The "Tongariro" Portable

Radio Record, Volume III, Issue 19, 22 November 1929, Page 28

The "Tongariro" Portable

A Design for a Powerful Set

(By

CATHODE

NXYONE who essays to deMisign a portable receiver is at mionce faced with the difficulty Bithat either performance or ‘ithe advantage of light weight must be sacrificed. In the design presented here it has : been considered desirable to aim at a .worth-while performance rather than at the reduction of weight, with the result that the completed receiver weighs somewhere round about 80lb. Not the kind of thing a mountaineer would care to add to his pack, but nevertheless quite convenient for the more usual motor of train picnic or the camp. Also, it must be remembered that a receiver which is conveniently portable over long distances makes for unsociability, since its performance is necessarily restricted to headphones reception by reason of the limited "B" voltage available; some people can tolerate the distortion from a badly-overloaded output valve feeding a speaker, but "Cathode" is not one of them, As will be seen from the circuit diagram of the receiver, four valves are employed, two of the screen-grid variety operating as high-frequency amplifiers, one three-electrode valve operating as a grid detector, and a final amplifer which may be either a three electrode valve, or, where more amplification is desired, a pentode. A point which may be new to many readers is | the use of a fixed tune coupling between the two screen-grid valves. The more usual method of using an untuned coupling incorporating a _high-fre-quency choke has been tried, but the fixed tune transformer (its frequency response curve very much flattened by | the 600 ohm resistance was found to give appreciably better results. Actually, this transformer peaks at about 400 metres, but the amplification is almost equal over the entire broadcast band. The dimensions of the carrying case fre really fixed by the dimensions of the batteries employed, and in particular by those of the portable accumulator if one is used in preference to dry cells. The writer unhesitatingly recommends the use of an accumulator for filament supply rather than dry cells,

since the consumption of filament current, while by no means excessive is yet such that only a short life could be expected from the ordinary No. 6 cells. Whether a two or four-volt accumulator is employed is a matter for ‘individual preference. Slightly better results may perhaps be obtained from

4-volt valves, but the smaller amperehour capacity of a 4-yolt accumulator of such a size as to fit in the available Space will necessitate more frequent recharging. One of the special types of portable accumulator is distinctly desirable, although a small light accumulator of ordinary construction may be readily adapted for portable use by the addition to its electrolyte of the recommended. quantity of ‘"Jelectro," a liquid which has the property of partially solidifying the acid electrolyte so that it assumes the consistency of a jelly and is free from an tendency to spill. It is, of course, possible to use the ordinary type of accumulator where the receiver will always be maintained in a vertical position, as, for example, where it is only intended to be moved from room to room, or from the house to garden, but where it is to do much real travelling the use of an ordinary accumulator will almost certainly lead to disaster. | As regards "B" supply, the usual

American heavy duty battery is at once ruled out on the score of its size and weight. Most Bnglish-made batteries are of 4 size convenient for use in a portable, the Hellesen being another battery with a high voltage and a fair eapacity packed away in a small space. Many Wnglish batteries have, in addition, to 120 volts "B" supply, a "0" voltage up to 9 ‘volts tapped off the same block. These are very convenient and compact, but if the dimensions to be given are adhered to, there will be no difficulty in packing away up to 135 volts "B" battery and a separate bias battery of 9 volts, The performance of this receiver makes it a serious challenger to the more conventional type of home receiver. Many people will wish to use it as a permanent receiver and may consider the rather short life to be obtained from the small batteries econtained in the case something of a disadvantage. The remedy for this is the provision of a separate block of heavy-duty "B" batteries or a suitable mains unit, for use when the receiver is at. home, the small batteries only being used abroad. A second "B" wander plug may be provided to avoid the necessity of disturbing the one inserted in the small battery, but, the positive plugs must be shifted from the small battery to the exterior "B" supply. It is quite permissible to move only the high-voltage tapping from the small battery to the exterior "B" supply, as the current taken from the other tappings (detector valve and screening grids) is so small that it will have no adverse effect on the small batteries. Some constructors may consider it worth while to put in a two or four-pole double throw switch and a suitable plug and socket arrangement, so as to make the change-over more convenient. These could be mounted immediately below the baseboard.

The Cabinet. The overall dimensions of the cabinet illustrated are: Length 16}in.; depth 173in.; width 8#in, As has been previously mentioned, the accommodation for batteries is fairly generous, and it is possible that the depth might be decreased a trifle. It must be remembered, however, that if the dimensions are thus modified, it will be necessary to put another turn or so on the frame aerial to maintain a proper tuning range, / The construction of the cabinet may be carried out in any light timber, a thickness of three-eighths of an incb | )

-being suitable. The overall dimensions have already been given and _ the sketches will render the mode of construction readily apparent. The wooden "panel" which carries the speaker is nearly 164 inches by 12 inches, so that only the upper five inches or so of the panel is exposed; thus the heads of the screws holding the two horizontally mounted valve-holders are hidden. The inner framework on which the’ frame aerial is wound and within which the receiver proper is mounted is but 5 5-8 inches in width; consequently there is a space between the receiver panel and that on which the speaker is mounted, the space being conveniently filled by a narrow wooden shelf,

This week "Cathode" presents the first instalment of an article on his portable receiver. It will be concluded in next week’s special portable number, However, there is ample here for the set- . builder to start constructing.

‘The fact that the receiver proper is "sunk" a trifle behind- the extreme front of the carrying case permits the attractive and strong-hinged cover shown in the diagrams. The back of the cabinet may also be hinged if desired, but in the original receiver the inner framework was permitted to slide three-eighths of an inch inside the cabinet (by narrowing the shelf in front of the receiver panel), the back then taking up this space and being secured by miniature turnbuckles. , It will be noted that the inner framework (which should be very: lightly. constructed) has a number of thin laths screwed or glued round its outer edges so as to leave a channel within which the frame aerial is later to be wound. The dimensions of this inner framework must be such that, with the laths iz position, it will just slide within the carrying case. Thus, if the

di rensions given are adhered to, the ini er framework will: be 15% inches by 164 inches over all; the dimensions minus the laths will, of course, be 153 inches by 164 inches, since the laths _ are one-eighth inch thick. Quarter-inch stock may be used for building up the SPRL ERR LE LEAL ELALERELAAGRAALELEALELELELILEALLALILI

inner framework, which, as has been mentioned, is 5 5-8 inches wide. It is not really necessary to dovetail the corners, which may instead be firmly screwed; even the outer carrying case may be securely screwed together if the constructor is not sufficiently adept with ‘woodworking tools to tackle dove- ’ tailing, The fret for the speaker is centrally located in the panel provided for its accommodation. It is practically impossible to cut this fret without a fret saw, so that it will be necessary for

the constructor to beg, borrow or steal one of these useful tools, or:else have the fret cut by a tradesman or friend. The designs shown need not be slavishly copied, and an opportunity presents itself for quite artistic treatment by those so inclined, It should be remembered that the ‘ret should neither be so ".olid" as to impede the radiation of scund,. nor yet so delicate as not to be sufficiently robust to stand an occasional knock. The speaker unit specified is supplied with very full instructions as to the mounting and making of a suitable free-edge cone, so that there is no need to spend space unnecessarily on a description of these operations. As regards finish, the timber may either be stained and suitably polished or varnished; or it may be covered with imitation leather glued on. The imitation leather is not very satisfactory

for covering the speaker fret, and it is perhaps best to leave a circle uncovered, and stain and polish the timber within this area. With the sketches given and the foregoing notes, the constructor should not have much difficulty in completing the

carrying case and inner framework, and making and mounting the speaker. It is suggested that the winding of: the frame aerial should be the next operation. THE main winding of the frame is a simple, straightforward winding of 18 turns of gauge 20 or 22 D.C.C. the turns being spaced by about the diameter of. the wire. Before winding any reaction winding on the frame, however, the constructor must decide once and for all whether ‘he is ever

going to use the receiver on an outdoor aerial or not. It will be seen that provision for use on an outdoor aerial has been made (dotted) in both theoretical and practical wiring diagrams. Whee the frame only is to be used, best results will be secured by having the reaction winding on the frame, but if this arrangement were to be used with an outdoor aerial, it would be capable or causing interference to other listeners if improperly handled, and moreover, would contravene the regulations governing receiving sets. For use with an outdoor aerial, the reaction winding should be transferred to the second high-frequency transformer, the use of an aerial more than compensating for the change, If it is desired to use only the frame aerial and have the reaction winding coupled thereto, this latter should consist of 9 turns of about gauge 26 D.C.%., spaced a little distance (an inch or less) away from the main wiring. The direction of winding is immaterial, since the leads to the reaction winding can readily be reversed if it is found on test that increasing the reaction condenser reduces signal strength. THD next step is the construction of the high-frequency transformers L2 and L38. The formers for these are built up of three discs or hard rubber, bolted together with a small brass bolt through their centre, the inner dise being 2} inches in diameter, and "the two outer ones 3 inches. In the case of the second, or tuned transformer, a "wing" may be left on one of the larger discs, this subsequently being used for mounting by means of an angle bracket, but the first or fixed tune transformer must be supported in a horizontal position at such a height as to clear the bulb of the first valve which, as will be seen, is mounted in a horizon: tal position. Thus. this fixed tune transformer must be screwed to a short piece of wooden dowelling which is in turn secured to the baseboard by: a screw passed up from underneath. At this point, the writer considers it will b~ convenient to stop this instal-

ment. In next week’s special issue the remaining constructional details will be given.

List of Parts for "Tongariro" Portable 1 Panel, 153 x 7 1-8 x 3-16in. 1 Portable Cabinet with baseboard and loud speaker panel. «1 Loud speaker unit (e,g., Blue Spot 66k.). 2 Variable Condensers, 0.0005 mfd. 1 Reaction Condenser, 0.0001 mfd (midget type). . 1 "On and Off" 4 Valve holders. 2 Grid leaks, 2 megohms and holders, 1 Resistance, 600 ohms. 1 L.F. transformer. 1 Dry Cell (Ever Ready "0" type). 1 Fixed Condenser, 2 mfds, 1 Fixed Condenser, 0.0001 mfd, 1 Fixed Condenser, 0.0002 mfd. 1 Fixed Condenser, 0.0003 mfd. 1 Fixed Condenser, 0.0005. mfd, 2 Fixed Condensers, 0.002 mfd. 1 Fixed Condenser, 0.003 mfd. 1 2-volt Accumulator. 1 "B" battery; 120 volts. 2 Valves, 2-volt screen-grid. 1 Valve, 2-volt detector, 1 Valve, 2-volt pentode, 5 Wander Plugs, 2 Spade Ends. Wire for coils as specified, Systoflaw or Glazite screws, ete.