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Power tubes and rectifier tubes become very hot when in operation and will burn the. hand. Sudden cooling of the glass will cause strains and may crack it. Glass strain makes the glass extremely fragile and subject to breakage from a slight jar. Avoid placing hot tubes on a metallic or wet surface.

145. Dry Batteries.-The PG-3, 4, 6, 7, 8 and 10 equipments use dry batteries for plate ("B") supply of the voltage amplifiers. All equipments use dry batteries for the grid ("C") supply of the voltage amplifiers. The dry batteries need to be replaced periodically, their life depending mostly on time of actual use. This is usually taken care of by the RCA service man, but in some cases it may be necessary that the operator replace them. The plate supply requires "heavy duty" batteries, because the current drain is appreciable, and the use of small batteries is not economical. No current is drawn from the "C" batteries, so small sized batteries are satisfactory for this purpose. Their life when used in an amplifier is practically equal to their shelf life, but they should be replaced at least once a year, to avoid any possibility of noise being introduced into the reproduced sound from this source.

The condition of "B" batteries can most readily be told by the voltage. They should be replaced when the voltage has dropped 15 per cent of the original voltage. The voltage readings -should be taken with the amplifier in operating condition with all the voltages on.

The polarizing voltage for the photo-cells is also supplied by dry batteries in all equipments except the Types PG-1, 2 and 13. Very little current is taken by the photocells, so that "light duty" dry "B" batteries are used for this purpose.

Old or poor grade dry batteries will cause a noise similar to static in radio. It is therefore advisable to use only good grade batteries and to replace them as soon as their voltage starts "dropping off." When the batteries get old, their voltage decreases rapidly, so that they may reach a condition which will cause noise in the loudspeakers during the running of a show if they are not replaced at the proper time.


Chapter Ten Pages
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