141. The Type PA-41A Amplifier Unit Used in the Type PG-13 Equipments. - The voltage and
power amplifiers of the PG-13 equipment are constructed as one unit. The voltage amplifier
utilizes three UX-112A Radiotrons. The output of the projector circuits is coupled to the grid
circuit of the first UX-112A of the voltage amplifier by means of an auto-transformer. The
output of the first tube is resistance coupled to a transformer which operates the next tube of the voltage amplifier, also a UX-112A.
The output of this is impedance coupled to the transformer which actuates the grid of the third voltage
amplifier tube (a UX-112A Radiotron). The output of the third UX-112A (output of voltage amplifier) is
resistance coupled to a push-pull auto-transformer which operates four UX-250 Radiotrons in a parallel
push-pull circuit. This is the power amplifier stage. The Type PA-41A amplifier is illustrated in Figure
Volume control is provided by means of a control in the input circuit of the second UX-112A Radiotron.
The filament and plate voltages for this amplifier (both voltage and power) are supplied from a motorgenerator set. The three unit motor-generator set used with the Type PG-13 equipments is described in
section 37. Grid voltages are obtained from small dry batteries ("C" batteries).
142. Voltage and Power Amplifiers Used With Theatre Phonographs.-The output of the pick-up of the
theatre phonograph is connected, by means of a transformer, to a stage of push-pull amplification. The
output of this stage is transformer coupled to a second stage of push-pull amplification. The output of
the second stage is transformer coupled to the input of the power amplifier.
Both stages of the voltage amplifier (Type PA-11) use UX-226 Radiotrons, four of these tubes being
used. The grid and plate voltages are supplied by a socket power unit (Type PK-4). This unit is similar
to the rectifier described in section 134, except that it uses one UX-280 tube instead of two UX-281
tubes. A UX-280 tube has two plates and, therefore, is capable of full wave rectification, each plate
being connected in the circuit as they would be if two tubes were used. The low voltage alternating
current for lighting the filaments of the UX-226 Radiotrons is supplied by a low voltage winding on the
rectifier power, transformer. The power amplifier unit used is the Type PK-1 described in section 140.