In a number of installations two converters are used, one machine being a 'stand-by" for emergency service only. In this case a four pole double-throw switch is used to transfer the power supply from one converter to the other (see Figure 32) . Pilot lamps are provided to show which converter is in use.

36. Motor-Generator Set for Types PG-1 and PG-2 Equipments. -The 4-unit motor-generator sets used with the types PG-1 and PG-2 equipments each consist of a 250 volt DC generator, a 15 volt DC generator, a driving motor, and a 1000 volt DC generator (see Figure 33). The type of motor supplied depends upon the power available at the theatre.

The 1000 volt generator has two 500 volt commutators, one on each end of the armature. These commutators are connected in series, giving a total of 1000 volts. Two 6 volt storage batteries are "floated" across the 15 volt generator. This arrangement gives a very steady DC output for lighting the tubes in the power amplifiers, and for lighting the exciter lamps in the sound head. The storage batteries are necessary because the direct current supplied from a DC generator is never absolutely constant, but varies up and down slightly, depending upon the number of bars in the commutator and the speed with which the generator is rotating. Such variations, called "commutator ripple," will cause a decided hum from the amplifiers if something is not done to prevent it. Dirty commutators may cause a sufficiently bad ripple to produce such a hum in spite of the smoothing action of the batteries. If this occurs, clean the commutator of the 15 volt generator (and in some cases, that of the 1000 volt and 250 volt generators) with fine sandpaper. In order to clean the commutators of the DC machines, it is necessary first to remove the iron shields. CAUTION: Do not clean the commutators with the power to the motor turned on, or with the main operating switch at the power amplifier panel closed.

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