Questions and answers

Radio Record, Volume III, Issue 29, 31 January 1930, Page 36

Questions and answers

Tone Harsh. [THE tone of my factory-built receiver is very, harsh, writes "Sonora" (Wellington): I find on removing the grid eak that it mukes very little difference to the functioning of the set. A.: Harshness is usually caused through faults in the audio amplifier. Is the correct of grid bias applied to the last valve? Has this valve lost some of its emission so that the grid bias is disproportionate to the plate current. dealer can test the valve, and if this is perfectly. good, we should advise you to get in touch with the agent for this receiver, as we do not recommend the amateur listener to interfere with a commercial receiver. The fact that the grid leak can be removed, and the set yet work, is not altogether unusual, but its removal has a tendency to spoil it. 2..The radio frequency side is known as the balanced bridge. Can this. be adapted to take two screen grid valves in the R.F. stages.

A.: Adapting a tuned R.F, receiver to take the screen grid is by no means a simple task. It is certainly not economical, and we would not advise you to attempt it, and if you are not prepared for a great expense, we should not advise you to have it done. A.C. Short-wave Adapter. qs there any likelihood of your publishing details of an A.C.short-wage adapter? asks "Dynamic" (Lower Hutt). A.: There is every likelihood. We shall do so either in the "Radio Record" or in the 1930-31 "Radio Listeners’ Guide." We. shall not, however, promise any particular but will do so as soon as possible. 2. Do adapters give as good results as, say, "Round-the-World" Tw 0? A.: There is no reason why an adapter used with a good amplifier should not give better results, but more trouble is eneountered with the adapter than with the set.

8. Could you inform me where I could get the D.X. clock? . A.: It was published in the Record’ dated January 6, 1928. Converting a Gramophone ‘to Panatrope. YyiLt you ss'pply me ‘with the details, including a diagram, of the method of converting a gramophone to a pana‘trope? I wish to retain the clock-work motor and the 10in. horn in the in the machine if possible, asks "Brandes" (Petone). A.: We should not advise you to attempt a conversion. Your best plan would be to obtain or build as amplifier and a dynamic cone speaker, and attach a‘ pick-up to the gramophone and work through this. It-does not seem reasonably possible to retain the horn in the gramophone. Speaker from a Crystal Set. I WISH to try to operate ‘a loudspeaker from my crystal set. What type of instrument would I use, and what would be the price? asks "T.G." (Wellington), A.: A very sensitive speaker such as Amplion in the horns, and Spheravox in the cones, should give results where signals come very loudly through the phones. This type of speaker is priced usually | between £3 and. £4. It must be under- stood that a cheap speaker eannot be used directly following the crystal except under unusual conditions. ' Unanswered Questions. " R.S." (Glenmore) sends us a little * note asking us if we have overlooked certain questions about the time of the Auckland Exhibition, or whether there are so many in ahead of his, his turn has not yet come. A.: Tf "P.R.S." will look at the last two issues of the "Radio Record," he will see that "Cathode" is devoting a special article to the explanation of his problems. A Hard Detector. I HAVE construéted "the Short-wave _ Adapter in the "Listener’s Guide," "J.W." (Bay of Islands), but although it will work with a transformer coupled audio amplifier using a PM5, it will not work with a resistance coupled amplifier using a 201A as detector. A.: There are two factors to be considered here. The 201A is a much harder valve than PM5; in other words, it will require higher voltage on the plate, the grid return to the positive, and more . tickler turns. The 201A may have lost a great deal of its emission, and _consequently reacts very tardily. Have it tested. Power N oise. HAVE a five-valve receiver in which’a great deal of noise is present when I operate a generator for the house supply, states "W.C.W." (Geraldine). A,: This cah be stopped by the use of two 2 mfd. condensers, with the centre point earthed. See an article in the ‘Radio Record" dated September 13, 1929, Resistance Value. wat is the value of the resistances to be used in connection with a centre tapped push-pull transformer for Osram P625 and P625A valve and how are they ealeulated? . A.: Consult the valve curves, and it will be seen that at 250 volts the grid

bias required is about 24 volts negative, With this bias, the output is about 23 milliamps., so that the resistance is tained by ‘Ohm's law from this. R=W/i == 28/24 X 1000 = approximately. 1000 ohms. For two valves in push-pull using the centre point for. bias, the equation resolves itself into 46/24 — 1900 ap. proximately. Applying this to P625A

valve, and the bias resistor for both push-pull and single can be determined. Valve Outputs. HAT is the probable difference in output volume between 615 byd 605? asks "R.H.F." (Brooklyn). A.: Very much depends upon the ‘circuit and the components used in it. It would probably be about twice in favour of 615, but of course, there is a great difference in the ability to handle volume, and to impart quality.