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NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
It is housed in the same box with the sound head selector switch but suitably shielded from the latter to prevent coupling. Also housed in this box is a network for controlling the polarizing voltage to the phototube and further space is provided for a second network for push-pull operation.
Provision for Volume Pre-Selection. — In addition to these parts, it is possible to install in this box a 16-db variable T-pad attenuator for volume pre-selection. This pad connects in the link circuit between the sound head and the voltage amplifier. This unit is not supplied as standard equipment.
The box which houses these parts constitutes the main sound head selector switch and volume control. It is commonly termed the "Main Fader." This main control unit, together with the extension unit and extension rods, are shown physically hi Fig. 14. The interior construction of the mam control unit and mercury selector switch is shown hi Fig. 15.
Amplifiers and Power Supply Equipment
Voltage Amplifier. — The voltage amplifier employed in this equipment is a two-stage amplifier, using RCA16 20 tubes as resistance-coupled pentodes in both stages with 18-db degenerative feedback on the second stage. Filament and plate power are both obtained from the 25-watt power amplifier with which it is associated. Its electrical characteristics are as follows: Input Impedance 250 or 500 ohms Load Impedance 250 or 500 ohms Frequency Response =t 1 db from 30 to 10,000
cycles Gain 55 db
Maximum Output Level +6 db (0.006 w. ref.) Output Noise Level —75 db (0.006 w. ref.)
25-Watt Power Amplifier. — The power amplifier consists of three stages, using one RCA -16 20 as a pentode in the first stage, oneRCA-1620 as a triode
hi the second stage, with a third RCA-1620 operated as a triode phase inverter. The last stage is a parallel push-pull stage, using four RCA-1622 tubes which are special tubes similar to the RCA-6L6. Two RCA-5U4G rectifiers provide the plate supply. In addition, one RCA-874 regulator tube is used to regulate the polarizing voltage for the phototube which is taken from this amplifier. Inverse feedback is employed in the last two stages to reduce the distortion at all output values below maximum. It does not increase the maximum output of the amplifier, however. A second feedback circuit is employed on the first stage to increase its output, so that there will be sufficient margin between the undistorted output of the first stage and the required input to the second stage.
A plate current meter is supplied on the front panel of this amplifier which may be switched to measure individually the plate current of each of its tubes or to measure the combined plate currents of the tubes hi the voltage amplifier.
The electrical characteristics of this amplifier are as follows :
FIG 15. Interior of MI9701 showing mercury switch assembly and one of the mercury contacts.