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The volume control is mounted at the center of this panel. It is calibrated from 0 to 40, and provides 20 steps of equal "gain," so designed that an increase of any one division on the dial gives the same apparent increase of volume as any other division. Along the bottom of the panel is a row of five metering jacks. These jacks, in conjunction with a test meter (to be described later), are used for measuring the plate currents and the grid voltage of the voltage amplifier. A phonograph pick-up jack is mounted in the upper left hand corner of this panel. This jack is used for making a connection from a non-synchronous phonograph to the input of the voltage amplifier.

Small dry batteries are mounted to the rear of the panel, for supplying the "C" bias of the voltage amplifier. See Figure 116.

169. Description of the PA-1 Power Amplifier Panels. -The power amplifier uses two panels mounted directly below the voltage amplifier panel. The upper panel has no controls, but has a hole cut into it which is covered with a mesh screen and allows inspection of the UV-845 tubes from the front of the rack. The controls for the power amplifier are mounted on the lower power amplifier panel as shown in Figure 115. For a description of the power amplifier unit see section 137.

The two meters on the outside, to the left and right, are plate current milliammeters for reading the total plate current of each bank of five UV-845 tubes. The plate current of the front bank of tubes is read on the meter to the right, and the current of the rear bank of tubes is read on the meter to the left. These meters are recessed in the panel and are protected by a glass cover, because they are in a high voltage circuit. - The center meter indicates directly the voltage across the voice coils of the loud speakers, and indirectly the volume of sound from the loud speakers. The two small controls under this meter are the grid bias or "C" voltage controls. Since the grid bias controls the plate current of amplifier tubes, these controls are used to control the plate current of the UV-845 tubes. The plate current of one bank of tubes should be regulated so as to be equal to the plate current of the other bank. If this is not done a high pitched whistle or some distortion may result.

170. Description of the PA-1 Metering Panel. -The metering panel is mounted directly below the lower power amplifier panel. This panel has three meters. The two to the left are exactly alike and are both connected to jack plugs through suitable cords. These meters are used in conjunction with the jacks distributed over the rack for measuring the various voltages and currents of different circuits. The two meters permit two simultaneous measurements from different jacks. These meters can be used on either rack making it possible to make four simultaneous readings if desired.


Chapter Thirteen Pages
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