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1044 HANDBOOK OF PROJECTION FOR
The filament of an amplifying tube is heated by current from a storage battery of approximately twelve volts pressure. It is extremely important that the filament current be maintained at exactly the value indicated in the instruction book supplied by the installation engineers. If the filament current be too high it will very greatly shorten the life of the tube without any corresponding benefit. If the current be too low, then, while the tube itself will take no damage, the sound volume will be low ; also the quality of sound may and probably will suffer.
The reason why the life of the tube is shortened by too much current is that the too-hot filament throws off electrons at an excessive rate, hence the oxide filament coating, which supplies most of the electrons, rapidly becomes exhausted. This of course reduces the flow of electrons, and finally brings about the destruction of the filament.
VACUUM TUBE FAILURE SIGN.— When an amplifying tube filament fails, it usually first weakens at one spot. This spot will glow much more brightly than the rest of the filament. Replace the tube with anew one immediately upon seeing this warning sign. If you neglect to do this, it is not at all unlikely the filament will fail right in the middle of a sound reel, which will be very embarrassing.
I again warn you to keep the amplifying tube filament at exactly the voltage indicated in your equipment instruction book.
Noisy tubes will occasionally be found. The noise is caused by the filament throwing off electrons at an irregular rate. It is only occasionally that a noisy tube will be encountered. Such tubes rarely give trouble any