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failed, as the others will remain alight. This is further discussed in the chapter on Fault Finding.
The correct way to remove a valve is by taking hold of its base, not by the glass bulb. Use a cloth to hold it (if it is very hot), and lay it down gently.
-8-3 3 8,
VALVES WIRED IN PARALLEL
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WIRED IN SERIES
Fig. 8
Note that in series wiring the failure of one valve will completely interrupt the circuit and cause the lot to go out.
The Loud-Speaker
The Loud-Speaker should be the least troublesome part of your equipment. It requires very little adjusting, and has only to be kept clean and not knocked about. But the speaker base is nearly always magnetic, and this should be periodically examined to see that it has not collected a few hair-pins or the nail-file you lost the week before. The magnetism is also likely to upset your watch, so do not wear it when handling the speaker.
All manufacturers of sound equipment supply loudspeakers which are a perfect match for their own projector sound-systems, so if an additional speaker is required, remember that it too must be a perfect match. This means that the speakers must be of similar resistance, impedance, and work on the same power-supply without impairing in any way the quality of the sound. The use of an extra loudspeaker, however, does not imply extra volume. You will get better sound, perhaps, but not necessarily more of it.
Any particular note in music (or other sound) may be described as a high (treble) note or as a low (bass) note of so many frequencies. The range of frequencies audible to the human ear extends from about 16 to 16,000 cycles per second. That pro
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