202. Description of the PA-19A Voltage Amplifier Metering Panel. -The test meter mounted near the top of the metering panel is a DC voltmeter. The meter has two scales, the upper scale is used in making voltage measurements up to 250 volts, the lower scale, used in conjunction with the meter push-button, measures voltages up to 25 volts. The double jack to the left of the meter is a phonograph pick-up jack, and is used for making a connection from a non-synchronous turntable to the input of the voltage amplifier. The first jack on the left is for use, in conjunction with the meter, in measuring the voltage of the storage battery. The second jack from the left is used for measuring the plate voltage of the voltage amplifier, and the third jack from the left is used to measure the plate voltage of the other voltage amplifier. The jack on the right is for checking the plate voltage of the photo-cell.

203. Description of the PA-19A Volume Control Panels. -These two panels are identical. One is used to control the volume when using one voltage amplifier and the other is used to control the volume when using the other voltage amplifier. The bulls-eyes at the centers of these panels indicate which voltage amplifier is in use. The knobs on the left are volume control knobs and the knobs on the right are the output fader controls. These output faders are used, when changing from one voltage amplifier to the other, to prevent a loud popping noise incident to changing over. They consist of variable resistances in the output circuits of the voltage amplifiers.

204. Description of the PA-20A Power Amplifier Rack. -Figure 122 shows the arrangement of parts on the PA-20A power amplifier racks. The four power amplifier units mounted on the back of this rack are of the PK-1 type and were described in section 140. These amplifier units are arranged in two groups of two units each for the purpose of control, but each amplifier unit can be turned "on" and "off" separately. The two units at the top comprise one group and the two lower units make up the second group.

The switch on the second panel from the top is an AC power line switch for turning "on" and "off" the power of the entire rack.

The third panel from the top is the control panel for the amplifier units No. 1 and No. 2. The rheostat knob near the center of the panel is used to control the AC line voltage input to the amplifiers. The meter in the upper right-hand corner is an AC voltmeter for indicating the voltage of the power input to the amplifier units. The two switches below the meter are single-pole power switches for turning the power "on" and "off" of each amplifier unit separately. The two jacks below the switches are monitoring jacks and are used for monitoring the output of each amplifier separately.

The fourth panel from the top is another power amplifier control and is identical to the one just described. It is used to control the power amplifier units No. 3 and No. 4.

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