The "Home and Country" Portable

Radio Record, Volume IV, Issue 25, 2 January 1931, Unnumbered Page

The "Home and Country"

Portable

y A light receiver for home or holiday use

By

Cathode

) HE design of a portable reby ceiver is a difficult task. To really justify the. term "portable". it must be a receiver of small dimensions and weight, yet it is invariably expected to provide genuine loudspeaker strength on a small inbuilt frame aerial-and that at considerable distances from a broadcasting station. Every summer beach has its dancing parties for which the portable must provide stimulus for the light fantastic toe, : Generally speaking, the difficulty of keeping the weight déwn results in the degeneration of the portable into a type which has come to be known by the term "transportable." This tvpe of receiver may have four or five valves with a correspondingly good performance; the cabinet has ample room for even large "B" batteries and an ae cumulator of considerable capacity. But such a receiver is definitely not portable. It can be carried from house to lawn at the expense of much personal effort. It can even form part of a motor party if the motor is large ‘and uncrowded. But as part of one’s personal baggage it is awkward; in w midget car it is an unmitigated nuisance. In "leveloping the present design the wthod of approach adopted was this: It was first sought to find the smallest and most easily built receiver which could be relied upon to give speaker strength on a small frame aerial up to 80 or 40 miles from a broadcasting station. The conclusion arrived at during this process was that a threevalver was the absolute minimum. It

was furthermore found that with the limited space available much more suzcess was obtained by amplifying at low frequency than by employing one of the valves as a radio-frequency amplifier, even though a screen-grid valve was employed. ~The fact that maximum amplification was desired prompted the use of two transformer-coupled stages in preference to resistance-capa-city éoupling, instability being prevented by certain precautionary measures which will be described later. Having reduced the receiver itself to the smallest possible proportions, it was néxt sought to reduce the bulk and weight of its appurtenances. Here, of course, we encountered a_ difficuliy which is usually regarded as insurmountable. A three-valver takes a fair admount of current. The set is sure tu receive regular use at home as well as abroad. ‘Therefore the batteries must be of considerable capacity if they are to last any time at all. And largecapacity batteries are very heavy. Wel, that does look like a bit of a hurdle, doesn’t it? -. ° . The solution is to equip the set with the very smallest size of batteries and reserve them purely for portable use. An ingenious and novel piece of design permits the plugging-in of a small mains unit when the receiver is used in the home circle, the battery being automatically disconnected when this is done. That disposes of any difficulty as regards the weight of super "B" batteries. ° OW as to the "A" battery or accumulator. There is no doubt that a two-volt accumulator is much betier than dry cells for filament supply, put it must be a small one or weight will be increased too much. Small accumuiators must be charged fairly frequentiy, so that a home charger would be a great convenience. Very well, we wiil incorporate a charger in the mains unit which is to supply the "B" current. It can be done very easily; and by using the same system of plugging-in the charger as is used for the "‘B" eliminator, the process of charging is, freed from any vestige of inconvenience. ' . Although a great deal has been done in the cause of weight reduction, performance has: definitely: not -been sacrificed. The frame circuit employed is a particularly efficient version of the Hartley circuit, and this is backed up by an extremely powerful audio ampiifier. Almost any amplifier employing two transformers will give considerable "lift," but in this set the system of filters in the plate leads ensure that each stage is operating under ideal conditions, so that the overall magnification is something exceptional. The filter circuits comprise: (1) A combination of series resistance and

by-pass condenser in the plate lead to the detector valve, (2) an output filter for the loudspeaker, and (3) an arrangement of radio-frequency choke and by-pass condenser in the plate lead to the output valve. The last is intended to prevent any radio-frequency component which may be present in the plate circuit of the last valve from feeding back via the loudspeaker to the frame aerial and causing instability. The net result of the three filters is that the receiver as a whole is quite stable and perfectly easy to operate, notwithstanding that it has two trans-former-coupled amplifiers and will normally be operated without any earth connection. The quality of reproduction is of quite a high standard and more than

justifies our making the receiver suitable for use in the home, The Carrying Case. The overall dimensions of thg; cabinet or carrying case are: Hei inches; width, 164 inches; and: 6% inches. The top bottom and/sides of the case should be made of half. inch stock, and as this portion of the construction carries practically the

whole of the weight it is advisable

to

dovetail the corners. If this

operation

is beyond the constructor,

equivalent

strength may be secured by

reinforcing

the cory@rs with thin sheet

aluminium

or bras®awell screwed to the wood:

0d 3 or

strips &t the same metals mav be.

taken

right rund the case, leaving the

ends

separated an inch or so so as

not. to

impair the efficiency of the frame

aerial

by coupling to it.

The front of the case is occupied by the control panel and by the frettedtimber which carries the speaker chassis. The speaker chassis should be of a fairly small and light type so that it will do no harm to use three-ply to support it. Hvery little bit of weight that can be saved will help in: making the set convenient to carry. . If doors are used to cover the panel and speaker fret as in the illustration -of Fig. 2, these should be as lightly constructed as: possible too. . The back of the cabinet may also be made of three-ply, and is detachable as a whole. The frame aerial is carried on ‘it, being wound on four strips _ of ebonite mounted diagonally at the © corners. Wach strip must have fourteen. sloping sawcuts, spaced oneeighth -of an inch, made across it part way through, the aerial wire being wound in these cuts. As may be guessed, the frame winding consists of fourteen turns, 26 s.w.g. wire with a double silk covering being suitable. The winding is tapped at the seventh turn, the tapping and the two ends are taken to a miniature terminal strip mounted _ alongside. the winding, the tapping to the centre terminal and the two ends of the winding to the two outer ter minals. Flex leads are then taken from these terminals to the proper points in the receiver, the leads being left long enough to permit of the back being opened to provide access to the receiver and batteries. ‘ The strips on which the baseboard rests should be a little over seven inches down from the top of the cabinet. The precise method of constructing the cabinet is a matter in which every individual constructor will wish to. be governed to some extent by his own ideas, but the general layout indicated in Fig. 2 should. not be de parted from too completely. , The ‘General Layout. HP construction of the receiver itself is very simple indeed ; moreover, we have been careful to draw. very complete diagrams so as. to eliminate any ‘possibility of a mistake... The fact that some of the components are mounted underneath the baseboard has made’ it necessary to include an extra drawing of the underside of the board, but there should be no difficulty at ail in following this, The two jacks for plugging in the "B" eliminator and "A"? charger were in the original model screwed direct to the baseboard. Some types of jack do not permit of this being done, and where this is the case the width of the panel must be’ inereased so that it will project below the baseboard sufficiently to mount the jacks. Where this is not done, the panel which carries the speaker chassis must be drilled to permit the insertion of ‘plugs into the jacks which otherwise, of course, are inaccessible. The jacks are shown mounted sidewise in Fig. 4, but this is only for the sake of clearness in showing the connections to them; actually they are mounted vertically. . The [THE choice of components is a matter for serious consideration in

‘ant portable set, as it is necessary to . strike a nice balance between small dimensions and efficiency. For instance, many variable condensers of very. high efficiency and mechanical excellence are also on the heavy side. Fortunately, it is not difficult to find

components which, while being. light, are also well made and efficient... Many transformers of small dimensions, but having excellent characteristics, are now available. If possible, it is. advisable to-use two transformers of different makes, as stability is apt to be affected where the two trans- . ome | . . . e , --

formers are exactly similar. The use of low ratio transformers is recommended, as these usually result in improved tone quality, while the sensitivity of the receiver is such that the loss of a little amplification is not serious. It will be noticed that certain of ‘the components are fairly closely spaced, It is just possible that if different components to those used in the original are employed, some slight readjustment’ may. be necessary. However, an effort has been made to provide room for most alternative components, so that it is very unlikely that anything of this kind will be called for, especially if very bulky apparatus is avoided. For the same reason as in the case of the transformers, it is advisable to use two different makes of radio frequency chokes. If home-made chokes are employed, use two which have a widely differing number of turns rather than two of identical construction. There is not very much space available for the second r.f. choke, so that this must necessarily be of a compact type. In next week’s article the remaining components will be discussed, and general hints given on construction .and operation.

Ae a A EE EE A i Components Required 1 Panel 16in, by Tin. by 3/16in. 1'Cabinet with baseboard 4iin, ‘deep. 1 0.0095 mfd. variable condenser. 1 0.00005 mfd. miniature condenser for reaction. 2 Double-circuit (closed) jacks, 1 On-off switch. . 8 Valveholders (preferably sprung type). 2 Radio-frequency chokes, 2 Audio transformers (low ratio). 2 2-mfd; condensers. 1 50,000 ohm, wirewound resistance, 1 0.0003 mfd. fixed condenser. 1 0.001 mfd, fixed condenser. 1 0.0003 mfd. fixed condenser. 1 2-meg. gridleak and holder. 1 20-Henry L.F. Chokes! (not too heavy). Wire'and Flex, Speaker, Batteries, and Valves, ce Ea a OE A aE ae at a