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262 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
is designed and the permissible variations of the circuit arrangement constants. JAN (Joint Army-Navy) specifications prescribe specific limits for electron tubes, and no tube outside the limits is permitted to carry the JAN marking. Unfortunately, published characteristics of commercial electron tubes represent " average" characteristics, and, although certain" tolerances are expressed or implied, such tolerances are "nominal" and do not constitute a representation by the manufacturer that all tubes sold are within the published tolerances. Since an efficient design requires that both tube variations and circuit variations be known and taken into account, the performance of an efficient design is likely to be "out of limits" if random tubes are used because there is no certainty that a randomly selected tube will be within prescribed tube variations. Where tolerances are too wide, the performance of the electronic device is relatively poor and/or it will have too many components, making it too costly for the performance obtained.
During World War II, it was American military practice to require that the performance of any electronic device be within specification limits with any electron tube of correct JAN type chosen at random. Unfortunately this excellent practice does not carry over into civilian equipment; it is necessary to inquire of the manufacturer of soundrecording equipment whether the claimed performance is obtained with run-of-the-mill commercial tubes, whether special tubes are required, or whether special selection of commercial or special tubes is required. In the latter case inquiry should be made as to the selection criteria and the "go" and "no-go" limits for each type involved. This is valuable practical information needed to obtain the most satisfactory operation of the equipment in service.
If the product of one particular tube manufacturer (e.g., Sylvania) would be better than that of another manufacturer (e.g., RCA) for a particular type (e.g., the 6P5G which is used occasionally as a selected preamplifier tube), this should be specified by the manufacturer of the recording equipment. The manufacturer should be expected to keep satisfactory replacements in stock for delivery to the customer. It is well to remember that in electron tube manufacture, just as in other competitive manufacture, the suitability of a particular type for a specific specialized purpose may vary over wide limits from one manufacturer to another.
To reduce to a reasonable minimum the wide range of performance variability due to electron tube variations, recording equipment manu