Push-Pull Amplification

Radio Record, Volume II, Issue 20, 30 November 1928, Page 28

Push-Pull Amplification

A Correspondent Questions the Pentode ea The following series of questions submitted by a correspondent, W.A.R., are of sufficient general interest to warrant their being devoted a little space: (Q. 1).-Would a pentode give satisfaction if used in the first stage of push-pull, and, if so, would not the high milleamperage, which I understand is 12 m.a.’s, cause saturation in the transformer? (A.).-The pentode is essentially a last stage valve, and cannot be used in previous stages because of the tendency, or rather the actual fact of the saturation of the following transformmer

er, although in this case more than half the D.C. current is flowing from the second grid, and misses the plate current. The correspondent is using AFD’s. Why not try a high-frequency or R.CG. valve in first stage, with a fairly high © ambplification and impedance? The AF5 has a high primary impedance and can be used with a valve of impedance of 10,000 to 30,0000hms, (Q. .2).---Would a pentode work in the last stage of push-pull? I often take out one of the P.M.6’s when listening on the *phones or listening to a distant station on the, speaker, obtaining the samme volume as I would if I had in both P.M.6's. (A.).-It would work quite O.K., in fact, there is no reason why two could not be used to get enormous amplification from one stage. Incidentally, when using only one valve in one side of push-pull always halve the C voltage ~

and treat it as though it was only a single valve in last stage. (Q. 3).-Will a pentode ‘handle as much volume as an ordinary power valve? (A,).-It will give a far greater output, put will not handle the same input. If both valves were put on test, each handling as much as possible, then the pentode would win, according to results from the speaker. (Q. 4).-Would two pentodes work in push-pull? (A.).--It is doubtful if your exponential horn would handle the output. (Q. 5).-Where could a pentode be used in my set to increase the range to best advantage? (A.).-A pentode could not be used to increase range in any way. The equivalent in R.F. work is, of course, the screen grid. (Q. 6).-I£ used in the first stage audio, would it operate on, say, 70 volts, and would the milliamperage drop down to, say, "6M.A’s’? I am compelled to operate from dry B hatteries. According to the milliamp. metre, I possess the two P.M.6 take, 11 M.A. at 185 V’s, and two 256 P.M., 2 5-M.A.’s. (A.).-If you are using dry batteries, my advice is to try a valve with high amplification factor, as I mentioned before.