POPULAR CIRCUITS

Sun (Auckland), Volume II, Issue 337, 24 April 1928, Page 16

POPULAR CIRCUITS

HOME-MADE SETS

The radio season, which is just ending on the other side of the world, has seen, in England at least, a return to the home-built set. Two of the largest valve manufacturers have combined with makers of other accessories and have designed sets using specified cornS ponents. The success attained i 3 rca- | Used when both these manufacturers I say that these sets have brought about | the sales of over 300,000 valves for i each manufacturer, making a total of well over the half-million. These two sets, which have achieved such popularity are the Cossor Melody Three and the Mullard Master Three. Both sets have been made locally and the results are good enough to anticipiate that many will try their hands this winter at the simple construction necessary to n ai:d these sets. The writer proposes to describe these sets in turn and to take the Master Three this week. The circuit need not worry the prospective builder as the whole set is carefully laid out in blue print form, and the position of the connecting wires is shown in such a way that one simply cannot go wrong. Another interesting point is that the set may be used for either short or long waves by merely changing a single coil. The set uses special coils, which are made by several well-known coil makers who supply the whole coil together with plug in base nicely rnaue on standard low loss principles. This point will appeal to the entirely new at the radio, happy as he or she will have no worries about number of turns, difficulties with reversed reaction coils or reaction coils which will cause oscillation over which there is not adequate control. Soldering difficulties are removed by the use of neat little connectors which make a ' perfect joint without recourse to the ! soldering iron and its eccentriciti?s in | the hands of the novice. I Thus we have the two main diffi- | culties of the newcomer removed, and I he can sit down and know that if he [follows the simple instructions he is i going to have a set that will not only j work but will also give satisfactory j results on all Xew Zealand stations l and good speaker strength on AustraI lia.

The tonal quality was the point which struck the writer most; resistance capacity units of excellent construction are used in the first audio stage, followed by transformer coil plug for the second stage. Economy in filament current is a feature, .3 amps being taken at either 2, 4 or 6 volts, according to the desire of the constructor. Another point is that it is the first set which might be called nretty. The components specified are finished in brilliant colours, and with the shining aluminium panel coloured connecting wires in neat and simple array, the inside is most striking. To those with experience, the specified list of components is not necessary, but to the absolute novice, the instructions, if followed in detail, will make a failure proof set, as the connecting links are made to suit these components and give a job confined to the use of a bradawl and a small screwdriver. The coils are the wellknown Colvenn, which are stocked Dv Messrs. Hartle and Gray. The H.F. choke, resistance capacity coupler, and L.F. transformer, are by R. I. VarJey, which are obtainable almost anywhere. Pye valve holders, Lewas coil holder, Mullard condensers and valves, A.B.C. Junit connecting links, are the chief components recommended. With batteries, speaker, etc., any good type will complete the little set. THE SCREENED GRID BROWNING DRAKE The Browning Drake is too wellknown to need any introduction, it being one of the most popular of all sets. For the first time since its introduction a radical change has been made in the design. Many months of research have culminated in two new designs, one for the new screened grid and the other for A.C. operation. The primary in the design for the new valve is left unconnected in the bought coils or removed in the home-made ones. The plate terminal of the R.F. amplifier goes through a .5 M.F. condenser to the grid end of the secondary of the regenaformer. The control grid of the new valve is connected to the neutralisation condenser, the other side of which goes to the secondary of the regenaformer tapping as usual. The shield grid or screen is connected to 45 volts H.T., and also to ground through a .5 m.f. blocking condenser. Neutralisation is effected in the usual way. An unusual feature in the use of this shielded valve is that it is important to have the stage shield in place while this neutralisation is done. The shield encloses the aerial coil, condenser and R.F. valve. This innovation should be interesting to B.D. users, as the writer's own results put the B.D. screened grid set as far ahead of other sets using the same number of valves. A.C. MODEL A second change in Browning Drake design is the introduction of an A.C. model using the TJX 227 heated cathode valves in the R.F. and detector stages and the 226 in the audio stages. A volume control is fitted by using a high resistance in the R.F. plate circuit. The amateur has an interesting winter ahead of him.