Microphonic Valves

Radio Record, Volume II, Issue 38, 5 April 1929, Page 30

Microphonic Valves

Shock Absorption LMOST the only disadvantage which the modern low filament consumption valve possesses is a tendency to be microphonic-that is to be susceptible to vibration so that a ringing noise is produced in the loudspeaker when the receiver is subjected to a slight jar. This effect is due to the vibration of the valve elements, and particularly the slender filament, under the influence of the shock. In extreme cases a fracture of the filament can resi. Many sockets are now fitted W¥1 special rubber or spring suspended bases to protect the valves from vibration, and the consequent annoying noise in the receiver. In many cases, however, valve sockets in use are not protected in this way. An excellent form of shock absorber can be provided by mounting the valves on a sheet of rubber cut from an old motor-car inner tube. The rubber should be cut in the form of a panel long enough to carry all the valve sockets in the receiver. The sockets are attached to it by small bolts, a large washer being placed under the head of each bolt to prevent the bolt head from cutting through the rubber. When the sockets are mounted on the rubber strip. the strip, should be mounted, slightly stretched, in a shallow wooden frame. It can be clamped to the top of this frame by strips of thin wood In making the frame to carry the rubber strip, it is important to see that it is not too deep to prevent the bottom of the socket from being pressed against the baseboard of the receiver when the valve is inserted, but deep enough to keep the bases clear of the baseboards even after the rubber has sagged slightly. as it will, under the weight of the valves. » It will be found advisable to use flexible rubber covered wires to make connections with the valve base. If bus bar is used. there is a danger that it will transmit shocks to the valves, and thus nullify the effect of the rubber suspension.