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266 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
is not available in an octal base, users are forced to keep unnecessarily large stocks of spare tubes on hand for replacements. It would be a great advantage to the user if the 1603 type could be made available in the octal base, since it would then be possible to design equipment with but a single type of electron tube socket for amplifier tubes.
All of these pre-amplifier tubes are designed for 6.3 v., 0.3-ampere heater operation; to make matters still more complicated for the user, most of the 12 v., 0.15-ampere nonmicrophonic tubes that are available have neither the octal nor the six-pin base but use instead a third type of tube base, the Loktal. For most purposes the use of three types of bases in this manner is most wasteful and tube manufacturers should be urged to " clean up" this unhealthy situation through the Radio Manufacturers' Association or other industry-standardizing body.
Unfortunately, there are too few special low-noise tubes available. The only other low-level tube worthy of mention is the 1612 (metal) ; this is electrically equivalent to the 6L7 (metal). Generally speaking, the variation from one tube to another is smaller for special tubes than it is for their commercial equivalents. Despite this, appreciable variations will still be found in the performance of special tubes when they are tested for critical applications such as in the first stage of a microphone preamplifier. Important variations are found not only in electrical characteristics, such as mutual conductance, etc., but also in noise output such as microphonic rattles, howls, random pops, hiss, hum, etc. The average of a group of special tubes such as the 1620, for example, is considerably better than the average for a group of commercial equivalents such as the 6J7. Even with special tubes it is good practice to have several pre-tested spares on hand so that it is possible to select the outstanding tube from the group for the most critical application. The best test for microphonics is the testing of a special tube in the particular socket of the particular amplifier (or prototype) in which it is to function. The absence of special tubes in a preamplifier can be considered a mark of inferior design. Any preamplifier that does not use nonmicrophonic tube types should be looked upon with suspicion regardless of the excellence of the design of the shock mounts for its tubes. The best nonmicrophonic 6.3 v. tube now available for use in the first stage of the microphone preamplifier or in the preamplifier associated with a film phonograph or other low-level input is the 1603. The absence of a 1603 tube in the preamplifier may be considered an unnecessary handicap in the attainment of good noise-free performance of the recording equipment that fails to use it.