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ELECTEON TUBES 267
If a recording system is to provide the highest practicable signal-tonoise ratio, it can not be accomplished with line frequency alternating current applied to the heaters of the low-level tubes even if special tubes are used. Appreciable hum due to emission and to magnetic induction as well as leakage occurs between cathode and heater of even the best special tubes ; accordingly, all low-level amplifiers should be heated from a unidirectional current source such as a storage battery or from a good low-voltage rectifier and associated filter. The performance of an alternating-current energized rectifier-filter type of low-voltage power supply that is properly designed is every bit as satisfactory as a storage battery and is far less of an operating nuisance. All modern 16-mm sound recording equipment uses unidirectional heater-current supply energized from the mains at least for its low-level amplifiers; still better design uses unidirectional heater-current supply in all equipment between the microphone and the sound recorder. The performance of preamplifiers in which the signal level is minus 20 db* or less can be improved materially if the noise-t>$.-life characteristics of practical tubes are understood, and suitable design precautions taken. Almost invariably the first tube in such an amplifier should be a 1603 connected as a triode ; almost invariably too its heater voltage should be about 5.3 v. and the voltage at its plate (not the supply voltage) about 40 to 50 v. The tube should have the customary shield can ; if a-c. fields are nearby, a thin permalloy shield should be wound inside the shield can. Heater current should be unidirectional ; either a rectified and well-filtered supply energized from the a.-c. mains or a storage battery will suffice. Operating the first tube under these conditions will result in materially lowered tube "rush" noise and tube hum, not only when the tube is first placed in service, but also with increasing tube age. A further advantage is that the 1603 has its grid cap quite distant from its heater terminals; under unfavorable humidity conditions, the glass envelope of the tube still provides a long path of high resistance and of low capacitance between the control grid and other terminals.
Amplifiers using alternating current for tube heaters can often be improved by operating the cathodes at a negative potential with respect to their heaters ; this reduces the effect of hum due to emission. If the operating level of the input stage is low and but a small space current is required, the heater voltage may be dropped from its rated voltage of 6.3 to about 5.3. Generally speaking, biasing of the heaters with re
* With respect to 1 milliwatt in 600 ohms (reference level) .