RADIO AND ITS ITS RECEIVERS

Sun (Auckland), Volume II, Issue 367, 30 May 1928, Page 14

RADIO AND ITS ITS RECEIVERS

RADIO, 1928 Possibly 1928 is going to see more radical improvements than has been seen during the last three years. The all-electric set is making an appeal, but it will be some time yet before the battery set is remove! from its preeminent position. There are several reasons for this, one of which is the first cost, which with good apparatus should be the last, another is the fact that when the electric supply fails the radio fails too. Yet another is the service question, where the skill of a qualified electrical engineer is needed and is quite a different matter from the servicing of a battery operated set. These difficulties will ©undoubtedly be removed, and one factor which in New Zealand is retarding progress is that America, the home of the A.C. set, is serve l with a supply of iiO volts, whereas our supply is 230 volts, a question which means complications for our local market and as yet only a limited number of manufacturers have made provision for this higher voltage supply.

Again, we have the new departure in high frequency amplification using the screened grid valve. The writer has received one of the new Mullard P.M. 14 valves, which makes the big sevenvalve sets seem unnecessary. Using one stage of tuned annode coupling of low-loss design with Litz wire, the Australian stations come in in good style by 5 p.m., and Wellington is better by day, as the volume is there minus the fading. The four-valve set with the radio and detector stages as described in The Sun is capable of getting ail that is worth listening to with pleasure, for the average man is not a super DX fiends, and wants radio as a musisal instrument, just as lie wants his cat to take him safely and speedily about, but not to be a possible winner at Muriwai.

This new valve fits the standard sockets and is made as yet for 2-volt or 4-volt supply only. The anode terminal is situated at the top with a neat ebonite terminal and the screen comes out to the usual anode valve leg, the control grid is connected to the usual grid leg. Filament current is low, .075 amps, the maximum anode or plate voltage is 150, and the screen The anode impedence is 230,000 ohms, giving an amplification factor of 200. It is necessary on account of the high impedance of the valve to use a H.F. coupling, which has a very high impedance, and this is best achieved by using the tuned anode circuit, and reduces all losses in this circuit to a minimum. The interelectrode capacity is negligible, and complete stability can be achieved without the use of stabilising devices, as neutralisation or potentiometer control. It is, however, essential to screen the various circuits to avoid stray electro-magnetic or electro-static coupling external to the valve, which would introduce instability. If due regard is given to the above considerations the amplifica Von is very much greater than that obtainable with the normal triode, and complete stability is assured together with simplified control. NEW SCREENED FOUR The writer had the opportunity of trying out the new Burndept screened four, which incorporates an Osram 625 screened grid with reaction controlled by a condenser. The set is built into a very nice mahogany cabinet, with the main controls on the front panel, and permanent resistors inside. The volume control is on the filament of the H.F. valve, which is completely screened from the other valves at the back sub-panel. The circuit, by a change over switch, will receive up to about 2,000 metres, and the writer was astonished to get four broadcasting stations above 1,200 metres, one at 1,600 was giving musical items at quiet speaker strength, but the others were only phone strength, and coming in and out in such a way that it was impospisble to get any identification. At about 9.30 p.m. a fairly loud station was picked up on about 1,200 metres, evidently Perth, in Western Australia. The optimist would hope that the others were European stations, but . An interesting improvement is the provision for a gramophone pick-up using the audio amplifier of the set as the output. The filament consumption was in the neighbourhood of £ ampre, at six volts, and only two high-tension leads are used. This is one of the first imported British multivalve sets which really suits our conditions and looks the part. The usual British set, with the exception of the Solodyne and Elstree Six. does not appeal - either in looks or performance. “A” ELIMINATOR

The writer, who has D.C. supply, has at last got an “A” eliminator which the power boards will pass. The principle is entirely new, and works on the thermojunction principle, and a heater is incorporated which, of course, will use either A.C. or D.C.

The efficiency is, of course, not as high as the A.C. eliminator, using a rectifier, but the cost would not be more than three farthings an hour. There are two, four, and six-volt types, with outputs of half and one ampere. A voltmeter is incorporated, and a resistance in series, with the heater gives adequate control. The voltage under full load of one ampere is 7.5 volts, which gives a full one-volt drop in the line between the eliminator and the set. Now. at one ampere, a resistance of one ohm is allowable without affecting the input of six volts to the set. Twenty-gauge wire has a resistance of 2 3.62 volts a 1,000 yards; 42 yards of this wire would reduce the voltage of the output to 6.5 volts at the receiver. Using the return of the same gauge wire, we could thus have the eliminator 63 feet from the set. The same eliminator is m Auckland using gas and petrol for the heater, and in these cases the eliminator would need to be in the kitchen or somewhere handy to a gas jet, and a range of 60-odd feet makes this type a proposition which will cause some thought. FIVE-ELECTRODE VALVE

With the aspect of a new fiveelectrode amplifying valve which is coming to the writer by post, three valves will be sufficient to get all that the average five-valve set will now get, with consequent economy. This new audio valve has three grids, the one nearest the filament being the usual control grid. The next, or auxiliary grid, is connected to a terminal on the side of the valve base, and must be maintained at a potential the same as the high tension supplied to the plate of the valve. The third grid, nearest the plate, is connected internally to one of the filament legs. The usual base is used, and it is thus possible to use the valve in an ordinary set, and the only addition is a wire from the extra terminal on the valve to the high tension positive terminal of the set. By this arrangement, the extremely high amplification factor of 82 is obtained, with a mutual conductance of 1.3 milliamps a volt, and an output equal to that of a super-power valve. Thus from a single valve the amplification of two stages of audio is got with the output capable of handling that of the usual super power valve. The first stage will thus be cut out, with a consequent saving to the set constructor.

The writer’s new set is going to be screened grid, H.F. detector, and

Conducted for THE SUN by C. M. TAYLOR, B.Sc., M.I.R.E.

one stage audio, and if the new valve is up to the screened grid the new set will be all that the average man requires. STORAGE BATTERY NOTES

If a battery is to stand idle for any length of time not exceeding about SO days, a refreshing charge will be necessary, as all batteries lose part of their charge while standing idle.

Each cell should be tested for voltage separately, and then the battery should be tested as a whole, at the two main terminals. If the total voltage of all cells added together does not equal the voltage reading of the whole battery at the main terminals, there is an indication of some one or other of the cells being reversed. This is exceptional, but if it happens the trouble should be remedied. Care should be taken to see that the polarity markings are correct—that is, that the positive end reads positive, and the negative end reads negative. When a battery has been a long time in transit from factory to distributor, it usually requires a thorough charging, the time required varying, but the more time taken the better the result. In case any single cell does not come up to charge with the remaining cells, this particular cell should be brought up separately. In that way the gravity of all cells is equalised. Batteries not in use should be inspected, and charged up at least once a month. Batteries, when new, may safely be stored wet, charged, for at least four months without any appreciable deterioration. When charging is carried on at night, or when the operator is not present, it is important to know that the charging is continuous, for if the power goes off the battery is not being charged and such time as is lost should not be counted up against the required time to effect the charge. Where motor generators are used there should be a protective device against reverse charging current, as for instance, if the power goes off the batteries will then start to discharge through the generator, reversing its polarity, and then should the power come on again, the batteries will be getting a reverse charge, which causes serious, if not fatal, damage. Terminals should be kept as clean and dry as possible. A good plan is to wipe off the tops of each battery occasionally with a cloth dipped in diluted ammonia water or soda water. To make “soda water” dissolve ordinary baking soda in water. After this is done, apply a liberal coating of vaseline to the terminals and all connections that are bolted. Be sure that all bolted connections are drawn up tight. Once the battery is properly installed it should require very little attention.

Keep the level of the solution in the battery at least 5-incli above the separators. Only water is lo§t and therefore only water should be added unless some of the solution has been accidentally spilled. * An inspection should be made at least once in two weeks at the start as the periods for adding water will vary according to the operation. In hot weather, evaporation of the water content is always greater than in cold weather. A battery when cold has less capacity than when warm. EFFECT OF DISCHARGE RATE? The ampere hour capacity of a battery is greater if it is discharged at a long slow rate or intermittent rate, rather than at a high short rate. This is because the slow rate allows the acid to penetrate to a greater depth in the plates and thereby increases the capacity. PILOT CELL READINGS Select one cell of the battery that is convenient and use it as the pilot cell. Gravity readings- of this cell will be an indication-of the condition of charge and discharge of the entire batteiy. To take a specific gravity reading—insert the rubber tube of the hydrometer in the battery cell after the rubber bulb at the top has been compressed. When releasing the jDulb, some of the liquid will be drawn up in the glass barrel. Draw up enough of the liquid to float the hydrometer. The reading is taken on the stem of the float at the level of the electrolyte. Be sure and return the electrolyte to the same cell it is taken from. Never take a hydrometer reading immediately after adding water to the cell, but wait until it has been mixed with the electrolyte by charging until the battery gasses. USE OF HYDROMETER A fully charged battery will have a specific gravity of from 1.225 to 1.300, depending on the type and service in which it is to be used. As the battery discharges the specific gravity lowers in direct proportion to the current or ampere hour capacity taken out. Therefore the hydrometer reading is a direct and true indication of the state of charge and discharge of a battery.