The Tongariro Portable

Radio Record, Volume III, Issue 20, 29 November 1929, Page 30

The Tongariro Portable

A Companion Recewer for the FHlolidays

(By

CATHODE

N our preliminary description published in last week’s "Radio Record" the cabinet and frame autenna of this remarkable set was described. Without reiterating,. we continue from where we left off. qT the case of the first or fixed tune transformer, the primary winding. which is put on first, consists of 45 turns of No. 40 D.S.C., and the second ary winding (separated from the pri mary by a couple of layers of dry paper or silk) of 100 turns of No, 28 D.S.C. Both windings are put on in the same direction, and the inner ends of prim ary «and secondary are soldered to gether, the combined lead then being taken to one side of the resistance RB, when a connection is made to "B+". The outer, or free, end of the primary is connected to the plate (or top terminal in the case of English and Continental valves), of the first screen grid valve, and the outer, or free, end of the secondary goes via the condenser C5 to the grid terminal of the valveholder for the second screen-grid valve. The tuned transformer accommodates

in its slot, first, a primary winding of 20 turns of No. 40 D.S.C., and, second, wound in the same direction over a couple of dayers of paper or silk, a sec: ondary winding of 64 turns of No. 28 D.S.C. The inner ends of the two windings are again joined by a drop of solder close to the transformer, but in this instance both ends are left long, one being taken to "B-+-" and the other to the moving plates of the second tuning condenser. The outer or free end of the primary is taken to the plate of the second ‘sereen-grid. valve, while the outer end of the secondary goes via the grid condenser C6 to the grid terminal of the detector valve-holder. This transformer will have to accommodate a reaction winding in addition if this is not placed on the frame; this Wind ing can be wound over the secondary

(separated from it by a couple of layers of paper or silk) and may consist of 20 to 30 turns of No, 40 D.S.C. The

to present itself was between, firstly; an apparently efficient design involving a considerable amount of. screening, and, secondly, a reduction of efficiency (by reducing "the primary turns on the transformer) until a satisfactory degree of stability was secured. In practice the efficiency of the screened receiver was not found to be ‘nearly so great as was expected, due, no doubt, to very heavy absorption losses due to the proximity of the screening to

It may be a matter for comment that the primary windings: of the two transformers seem too small to mateh the high impedance of the _ screen-grid valves. As a matter of fact they are. The only choice which seemed

specified, in combination with the natural stabilising effect of the high-re-sistance fixed-tune stage interposed between the two more efficient stages, provides a very satisfactory degree of stability. If, by any chance, a ten-

dency to oscillate is encountered in any particular case, it can readily be uelled by connecting a resistance of a few hundred ohms in the lead to the control grid of the first screengrid valve; it is very unlikely that this will be necessary, however, ’ Mounting the Components. HE routine work of mounting components and wiring up does not call for much comment, as & complete wiring diagram is reproduced. It is necergary to duplicate the placement of components fairly accurately, space being at a premium; in particular, great.care must be taken not to mount dny baseboard components where they will interfere with the insertion of the first screen-grid_ valve or the pentode or other power valve, which are mounted horizontally. /A word of comment is also called far in regard to the choice of compohents, The variable condensers must

be of fairly small dimensions. there being several English makes, notably the Formo and the Utility "Mite," of sufficiently diminutive size. As to transformers, if quality is a consideration, as well as small size and weight, one is practically limited to the Igranic type J or the Mullard "Permacore," , both these instruments using a special core material, . A little difficulty may be experienced in obtaining a single cell mounted on the baseboard just back of the panel and supplying’ bias to the screen-grid valves; therefore it is well to mention that a couple of springy clips may be made to contact with an ordinary flashlamp cell strapped down to the baseboard, the zine container of the cell being the nega‘tive terminal, Wiring should be carried out with No. 18 or 20 tinned copper wive, enclosed in Systotlex or-other sleeving, or with Glazite. The connections ‘from the high-frequency transformers are continuations of the fine wire windings of the transformers themselves, these being enclosed in sleeving wherever possible. A certain part of the wiring (in particular the connections to the large fixed condenser C9, which is the only component mounted beneath the baseboard) is located under the baseboard, and is shown

dotted in the diagram; light dotted lines show the optional connections for use with an outdoor aerial, Tags are’ provided for connection to the plates of the two screen-grid valves and the priming grid of the pentode, and these tags are clearly shown. The leads to the speaker lead through a hole in the baseboard just alongside the condenser Cll, as shown. With the additional remark that wiring is, wherever possible, "point to point’-that is, taken by the most direct route between connections-the yery complete wiring diagram should enable anyone to complete the wiring correctly. Power Supply. ATTDPRIES have been already ‘dealt with. The maker’s directions .as to voltages to be applied to the yalves should be followed as far ‘as possible. With the Mullard P.M. 12 (sereen-grid),. P.M. 2DX, and P.M. 22 (pentode), used in the original receiver, the full 120 volts was applied to the plates of all valves except the detector, to which 70 volts was applied, _ the full potential being, of course, also applied to the primary grid of the pentode, since this is connected to the common B+2 connection. The screening grids of the H.F, valves were given 80 yolts, and the control grid of the pentode 73 yolts negative bias. Any 2-yolt valves of the proper type may be used, the maker’s recommendations aus to plate and screen voltages being followed as far as possible. An.ordinary power valve may be used in place of the pentode if desired, with some loss of magnification; it will probably be necessary to increase the negative bias voltage if this chaiige is made; the tag which normally connects to the priming grid of the pentode will, of course, be unused, . _ With all wiring thoroughly checked oyer, the batteries may be connected and the valves inserted for a trial. Do not ‘orget to connect the tags to the extra terminals of the three valves to which they belong. Do ‘not forget, either, to reverse the connections to the reaction winding if necessary, as explained above. The directional properties of the frame antenna may be a trifle disconcerting at first. However, it will soon be learnt that rotation of the receiver has a yery definite bearing on results. A note has already been made on the method of coping with unwanted oscillation; it my’ also be borne in mind that an earth connection, even of the most makeshift kind, is of benefit, not only in increasing the range, but in attaining complete stability. This article has already run to some length, and space does not permit of any further details of adjustment and operation. If the number of inquiries merit it, a further short article may possibly be prepared to clear up any little difficulties which individual constructors may encounter with their particular versions of the receiver. In order to forestall one Possible inquiry, it may be mentioned here that the American UX 222 screengrid valve is unsuited for use in the present receiver, even though a 4-volt accumulator be used, unless a substantial modification of wiring is made. The reason, for this is that the UX 222 has the control grid connected to the top terminal instead of the plate; all English and Jontinental screen-grid valves have the plate connection at the top. ‘ .