MICROPHONIC VALVES

Manawatu Times, Volume LIV, Issue 6863, 19 March 1929, Page 4

MICROPHONIC VALVES

USE OP RUBBER SUSPENSION. Almost the only disadvantage which the modern low filament consumption valve possesses is a tendency to he microphonic, that is, to he susceptible to vibration so that a Tinging noise is produced in the loud-speaker or telephones 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 result. > Some sockets are now fitted with special rubber or spring-sus-pended bases to protect the valves from vibration and the consequent annoying noise in the receiver. In many cases, however, valve sockets 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 ia important to see it is not too deep, to prevent the bottom of the socket being pressed against the baseboard of the receiver when the valves are inserted, but deep enough to keep the base clear of the baseboard, even after the rubber has sagged slightly, as it- will under the weight of the valves. It will bo found advisable to use flexible rubber-covered wire* to make connections with the valve base, otherwise there is a danger of shocks being transmitted to the yplves and thus nullifying the effect of the rubber suspension. :