140. The Type PK-1 Power Amplifier Unit.-This unit is used in the Types PG-3, 4, 6, 7, 8, and 10 equipments. The output of the voltage amplifier is transformer coupled to a pair of UX-250 power Radiotrons connected in a push-pull circuit, and the output of this amplifier is transformer coupled to the stage and monitor speakers.

All of the filament, plate, and grid voltages are supplied from a power supply unit which operates directly from a 110-volt 60-cycle supply, hence its designation of SPU, or socket power unit. In the power supply unit, two UX-281 Radiotrons are used to rectify the high alternating voltage from the power transformer. This rectified voltage supplies the high DC voltage for the plates and the lower DC voltage for the grids of the UX-250 Radiotrons. The power transformer also supplies low voltage AC for the filaments of the UX-250 and the UX-281 Radiotrons.

Incorporated in the same unit with the rectifier and amplifier is a Rectox rectifying unit which is used to supply the necessary DC power for the fields of the stage speakers. This amplifier unit, with its field power supply unit, is shown in Figure 95.

The PG-6 equipments utilize four power amplifier units of the type described above. The PG3 and PG-7 each use two units, and the PG-4, PG-8 and PG-10 equipments use one power amplifier unit. Each power amplifier may be used to operate one, two, or four stage speakers.

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