AN INNOVATION

Evening Post, Volume CXI, Issue 142, 18 June 1931, Page 21

AN INNOVATION

VARIABLE-MU VALVES

MucH Vsnriosity has Venn, aroused by tho appearance of a new type of screengrid receiving valve, which was haraiy out of tbe egg before, it was into sets and on the market. It is called the "variable-mil"'valve, and its especial virtues are its excellence as a volume controller and its ability to handle signals without accepting . '.'cross-talk." Cross-talk is a peculiar form of interference which is fortunately not very troublesome here. A receiver may have a-blank tuning spaco between two sta-tionsj-but when one of them is tuned in, the other also becomes audible. it ia generally tinderstood that the mn of » valve, in other words its amplification factor,, depends upon the spacing '. of; the grid wires, that is, the distance between the wires themselves, and also the distance of the grid from the filament and from the plate. H6W, then, e»n the mv be altered while the valve is in u*e? 1o simplify the explanation as much as possible, the trick is to combine in one' valve a combination of grfdi: toe grid wire is wound so that ptrt of it is close-spaced and part of it widely epaeed. The closely; •paced part gives a high mv, and the widely Bp»eed part a Tow mn. It is in the nature of a high mv valve that it will work only with a small amount of negative grid bias, whereas a lowma valve will operate with a; large grid bias. Hence, if tho.bias is varied over a large range, starting with a high value, we get the valvo operating

first with low but increasing "gain," until the bias is sufficiently small to bring the'high-nut portion o£ the grid into operation. Further reduction of tho bias increases the amplification, given by this portion of the grid. This enables the amplification to be adjusled over a much wider range than with a standard valve without introducing distortion. It is not commonly realised that one of the most difficult problems faced by the designer of a highgrade receiver is an adequate volume control that will avoid "introducing distortion: it is a lesson spon learnt by anyone who builds his own set, and tho point ia usually dodged by making a more or less satisfactory compromise. Tho now valve seems to have got over the difHculty. •-■--■