A Corner for Beginners

Radio Record, Volume II, Issue 37, 28 March 1929, Page 12

A Corner for Beginners

Changing Valves. FROM time to time, correspondents write in regarding this subject, sometimes in difficulty regarding neutralising and sometimes to know the different types of valves for each position, In general if the set has | peen functioning well before the valves began to weaken or were burnt ont, , replace by exactly the same type of | valve, particularly so if the receiver | is a factory made one. When home-

made the constructor usually understands something about the relationship of his valves to the other components of the set, and so he can consider "matching." But for the beginner, one would strongly recommend that valves are replaced as they came out. The main point to watch in this direction is the substitution of American valves by English or Continental. Not that this cannot be done, but the beginner seldom makes a success of it. In order that a set may not interfere with its neighbours, and give good reproduction, it has to be neutralised, and it is neutralised for particular valves, and if these are changed the neutralisation is not now correct, and the set either makes itself a nuisance or it will be noticed that reproduction is not as good as it used to be. There ure certain makes of American receivers which cannot take other than American ralves without considerable alteration and readjusting, so that it is indeed

unwise to change. The users of American sets can be quite safe in using the 201A type of valve for all positions in the set except the last, when something mere powerful, e.g., the 171 could be used with advantage. With English and Continental valves, however, there is a greater degree of specialisation, and some consideration must be given before the valves are selected. As the number of these is fairly great, the beginner should consult a dealer before he makes his change. The use of one of these valves in the wrong place will entirely spoil reception. Generally speaking, valves with a high impedance and a high amplification factor are used in the radio frequency stages, and low impedance valves with an amplification factor are used in the latter stages. The last "alve need have only a very small amplification factor, ustially 10f

greater than three, e.g., 603 in Philips valves, PM252 (Mullard), and DEP250 (Osram). These power valves usually require a high plate yoltage, not 1ess than 150 volts, to work satisfactorily, while their filament consumption is regularly higher than that of the ordinary valve. Very complete valve tables have been compiled for the new edition of the Radio Listeners’ Guide. The valves treated are all those in general use in New Zealand, and the amateur can by referring to the table find in his own particular make the valve for any position in his set.

The Value of Condensers. (THE following is the use of the values of combined condensers. These were published some time previously, but recently there have been. several requests in this direction, so it has been considered advisable to reprint them. From the tables it will be seen that if condensers are placed in parallel] the total capacity is equal to the sum of the two condensers so connected. The value of the condensers in series is rather more difficult to obtain. The fact that condensers in parallel is equal to the sum of their capacities is very often very useful in building up condenser banks for battery eliminators. It is very difficult to get. precisely the capacity of condenser combined with the correct voltage test, so that it is necessary to combine these. The capacities in the following table are all expressed in microfarads (mfds.). The first two columns contain the values of the two condensers to be combined, and the other two columns give the resultant capacity of the combination.

Single Dial Control. ONE of our correspondents this week has raised the question of single dial control, and as this more or less interests all, it is considered worth a a little elaboration. By single dial control, it is,ymeant that all the variable condensers in the set are so fastened together that one dial operates all the moving plates. Each condenser controls a transformer or coil in the radio frequeney side of the set, and the duty of each is to adjust that coil so that it may receive the variations as they come frem the air, in other words, that the set may be tuned to

he station’s wavelength. Coils differ, ;o it would be expected that the readng for each coil will be slightly dif‘erent from that of another, and this s the problem of single dial control. This variation is particularly noticeible in the aerial coil. When a single coil is used in the 1erial circuit the inductance of the zerial considerably affects. the injuctance of the aerial coil, with the §=6r result that the condenser reading on that coil -should be slightly different from that on the other crid coils. To overcome this the aerial coil should be loosely. coupled to its srid coil. That is, there should be two separate windings. Where a single coil is used, and it is desired to incorporate single dial control, a midget, or balancing condenser, should’ be placed across the first grid coil. By adjusting this, the tuning can be m very much finer, and inaccuracies # anced out. It is very difficult to say if single dial control will replace multiple dial control, and some of our leading radio authorities claim that-it will not. Certainly single dial control cannot. be as selective as multiple dial control, but when balancing condensers are used the difficulty is overcome, but is it one dial control? Dull Emitters and Bright Emitters. [THE terms dull emitters and bright emitters are used rather loosely. Actually there are no bright emitters on the market at the present time, understanding by bright emitters those that consumed about 1 or 1.5 amperes and were indeed, lamps. Now, there are two general classes of valves: those consuming between .1. and .25 amperes, and those consuming approximately .06. For the sake of comparisons, the .06 type can be referred to low filament consumption valves as opposed to the higher filament consumption valve. In all ways, dull emitters of the two types mentioned are superior to the bright emitters that paved the way to modern reception. The differences betwen the two types of dull emitters are generally that although very economical in "A" battery, the low filament consumption valves are heavier on plate current. Where an eliminator is used, this latter is of little disadvantage. -~

Separate Capacities. 0001 .0001 0001 0001 0001 0001 .00025 00025 00025 .00025 .00025 .0005 .0005 0005 .0005 001 001 .001 002 .002 005 and and and and and and and and and and and and and and and und und and and and and 00025 0005 001 005 005 .006 0005 001 .002 005 006 001 002 005 006 002 005 006 005 006 006 Combination. Series. Parallel. .000071 .000083 .000093 .000095 .000098 .000098 .00016 -0002 00022 .00023 00024 .00033 0004 00045 .00046 .00066 .00083 .00085 001+ 1115 0027 00035 0006 0011 .0021 0051 .0061 .00075 .00125 00225 .00225 00625 0015 .0025 0055 0065 .003 006 007 007 .008 011