RADIO CLUB

Timaru Herald, Volume CXXXVII, Issue 19803, 19 May 1934, Page 14

RADIO CLUB

' TECHNICAL ARTICLE No * By G. H. S. We hear many discussions on tt tone of modern sets. Why is it tha with the great advances made in rad: recently, tone has not advanced also i The tone control proves this. Is it du j to the use of penthodes? If so why Is a penthode not as good as a triode Can the penthode be made to give a good tone as the older sets with triodes These are but a few of the queries w hear. With the now almost universal us of the penthode as an output valve, j appears rather ridiculous to state tha half the “rottenness” of present day re ceivers is directly due to the use o j these valves. Such, however, is th j case. A penthode can be made to giv jas good tone as a triode gives. It i j many times more efficient than th triode, but in that efficiency is th cause of all its troubles. Theoretically a penthode, used with moving-coil speaker, is the perfect in strument of reproduction. Practica factors, however, over-ride this possi bility. In 1929 when set manufactur ers in America were faced with th great slump, they turned to the mante set as a means of keeping their fac tories in operation. The mantel set meant half-size sets with full size per formance coils, valves, speakers, an< transformers were all reduced ii physical size. The valve-maker: brought out the penthode, a valve witl the efficiency of two triodes when use< with suitable components and properlj matched by the speaker. The speaker following a triode suci as a 245 can be 100 per cent off correc matching load, and the tone is stil good. With the penthode the matching load is really critical and must b< correct, and herein is the cause of th< “rottenness.” If the matching is incorrect, harmonics are amplified out ol all proportion to the fundamental tones, and new harmonics are generated in the valve circuit, which are again amplified. and the result is that harsl noise we know as distortion. To prevent this action, individua laboratory matching of valve to out put transformer is required—which ii certainly not done in most factory productions and cannot be done by home builders, so we see why theoretically perfect results are seldom realized in practice. In most of the big sets now being built an improved class of triode output system known as class A is being used. This is similar to the old style push-pull 245 s, but has about one per cent distortion as compared with about 3per cent for the 245’5. An average penthode output stage has from 9 to 15 per cent, distortion, and it is this distortion only—which is in the high notes and which spoils the reproduction—that makes the so-called tone control such a great advantage in the modern set. A slight misprint appeared in our previous article. It should read:— O' er lhe bottom of the secondary winding and winding in the same direction, wind 25 turns of 36 to 40 ga. wire This fine wire can be enamel insulated. The size of this wire is the heart of the coil.