137. Power Amplifiers Used in Type PG-1 and PG-2 Equipments.-As stated in the preceding section, the output of the voltage amplifier is impedance coupled to the power amplifier input transformer. In the PG-1 equipments the input to the power amplifier is connected to the grids of ten UV-845 Radiotrons in a parallel push-pull circuit. The power amplifier of the PG-2 is similar except that only four UV-845 Radiotrons are used.

These power amplifiers derive their filament, grid, and plate voltages from the four unit motorgenerator set described in section 36.

138. The Type PA-12A Voltage Amplifier Unit.-This unit is used in the Types PG-3, 4, 6, 7 and 8 equipments. The output of the projector circuits is transformer coupled to the grids of two UX-210 Radiotrons connected in push-pull. The output of this stage is fed successively through two additional stages of transformer coupled push-pull amplification, each stage utilizing two UX-210 Radiotrons. Figure 94 shows the arrangement of the tubes. The two tubes on the right are in the input stage and are normally kept covered with metallic cans. The intermediate tubes and those on the left are respectively in the second and third stages of voltage amplification.

Volume control is effected by controlling the output of the second push-pull stage.

Filament current for the Radiotrons of this amplifier is supplied from Exide Type MVJ-13 storage batteries. See Chapter IV. The plate and grid voltages are supplied by standard "B" and "C" dry batteries.

139. The Type PA-12B Voltage Amplifier Unit.-This voltage amplifier unit, used in the PG-10 equipments, is similar in most respects to that described in the preceding section. It, also, utilizes three stages of transformer coupled push-pull amplification, but, in this case, the less powerful UX-112A Radiotrons are used, and the volume control is effected by controlling the input to the second push-pull stage.

The filament, grid and plate voltages are supplied in the same manner as described in the section above.

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