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ELECTRON TUBES 261
outward appearances as by measurable performance characteristics such as the excellence of signal-to-noise ratio, low distortion, and a wide frequency range that is limited only by the band-pass niters (Fig. 32) intentionally introduced to limit the transmission to the desired frequency range. Thus it is now common practice to make all amplifiers used in modern 16-mm recording equipment with good fidelity to 15,000 cps ; in fact, manufacturers recognize that the wider the frequency range of the recording system in comparison with the frequency ranges actually employed in commercial recording, the more predictable and reliable will be the final result. Thus, although it may be rare to employ commercially in a film recording any range wider than range F (110 to 5300 cps), the basic equipment supplied by the major manufacturers has good response in the range from 50 to 15,000 cps and low intermodulation distortion throughout. As demands for range increases are made, they can be met by providing new band-pass filters rather than completely new equipment.
The cascade amplifier, although no longer composed of filament-type triode tubes, invariably has heater-cathode type electron tubes in all audio circuits between the microphone and the recording machine. For convenience in audio mixing and in using equalizers at the most suitable audio levels, amplifiers are now made in three portions :
(1) A preamplifier — used to increase the level from the microphone about 25 to 50 db before the signal is applied to the attenuator ("mixer pot") of the mixer panel. If used, the output of a re-recording film phonograph may be connected to a second attenuator of the mixer panel to effect mixing with the microphone output.
{2) An intermediate level amplifier — sometimes called a main amplifier — used to increase the level from the mixer panel output some 40 to 70 db more. At this higher level the signal passes through the equalizers to the output amplifier to the noiseless recording ("bias") amplifier and to the compressor.
(3) An output amplifier — the output is connected to the light modulator of the sound-recording machine.
Electron Tubes. Rugged tubes of low microphonic sensitivity (such as the 1603, 1620, and 5879 types) are usually found in all preamplifier and low-level stages such as those immediately following the microphone and the re-recording film phonograph. The importance of well-selected electron tubes is hard to overemphasize, since the loss of a few minutes of recording time will more than offset the cost of even the most expensive electron tube used in the equipment. Over-all performance depends in great measure upon the performance of the electron tubes used.
The performance of an electron tube in a circuit arrangement depends upon the permissible variation of the tube for which the circuit