. The Bell System technical journal . 9 ma and zero in the other. The dissipation in each coil for(luiescent current is about watt, and the full signal power is watts. The requirements that the frequency response must extend to dc andthat the control power consumption be held to a minimum suggest theuse of a dc push-pull output stage. The cjuiescent dc plate current isused as the magnetizing current for the solenoids instead of providingthe field by a permanent magnet or separate coil. In spite of the smalloutput current available from the amplifier, relatively large forces anda high r
. The Bell System technical journal . 9 ma and zero in the other. The dissipation in each coil for(luiescent current is about watt, and the full signal power is watts. The requirements that the frequency response must extend to dc andthat the control power consumption be held to a minimum suggest theuse of a dc push-pull output stage. The cjuiescent dc plate current isused as the magnetizing current for the solenoids instead of providingthe field by a permanent magnet or separate coil. In spite of the smalloutput current available from the amplifier, relatively large forces anda high resonant frequency are realized. This is accomplished by an effi-(!ient magnetic circuit and low armature mass. The opposing solenoidconfiguration described makes these features ])ossible. The magnetic circuit used in the J-7 valve has very low reluctancefor a sliding armature type actuator. This low reluctance is accom-plished by providing ample thickness in the iron parts and employing 720 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957. Fig. 8 — Internal view of the J-7 valve body. very short gaps of considerable cross-sectional areas. It has been sho^\^lelsewhere^ ■ ^ that if the distribution of reluctance of the magneticstructure is symmetrical along its longitudinal axis, minimum leakageflux and hence maximimi force would be developed if the working gapwere at the exact center of the coil. To a close approximation, the valvesolenoids are symmetrical in this manner. The references show that themaximum pull is not sensitive to small changes from the optimum loca-tion of the working gap. The gap in the J-7 valve is displaced towardthe center of the valve from the location of maximum pull. The slightreduction in magnetic force was accepted as a suitable compromise forthe resulting reduction in the mass of the moving pole piece and thecorresponding increase in resonant freciuency. The gap between the solenoid pole faces is inch when there isno signal and the valve is cen
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