A Corner for Beginners

Radio Record, Volume II, Issue 39, 12 April 1929, Page 13

A Corner for Beginners

Neglected Instructions. ITH all eliminators instructions are issued regarding their operation. Foremost among them will be found invariably, "Turn on the filaments before the eliminator, the eliminator before the filaments." For some unknown reason this is frequently neglected, usually resulting in burnt-out components. , A case arising from neglect of: this precaution has come before the notice of the writer. A unit was installed and connected to the lights by means of a two-way adapter and plug. Between the plug and the eliminator there was not a switch of any description, so that the eliminator was constantly connected to the light while it was burning. Current was thus passing through the unit and incidentally through the set all the time the light was on, irrespective of whether the set was turned on or off. This continued for some time, and then it was found that the set began to become considerably weaker in volume and yery harsh in tone. It was rather unfortunate, for this was far back in the country where radio service was difficult to obtain. On examination it was revealed that one of the anode resistances of the set had been burnt out, and that actually the set was operating from 5 volts only. On changing round the resistances, so that the power was cut off the second last. valve, fair ‘volume was received, and the use of another new resistance cured the trouble. Really the owner of the set was more fortunate than unfortunate, for very many cases are on record of extensive damage being done through the eliminator being left connected. By-pass condensers usually suffer, and quite frequently more than one resistance is burnt out. Causes of Distortion. DP 1STORTION in wireless receiving sets is more common than is generally admitted, and it can be noted even on very good sets that are acclaimed to be almost perfect. Most good sets leave the manufacturers’ hands in such a condition that, given a fair trial, distortion will not take place, but when the batteries begin to run down and valves begin to grow a little dull, distortion gradually creeps in until it is realised that the set is not as good as it used to be, though no reason can be offered. When this is the case or suspected to be the ease, the valves should . be taken in to a dealer to be tested, and any weak valves thrown out, and like-

wise the battery should be tested. For good reproduction frequent testing of both valves and batteries is essential, meaning by frequent six months or less in the case of the batteries, and after 12 months in the case of the valves. A good make of valve will last for some considerable time as compared with a poorer make, although the characteristics of both types of valves are the same. Particularly is tone ruined by a poor detector and a poor last stage valve. weak or cheap detector will cause a humming noise in the speaker that may develop into a low-pitched squeal. The detector is then spoken of as- being microphonie and should be replaced by a new valve or by one of a_ better make. There are several standard makes among which it is almost if not impossible to discriminate, but beyond these few makes there are a large number of second-rate valves that do not give | satisfactory service. Where several valves are used as radio and audio frequency amplifiers, an adequate power valve must be provided for the last stage. By a power valve one means one of the 171 type. Actually, UX1714A is a Radiotron valve, but it is usually taken as a standard, and its equal can be obtained in all good makes. If the beginner specifies 171 type in the make he particularly likes, his dealer will be able to supply the necessary information. If a valve of this type is selected adequate plate voltage must be applied, that is, trom 185 to 180 volts. With batteries this would be rather expensive, but with an eliminator there would be no difficulty, as the voltage could be quite easily regulated, and in some of the later types grid bias can be adjusted at the same time, so that purity of tone can be assured. Where bias has to be applied through a battery, it will be necessary to provide from 223 volts to 40 volts according to the particular make of valve used. Failure to provide adequate grid voltage means that full benefit of the power valve is not being realised. The beginner might ask himself, "When might I use a power valve?" It can be taken for granted in this respect that if five or six valves are being used on a powerful station such as 2YA within a radius of anything up to 150 miles, other things being equal, any valve smaller than the 171 would not be able to handle the output satisfactorily, and a certain harshness of tone would be present. A ease illustrating this has been recently brought to the notice of the writer, A factory-made set using six valves had been in operation for some considerable time, but on 2YA and even on 2BL and 2FC on good nights, there was a certain harshness, and it was necessary to dull the filaments of the valve considerably and so weaken the volume in order that music could be listened to. The valve in the last stage was of the 112 type and biased by a 43-volt battery, which had almost run flat. A few changes in the circuit were made and grid bias was placed upon the second last valve, which had been a 201A. ‘This was replaced by the 112A and a power valve of the 171 type used in the last stage, Actually the valve was of "660" manufacture, valves which as yet have not appeared on the New Zealand market. The result was very gratifying. Instead of the harsh, unnatural tone, the music came in sweet and mellow, the bass fully recorded and the treble notes clear and pure. The result was really outstanding, and fully justified the inclusion of such a valve in the last stage. Actually, volume is slightly weakened, but to so little an extent that it is fully worth while. Even with this alteration, it was found that volume had to be con‘trolled on the main Australian and New Zealand stations.

Regulating the Eliminator. ON most battery eliminators there are two or three knobs which control the plate voltages. Many listeners neglect to adjust these for the reason that they do not understand their operation,. and are afraid of destroying their valves. It is very important that these be correctly adjusted, especially the one con-. troliing the detector valve. Voltage on this valve -should not exceed 50, but it will be found that for each detector there is a maximum voltage at which it will work satisfactorily, and this maximum varies. To adjust to this maximum tune in to a station that is coming in at medium strength. Then steadily turn the knob marked "detector," or "plus 45," and listen carefully to the result. ‘ It will be found that up a_ certain point signal strength increases with tone, but beyond this, although strength may increase, tone will rapidly decrease. ‘This means that the optimum point has bene passed, and that the voltage on the plate of the detector is too high. The knob should now be turned in the opposite direction, until the point of quality and volume are at a maximum. The eliminator should be left at this for some time, but occasionally it is necessary to repeat the test. In a similar manner, the knobs controlling the audio side should be adjusted. Turning the knob marked "Audio," listen carefully for the maximum point, but if the eliminator provides grid bias as well this should be adjusted with the audio control, so that the point where quality and volume is at its greatest can be readily ascertained. ;