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July, 1944 MILITARY AMPLIFIER DESIGN 17
not keep out moisture as the wax cracks open during sudden shifts in temperature, such as may be experienced in aircraft.
The wire in military equipment is not the push-back or rubbercovered wire used in ordinary commercial construction. The preferred type of wire has an inner insulation of either 2 overlapping tapes of cellulose acetate butyrate, or of a vinyl compound, or has an extruded vinyl insulation. The outer braid is usually a double braid of fiber glass. The wiring is usually made up in harnesses which are preassembled after thorough checking. The forms on which these cables are made quite often have colored lines running between the various pins, so that the most inexperienced operator can assemble the cabling with little trouble.
Laminated switches of the home vacuum-cleaner type found in commercial construction do not stand up in service. Molded housingtype switches are preferred.
Tube sockets of the gang type are also out for military purposes. Not only do they not lend themselves to the use of tube clamps which are a necessity to withstand vibration and shock, but they also form a perfect leakage path between tubes when moisture enters the phenolic strips.
With regard to the choice of tubes, the military amplifier designer does not have at his command the large and almost endless choice with which he can play in civilian design. Equipment designed for the services must have all tubes, including the photo-tubes, chosen from the Joint Army-Navy Preferred List of Radio Electron Tubes. Exceptions to this list are allowed only in very special cases. To design equipment using tubes which must be specially selected is considered an unpardonable sin of the highest order. Equipment must be designed so that any tube with the proper type number will work when plugged into a socket, regardless of whether or not the last 2 db of output is sacrificed.
Incidentally, not only in the choice of tubes, but in the choice of all parts are the services insisting on the use of standard, commercially available parts. The problems of stocking a multiplicity of special parts have been such that it is very easy for a designer to get into the "doghouse" through using special parts.
Regardless of the type of parts that is chosen, manufacturers of equipment for the Armed Forces must have very thorough incoming inspection, checking each individual part as received. Slip-ups and mix-ups do occur in parts manufacturers' factories, as well as in