against the gas, which makes room for it by escaping through an asbestos membrane into an adjoining chamber. The time required for the gas
Note: Figure 1 of this article appears on the second page following.
to get out of the way gives the filaments time to heat and, especially, gives the mercury in the rectifier tubes time to vaporize, thus avoiding any possibility of a short-circuit in those tubes through the presence of liquid mercury.
The time-delay relay in some amplifiers of this type does not depend upon the action of the plunger in oil, but upon the curvature of a heating element. All metals expand under heat, some more than others. If a strip made of two metals of different expansion rates is heated, it will curl, the metal that expands more being on the outside of the curve, and the one that expands less on the inside. Thermometers are sometimes built on this principle, especially for high temperature work; and so are the automatic relays that shut off a flat-iron when it becomes hot enough, and heat it again when it cools.
In some amplifiers of the type of Fig
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