Questions and Answers

Radio Record, Volume III, Issue 13, 11 October 1929, Page 26

Questions and Answers

Fumes from an Accumulator, I WISH to use an accumulator in my . ‘bedroom, states "H.C." (Wellington), are the fumes dangerous? A.: When discharging, an-accumulator gives. off hydrogen and oxygen. These ‘are not harmful, but hydrogen is inflammiable, so that it would not be safe to smoke in bed and dust the ash too near the cells. .Similarly one would have to be careful of stray matches. Barring these observations there is little danger. Concerning Dry Batteries. "s D." (Kilbirnie) asks the following * questions telative to the improvement of the battery system of his set :- 1. How to efficiently connect up dry -eells for the "A" battery. ‘ A.: The best method: is by series paralJel. This arrangement keeps the voltage the same, but doubles the capacity, giving the battery probably three times greater life. A diagram is included showing hiw a 4-volt bank would be arYanged.

= iow to Insert a "OO Dattery cell ‘fm the circuit of a 2-valve set. A.: Examine the last audio transformer and it will be seen that there

fs a terminal labelled "GB," "F-" or "CO," A lead from this in an unbiased stage goes to filament negative, Disconnect this and take it instead to the negative terminal of a small battery the voltage of which is equivalent to the bias to be applied. In a 2-valve set this would not be more than about 4.5. Take the -positive of this battery to "A-." Set Fails to Get Certain Stations. LTHOUGH' my set can receive a large number of stations, I cannot get 3AR and 2BL, writes "K.V.B." (Wanganui). I have a long aerial and ean get 3UZ and 3L0O. A.: Does the correspondent mean that he can hear these stations. -but that he cannot get them at satisfactory strength, or does he mean that the set goes dead when tuned to their frequency? In the former case little if anything can be done, the failure being put down _ to locality effect. In the latter case it appears as though the condenser vanes are touching and causing in internal short circuit, making the set go dend. Examine the condensers for dust and closeness of the vanes. Valye Combinations. HAVE an American factory-made set, writes "Cockey" (Taranaki), ang I am using PM6 in all stages. Could lL improve my set by using PM6D as detector or by using a pentode in the last stage? ° A.: PM6 is.:a semi-power valye and should not be used in all stages, . se PM5 in R.F. and first audio and PM6D as detector. The introduction of a pentode in the last stage will certainly strengthen the weaker stations.

4 Crystal and Amplifier. _ OULD I add an amplifier to my crystal set? asks W.D.K. (Dunedin). A.: The accompanying sketch shows the manner in which this may be brought about, Probably the adaptation of your set to the one in the diagram would be an advantage. If you do not wish to make this alteration simply add the amplifier to the two terminals of the ‘phones. An ordinary valve may be substituted for the tetrode (A441) shown in the diagram. We have returned your diagram suitably inscribed.

Voltage Tappings on an Eliminator. [THE voltages are not given on my new eliminator, states T.G. (Silverstream). Coyld you give me approximate readings of these. _ A.: "B" +, 150 volts at 25 mas. Detector as follows :- Tap 1: 22 volts at 5 mas. or 40 volts at 2 mas. Tap. 2: 45 volts at 5 mas. or 80 volts at mas. Tap . 3: 60 volts at 5 mas. or 100 volts at 2 mas. It will be seen that when an average detectot is used the voltage will be about 30. If further breaking down is required, insert a veritable resistance in the detector lead, and put in a by. pass detector between it and "B’"-a suitable fixed resistance would be, 500 ohms fixed. Transforming a Set. CAN you give me the information as to how I might transform a set with an eliminator built for 110 to 125 volts AC, 25 to 50° cyeles, to work off 230 volts 50 ecycles?-Harmonik (Taumarunui). 1 A.: Construct a transformer with a primary. built to take 280 -volts 50 cycles, and to deliver 125 volts. Such a transformer has been deseribed in the "Radio Listeners’ Guide." ‘The difference in frequency will not be a detriment to the working ‘of the set. For the guidance of the constructor the following is an extract from the’ ‘Guide’: Core 1 x 1 inch primary turns, 1850 wire, 26-82 SWG. Secondary 1000 turns. Turns per volt, 8.5.

Efficiency of Push-Pull. "PDADIO" (Hastings) asks the follow- *. ing questions relative to the use of two 171A’s in push-pyll. 2 (1) Would two of these valves in pushpull require twice the amount of curregt, bias and voltage of a single valve? " ° ‘A.: See our atticle in the special issue. ' The voltage will remain the same, . the bias may be the same or double, according to the operating principle adopted by the user, the plate current will pe double that of one, while the output will _be three times. as great. vos (2) Could a standard amplifier be °

worked directly from the set by plugging into the output jack? _A.: Not if the set has already two or more audio stages; the usual practice is to plug in immediately following..the detector. valve, or if.the amplifier has a pull-push stage, following the first studio. _ Replacing the Audio Transformers. I WISH to replace an audio transforA mer, writes W.R. (Martinborough), which ratio will I get?

A.: A 34 or 4 to 1 value is not critical, Purchase a small first grade transformer. 3, Can I use a pentode valve in. the last stage without altering the wiring? * A.: Simply plug the pentode into the last socket and connect the auxiliary terminal to the highest "B" voltage available. A Prohibited Circuit. qs this a prohibited circuit? asks F.J.M. (Pahiatua) of an enclosed cireuit. A.: Yes, The grid of the first valve is connected directly to the aerial, It would be permissible to use this off the loop ér if the aerial was brought in to the aerial coil, say at the éighteenth turn. Batteries run down: Rapidly. M* batteries are running down far too rapidly, complains D.B.B. (Whakatane). I have inspected the set and can--not find a short circuit. There is a. fixed condenser between ' "B" and the ground. If I disconnect this condenser the set seems to go better, though it howls if I turn the filaments too high. Can you procure for me q diagram of the set? A.: It appears that the condenser has been punctured, and that "B"’+. is connecting directly with the ground. This would cause the batteries to run down very rapidly. The howling when the + condenser is disconnected would indicate that the neutralisation is affected by this defective condenser,. The rupturé may be only partial, resylting in a high resistance counection, so that the condenser is partly functioning. Other than testing the condensers by the voltmeter battery (or ’phones and cell) test little

‘ean be done by the correspondent himself. We cannot supply the circuit of a trade receiver. Microphone Amplifier. NITHNDING to construct the microphone button amplifier, I should like constructional details of the 1 to 100 transformer, asks "H.L." (Wellington). A.: This would not be satisfactory. This type ‘of amplifier has gone out of date long ago, and you would be disappointed with the result. However, if you wish to make the transformer the following details should suffice. Obtain a set of standard stampings (Ballingers), and wind, say, 1000 turns of 42 gauge wire for primary. 'The secondary would be -100,000 turns of 46 wire. In the opinion of the writer this would ‘be a waste of time-and money. Output Choke v. Transformer. CH is the most satisfactory manner of operating the output of my ‘set, asks "G.B." (Gisborne), as a choke eby connecting the primary and the secondary and using the centre as a centre tap, or, as a straight out transformer. ~ A.: Using the centre has one or two serious limitations. ‘The secondary has probably finer wire than the primary, hence the impedance would be. different. This would cause unbalance. The higher impedance on the secondary would no doubt be the cause of uneven plate voltage while with current passing through the secondary would probably burn it out before: long. ‘The transformer would be alright in the intervalve stage of the set, but all the amplification would be in the valve itself. The ratio would no doubt be one to one. Valve Base Coils. "\:A RE eoils wound on the bases of valves as efficient as those on a three-inch former?--Wavemeter (Blenheim.) A.: Theoretically, no; but the greater convenience of these wins them favour in all recent types of sets. Actually there is little difference between the operating efficiency of a set of these and a set of the larger coils.

2. What are the specifications of a valve base coil to cover the broadcast band when tuned with a .00014 condenser? A.: Two coils are necessary. (a) To tune between 550 and 325 metres a winding of three inches of 32 S.w.g. d.s.c. (225 turns). (d) To tune below 325 metres a winding 14 inches in length approximately equal to 100 turns. If the base is not long enough, extend it by strips of celluoid, 8. Could you give me the details of a wave-meter? (a) The publication .of such details would, at the present stage, not warrant the space. The details can be found in _1*Radio Amateurs’ Handbook," obtainable s from radio booksellers, or failing them, from the Te Aro Book Depot, Wellingon. Power Line as Aerial. LATELY 2YA has been coming in irrespective of tuning, states R.M. (Hast Coast). This does not affect reception, for I can receive the American stations, (a) It appears that the mains are actjing as an aerial. Place R.F’. chokes in _the back lines from the mains. Adapting a Filament Winding to A.C. HAVING made the original "B’" eliminator, which, by the way, is giving excellent service, I now desire to use the filament wiring, provided but not in use, to operate an A.C. valve---A.M.K, (Weljington.) A.: The: filament wiring can be used with a four-volt valve taking up to 3 amps, The introduction of an A.V. volt-meter would save an amount of trouble. Wour-volt A.C. valves are at this time made only by Phillips. P.M. series are identical with the American. Use the indirectly-heated type (W415).