For Tonal Perfection Build the LOFTIN FOUR

Radio Record, Volume IV, Issue 21, 5 December 1930, Page 16

For Tonal

Perfection

Build

the

LOFTIN

FOUR

=F now come to the ultimate set of the Loftin-White seriesthe "Loftin-Four." ~ There is no doubt that this is the most powerful set we have described in our constructional columns o and its distance getting ability must make it a favourite with all those who are in the a.c. areas. Of course, before we go any further we know that we are not going to satisfy all our readers who are waiting to build. Some we feel will expect that the ‘"Loftin-Four" will be a better dx.er than a modern superheterodyne, will be as easy to build as a Browning-Drake, and will be very cheap, Let us right from the onset dispel all these ideas. The set is no more powerful than an ordinary a.c. set using three of four screen grid stages and the usual transformer of resistance coupled amplifier.

We do claim that as far as distance is concerned, this set will be equal to a screen grid set with five valves but, and here is the whole point, the quality will stand out sheer from anything else in the same class. It will be found that the direct coupled system gives the same amount of amplification as the conventional leak detector followed by transformer coupling, but does not possess its inherent limitations of broad tuning ‘and distortion. Greater selectivity is possible by this modern arrangement, and for this reason there is little need to choke back the amplification of the r.f. stages for the selectivity considerations. Translating this into practical terms it means that we can increase the ratio of primary to secondary turns in all the r.f. transformers and yet keep the set selective. In fact everywhere but right under the shadow of powerful stations, the only

limitation to the increase in the numbers of primary turns is the increasing instabitity of the set. No Use for Reaction. NOTHER considerable point in the design of this receiver is that we say good-bye to reaction. And we are not sorry to do so either. Regeneration can be used on this set but it is not satis-factory-it has been rendered obsolete with the new valves that alone have made a set of the nature of the Loftin-Four possible. Again we can recommend this set to constructors and know that we are not going to be swamped with queries from fellows who have encountered a multiplicity of difficulties, including hum. The unique arrangements made to combat it have been entirely satisfactory, and only where some deviation from specifications either intentional or accidental are made is there any cause for complaint. When we have finished talking about the merits and construction of this set we shall point out a few likely sources of trouble, and by studying them there is certainly no likelihood of anyone being stalemated.

i The Circuits. OW, the first thing to do in describing a set is to present a multitude of circuits and confuse everyone who attempts to reach a decision as to which one to build. We think we have quite effectively done this, so shall pass on and try to unravel all these circuits and tell constructors the particular virtues of each so that they may intelligently decide which one to build. Our whole aim in this series has been to give constructors a choice so that they might have what they want. Long experience has shown us that if we give a circuit with one particular set

of constants and no alternatives the will be a host of work coming in r designing for constructors who wai something different. Of course we cou get over the difficulty by saying th what we have described is the very be and there can be no alterations. All th is very good for trade and saves us wor But the little difficulty is that other a rangements will do, so this time we a trying to forestall all these questions | giving the experimentally minded a nur ber of circuits and then he can plea himself. There are still more to con .g., that containing the E406 in the la ylage, but we cannot include it in tt series because although the characteristi can be worked out on paper, they ha yet to be put into actual operation-b re is on {the way. The Output Valves. (CONSIDERING the whole set as _ the full circuit diagram it will that in the last stage we ha

natives there oming in rewho want urse we could saying that the very best ions. All this saves us work. that other ars time we are 2 questions by 1inded a numhe can please nore to come, 106 in the last ude it in this characteristics yer, they have operation-but ves. 1ole set as in um it will be tage we have

used the 250-valve. Our reason? Because we thought it the best. But we can see a kick coming from many constructors, for if they use this valve there is nearly £4 gone in one hit. Yet it is the valve for real quality, especially for the bass for an r.f. choke ‘must be used to overcome r.f. components entering the speaker or otherwise causing instability, there is a marked tendency to cut off the higher notes. Nevertheless, with a well designed choke this will not happen to any extent, and the tone will be remarkably mellow. At the same time the set will be able to handle a great deal of volume without overloading. The 245 costing about one-fourth of the 250, can also be used in the last stage, but it is possible to overload this with fairly strong signals. For a medium sized room that is not heavily damped with carpets and heavy hangings, this valve will take the load quite well. It is somewhat lost in a heavily draped big room, ‘The constants for a circuit using this type of valve have already been published for the "Loftin-Three," and they can be used in the present circuit without further adjustment. A parallel arrangement will be described next week. The two r.f. valves can be considered as a separate tuner following the general outlines given last week or it can b used in the rebuilt circuit or the adapted circuit. One important consideration is necessary and that is the introduction of an r.f. choke in the output of the power valve. It would be better in the plate circuit of the detector, but it would cut off some of the high frequencies. No doubt some who are after the bass notes will prefer it in this position, but we do not like them well enough to sacrifice the high notes. The pentode in the last stage will appeal to a large number who want high

gain, and the characteristic strident tone of that valve. Furthermore, its use allows the r.f. filter to be moved into the place where it will be more effective, to wit, in the plate circuit of the detector. As we pointed out previously the filter will cut off the high notes, but this valve tends to emphasise them, so we arrive at some kind of a balance. Using High-voltage Speaker. FURTHER point that may be of interest to those who have speakers

of the high voltage type is . that these can be used in the main chain of resistance to replace one or part of one of the resistances. Most of these are designed to pass a current in the vicinity of 30 milli-ampres, that it may be conveniently placed in the main chain without placing any further drain on the output transformer. It will be necessary to find out the resistance of the winding though it is generally about 2500 ohms. and (Continued on page 29.)

The Loftin-Four (Continued from page 17.) it can fit in the main chain 3500 ohm. position of the main sketch. To find out the resistance divide the voltage given by the current taken by the field multiplied by 1000, and there you are with everything cut and dried. The Power-Pack. N the first series of Loftin-White articles a power-pack with one-inch corded transformer and choke, was specified. This power-pack is suitable for a single 245 output valve, or other valve requiring a plate circuit of not more than 32 mile, or so. A pair of 245’s. or a single 250 would require 64 mills in the 1 a and 55 in the latter. It wuold 16% ‘be advisable to attempt to supply this ‘demand from the power-pack already specified even with increased secondary turns, so the specifications are given for a larger transformer and choke specially suited to supply a single 250 valve, or a pentode. The transformer and choke both have cores built of Itin. stalloy the transformer core built to 13-8 thick, and the choke core to 1}in. in order to increase the inductance as only one choke is used, a centre-tap being provided for connection to a 1 m.f.d. smoothing condenser. For the 250-wvalve a half-wave transformer is used, giving 650 volts from the high voltage secondary consisting of 3250 turns of 32 s.w.g.. enamelled wire in one continuous winding without centre tap. -‘ To give 450 volts full-wave for a pair of 245’s, the secondary winding should consist of two half-width windings side by side of 2200 turns each, or a continuous. winding of 4400 turns tapped at 2200, for which 34 wire could be used. Stalloy sizes for the transformer are 43 x 14 and 24 x 14, with windows 31 x 1 1-8 and spool ends 3fin. square, clamps drilled 44 centres for 4-in. bolts. The primary winding will consist of 1100 turns of 24 s.w.g. enamelled. wire to carry 100 watts, as allowing 60 maa. for the 250 and r.f. valves, and for the filaments of 250, 281, and four 224 valves there is a total demand of 74 watts. without allowing for losses. Filament winding of 18 d.c.c. should be as follow :- 74 volts**(250) .....eeee5 45 turns 23 volts (224) ..... coooe 15 iy, 2% volts (1227) ...ee0.. 15 ,, 24 volts (2227’s) ...... 17 ,, ¥ volts (--) ........ 23 Cs, Voltage is best maintained for heaters by running only one 227 from each winding, but two may be run if the extra two turns are provided to maintain voltage.

_ The rectifier and screen-grid detector should both be heated from separate windings, and in the case of the rectifier this is imperative. It is permissible to parallel several windings of equal voltage and supply the detector and several r.f. valves from common leads. The windings must be paralleled in correct phase, which is tested by conrecting two windings in parallel; if they are correctly connected, a 4 or 6-volt flash-lamp placed across them will light, but if the windings are opposed, the lamp will not light, and one winding must be connected the reverse way. The power valve must have a separate winding. The usual primary fuses and one secondary fuse must be provided, The Choke. HE core is of the V pattern, with a piece across the open end, giving two gaps. The clamp at the gap end is lin. wide, overlapping the ends of the V by tin. The two gaps must not be less than 1-32in. each, the space being filled with ‘cardboard. The window is 2% x 13/16. Four sizes of stalloy,are used, 3 5-16 (across end), 3%, 24 and 2in. by Itin. wide, one heap of each size lin. high. Spool ends are 3 1-8 x 215-16in. Put on 7000 turns of 32 wire,. tapping at 3500. More than 7000 may be put on if there is room-up to 500 extra-the fact of the tap not then being exactly in the centre will not matter. Two pounds of

wire. will be required, and the inductance with 60 mua. passing will be 34 henries. The dc. resistance of 7000 turns will be 450 ohms., and the voltage drop as follows :- 50 m.a. passing drop ..... 22 volts 60 m.a. passing drop ..... 28 ,, 70 m.a. passing drop ..... 32 ,, 80 m.a. passing drop ..... 36 4, If it is desired to use a factory-built choke, the Ferranti Bl is the most suitable, having a d.c. resistance of 500ohms., and in inductance of 18 -henries with 50 m.a. passing, dropping 25 volts. These chokes are not provided with a centretap. ; Next Week. E set out on this article with the best of intentions to give the complete instructions for the construction and operation of the four-valve set, but we find that the number at the top of our manuscript tells us that the allotted space is already nearly exhausted. Space has slipped away in discussing many points that, were they left untreated, would wotry constructors, so we have gone to some length to set everyone at rest. For those who want to purchase their parts we give a list, and they can collect these during the week, for we can assure them there will be no more theoretical considerations. We have now all the circuits, with the exception of the parallel output, we shall publish for a few weeks, and any new

ones can be easily made with a few alterations to the existing ones. Before we conclude this week’s instalment it may be worth mentioning that the coils are the same as for the LoftinThree. Complete kits of these are readily available at all dealers, but there 1s no great difficulty in making them. About that part of the business we shall have more to say next week.