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264 IX. SOUND-EECOEDING EQUIPMENT AND AEEANGEMENT
"So far as pops are concerned, the test permits with a 30 to 10,000 cycle-persecond amplifier two pops per minute. If more pops than this show up, the test is continued for a second minute and the tube rejected if the second test repeats the first. ... So far as the life of the 1603 and 1620 versus the prototypes is concerned, I would expect little inherent difference between the two classes of tubes in this respect. . . . The demand . . . for tubes of this kind is relatively small and does not justify very much expansion in the number of tubes (types) at the present time."
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To obtain outstanding performance of sound-recording equipment it is necessary for the recording engineer to have available tube-testing equipment and facilities, and to know how to use them. Tube "hiss" is the noise that should be predominant in a monitor loudspeaker when the gain of the system is advanced, say, 30 or 40 db above normal operating gain. To keep the hiss to a minimum, tubes should be selected for the low-level stages, and more particularly for the first stage. Practically speaking, tube hiss is a function of two factors primarily: (1) the gain of the tube measured as amplification factor (/*) or as mutual conductance (Gm) and (2) the gas current in the control grid circuit of the tube. All other factors being equal, the best tube for minimum hiss is the tube with the highest gain and with the least gas. Gain can be measured on test sets such as those made by General Radio and by Weston; gas can be measured by setting up an equivalent circuit (a circuit to simulate the circuit actually used, using the same kinds and values of condensers, resistors, and coils) and connecting a microammeter (range 0-5 microamperes) in series in the grid return circuit to the cathode of the tube. If, say, a dozen 1603 tubes are checked in this manner, individual differences will be quite significant, and the "best" tube of the lot can be selected. The tube to be placed in the socket of the first amplifier stage of the equipment is the tube that shows the least sensitivity to microphonics and is the best under the above gas test.
Among commercial-output electron tubes, the 6V6GT/G is one of the most satisfactory. After being turned on, it "settles down" in a matter of a minute or so and stays quieter for a longer time than most other similar types. Even in this excellent type, however, there is a wide variation with respect to random "pops" from tube to tube of a single manufacturer and from one manufacturer's product to that of another. Some of these "pops" occur as a result of expansion of internal parts due to heating. Commercial 6L6 tubes (both metal and glass) have had a reputation for variability and high noise level, the metal type seeming to be the worse offender. Should an output tube of such power be re