The "Shielded Five" Neutrodyne

Radio Record, Volume II, Issue 35, 15 March 1929, Page 28

The "Shielded Five" Neutrodyne

A Trouble-free Receiver with Great Capabilities

"Bo

PENTODE

‘HE writer has often been asked which circuit and arrangement in his opinion is the most efficient using standard valves, coils, etc. It is ne) a difficult question to answer, «> because of the multitude of "stunt" circuits available. Details are seldom given’ with the question as to the number of valves proposed to be used or to the depth into his pocket the prospective builder is prepared to go. There are many who just require diagrams of two or three valve sets and this class ean be soon satisfied by

= running through the numerous smak circuits that have been proved to be suitable for this country. When, however, it comes to a multivalve receiver of five or six valves the matter has to be given more careful consideration. Not a few of the published circuits of these larger sets are quite suitable for an experienced builder to tackle, but the man who anticipates building his first receiver has to tread warily, choosing a circuit having a good latitude of safety for any mistakes in lay-out, etc, due to inexperience. How many otherwise enthusiastic radio builders have been turned right off wireless through a failure. when building their first set. More experienced workers have also often been frankly puzzled over the indifferent results from a multivalve receiver. The writer has been in close touch with many set builders and 99 per cent. of the poor results obtained from a home-built set has been beyond doubt due to the general arrangement and not the cirtuit. The poor theoretical diagram gets blamed every time and the common ery is that "this and that circuit is no good, I’ve tried it." What is a Good Receiver? RETURNING once more to the question of the most efficient receiver. "Pentode" considers that the straight five-valve shielded receiver, to be now, described, to be one of the most efficient. Of course, this is just using standard three electrode. valves, standard coils, condensers, etc. Speaking of general efficiency, it is meant to include ease of control, reliability and certainty of results. When correctly neutralised it will not oscillate. The actual circuit is by no means recent and there is little difference between this and published details of most five-valve receivers. It is the general lay-out that differs, and prehave been taken go that

when the set is finished the chances of trouble are very remote. As it will be seen, each stage is completely shielded with by-pass condensers to confine each set of oscillations to their own compartment. The wiring is extremely simple as each screening box is wired up practically the same. If the shields are securely fastened together by brass bolts and the A-lead of each filament secured to the shields then these will act as the filament return and render

much wiring unnecessary. If copper screens are to be used all connections can be soldered. It will be noticed that the ends of all the coils are earthed to the metal shield as well as the moving plates of the three variable condensers. The grid leak and condenser of the detector valve are not connected in the usual way, i.e., in parallel, but separate and in series. The condenser is in the usual place, but the free end of grid leak connects to the positive lead from the A battery. Perhaps the easiest way to do this is to fix two small clips to the grid and filament terminals on the valve socket and clip the grid leak between these. A radio frequency choke is included in the plate of the detector valve. Although this is not absolutely necessary, it isolates the radio and audio frequencies and

very often makes the receiver far more stable in operation. Especially is this necessary if long speaker leads are used. A rheostat is used to control the volume by regulating the filaments of the first two valves. In the positive lead controlling the detector and audio filaments ig inserted a fixed resistance of

small value (an amperite may be used and the value of this can be easily calculated when the type of valves to be used is known. It is a great mistake to try to control volume by dimming the audio-frequency valves. Whatever the volume these should be kept operating at their correct temperature and all the control be done from the radio-frequency end. The main switch is arranged to work in conjunction with the speaker, plug and jack. To withdraw the plug cuts off all batteries. A list of components is given, as this often proves helpful when buying. Making the Shield. (THE screening box will first be described for those who desire to make their own. It will perhaps be easier if it is stated that it is merely a metal box, Gin. high, 15in. long, and Qin. from back to front, with a movable lid, and divided into three compartments each five inches wide. Lay the sheet of aluminium, copper would do just as well, on a flat surface, and mark out before cutting. Although a sheet only 15in. wide is necessary, the sheets are usually of a standard width, 24in., and a strip will be left over. This is unavoidable if the whole is to be made from one piece. The sizes and shape are given, and a Zin. overlap should be left as in pre-viously-described screens. The front is cut in one piece so that it will have no rivets or bolts to prevent it from lying flush with the front panel. The partition separators are best bolted into place from the back and screwed to the baseboard through the bottom of the main metal can. There is no necessity to fasten the front edge unless the whole is made of copper, when

it can be soldered, including all the seams. To bend the aluminium clamp firmly between two straight pieces of wood, wit the line along which the bend is to be made just showing. Now bend sharply and lay a flat piece of wood along the hend which can be tapped gently. When the box is bent to shape,

hold firmly while holes are drilled through the joints to take small nuts and bolts or rivets. These joints along the bottom can be fixed by screwing directly into the baseboard. Before finally fixing the partitions into place, drill holes where indicated through which the insulated leads go from one compartment to the next. These should be about 3-16in. diameter, and about 2in. from the bottom. ae first partition will need three holes ar the next two holes. The holes round the outside can be drilled afterwards. The lid comes next, and should be made quite flat, so that it will rest down evenly, making a more or less radio tight joint. Mounting the Components. BEFORE fixing to the baseboard and panel, mount the three variable condensers, or one in each compartment. If of the single hole mounting type one hole is sufficient, making sure it is in such a place that the moving vanes cannot foul the metal screen. In the middle compartment, between the left-hand and centre condensers, is mounted the rheostat. There will be room for this at the bottom left-hand corner of the centre division. If not, then a portion of the metal screen wi}. have to be cut away. Be careful an note if either of the terminals or th centre spindle of the rheostat is liabie to touch the metal screen. It must be carefully insulated therefrom, even if a large hole has to be cut in the screen. Dismantle these components, and after screwing the panel to the baseboard lay the screening box in position. This has to be towards the left-hand side, and close to the panel, with a sharp scribe mark through the holes in the aluminium front to the ebonite panel, showing where holes have to be drilled to mount the condensers and rheostat. Before attempting to drill for the speaker jack, arrange the components on the audio side and let the jack lie

between the second transformer and the last valve. The various 3-16in. holes can be drilled around the outside, half an inch from the bottom, in the positions shown in the layout diagram. Making the Coils. BHrorn anything further can be done to ihe set, the colls have te

-- be made. These are solonoid coils ,with a primary wound over one end the two windings by two or three layers of empire cloth or similar insultating material. Commencing #in. from the end, wind on 85 turns of the 24 DCO wire on each of the three formers. Fasten the ends of the wire by a soldering lug and small nut and bolt. A strip of empire cloth one inch wide is now wound around one end of each coil, directly above the last turn, on which the primary winding has to be wound. Wind on two or three layers of the

empire cloth and fasten with a little secotine or celluloid cement. For the aerial coil wind on 30 turns tapped half-way of the 24 DCC wire. The two remaining coils wind on 80 turns centretapped of the 80 DCC wire. The centre tapping in each case can be a twist in i wire and after cleaning off the Insulation a length of wire soldiered on. The two ends of all the coils can be easily fixed with a length of cotton. Give the whole a thin coating of celluloid solution and allow to dry. Small brass brackets bent twice and fixed to the coil former by small nuts and bolts hold the whole coil quite rigid. Arrange these supports so that the coil stands about lin. from the bottom. All coils should be wound in the same direction.

Commercial coils of the type used for standard neutrodyne circuits may be used profitibly. The Assembly. Now, drilling through the metal bottom of the screening boxes, mount the various components by screwing through into the baseboard. Immediately behind the variable condensers arrange the valve sockets as close to the right hand partition. This leaves more room for the coil, which should not be too close to either of the

metal sides or bottom, A point to watch before fixing the valve sockets is to see that the contacts are not pushed down by the valve pins and touch the metal underneath, If this is likely to happen a small square of celluloid or thin ebonite will have to be fastened between the valve socket and the metal work. Many commercial varieties of neutralising condensers will be found to have a small fixing screw underneath which will need to be sunk in further to prevent a short to the metal screen. The variable condensers will fasten the screen to the ebonite panel. Out of two small strips. of ebonite make two terminal strips, one to hold three terminals and the other seven binding posts. Soldering lugs clamped under the small nuts is preferable to other methods of connecting to the various leads.

--- ---- — The strip holding three terminals has to be fixed directly behind the first Screening compartment, and the other strip runs along the back behind the audio end. Reference to the diagram will indicate the positions for the two audio transformers and valve sockets. In the diagram will be seen a small resistance marked R2, and the value of this depends upon the valves to be used. Anyone can work this out by the following formulae and an example is given :- vi_y2 . = Resistance in ohms. Where V1 equals voltage of battery. _ V2 equals voltage required for the valves. © equals total current taken by all the valves shown as amperes. Example :-- Vis 6 volt car battery. V2= 5.5 volt valves. © = 5X.25 amperes. (Five is number of valves.) 6-5.5 2 == -_-, 5X .25 1.25 =: 4 ohms. Therefore the value of resistance R1 will have to be 4 ohms in this particular case and will have to be capable of carrying 14 amperes. In the case of amperites these have been worked out as per table. The easiest way to do this is to obtain a 6-ohm resistance of fairly heavy construction, and use 4-6 or 2-8 of the total resistance. Dismantle the rheostat and unwind the wire that is not required, fixing a soldering lug at each end of the Jength of material on which the resistance is wound. Being light in weight, it is self-supporting, and will "stay put" when wired into the receiyer. Wiring the Receiver. K\VERYTHING is now ready to be wired up. Sundry crosses will be seen in the diagram. This indicates where a lead is fastened direct to the metal shield. Perhaps the easiest way to do this to aluminium is to fasten a soldering lug underneath a screwhead, and solder direct to this. Some builders may use variable condensers with metal end-plates, but even if this is the case, a wire must be connected between the condenser and coil, and run to the screen, and not depend upon the end-plate connection of the moving plates.

The radio frequency choke has not been mentioned previously, and if this component is to be made, details will be found in vol II, 28, or 250 turns of 380DCC wire on a 1-inch former, wound in sections, will prove quite suitable. However, numerous good commerciallymade chokes are available. JN SULATED sleeving is preferable for wiring up this set, as thinner insulation is liable to break down when in constant contact with the metal work. As will be seen, the earth terminal is fastened direct to a bolt in the screen, and the two aerial terminals to the small primary coil wound with the 24-guage D.C.C. wire. One to the centre tap, and the other to the end nearest the grid end of the secondary coil. In the case of the other two coils, the centre tap of the primary runs to B+ 90, and one terminal of the .5-mfd condenser in each compartment. The remaining wiring is easily followed from the diagram. Testing and Adjustment. OW comes the testing and adjustments after it has been wired up. Insert the five valves in their correct sockets. Valves having an impedance of between 6000 and 138,000 will be found best for the first four positions, and a power or semi-power valve for the final stage. Connect up the A battery, and make sure that all the valves light up when the speaker plug is inserted. Connect up the © battery, and try 224 volts B on all the B+ terminals. It will be evident at a glance if anything is wrong. Now, put on the full voltage, and try out the aerial. The chances are that loud howls will greet the builder. With a wooden or ebonite rod shaped as a screwdriver, adjust the two neutralising condensers. Make these adjustments with the receiver tuned to the station of the lowest wavelength it is desired to receive. Adjust until the receiver will not oscillate, wherever the dials are set. If difficulty is experienced to neutralise with the lid off, drill two holes in the lid directly over the tops of the néutralising condensers, and insert the insulated rod. The writer has never had the slightest trouble to balance this set, and he has built quite a few of them exactly as described in this article. When once built and adjusted, it is a trouble-free set. Tonal qualities are excellent, with a marked absence of background noises due to slightly oscillating R.F. stages.

PU CL Components Necessary for Shielded Five. °

Crnree Screening boxes, 9 x 5 x 6 ' (approx.). 3 Ebonite formers, 34in. x 2in. dia. 4Ib. 24 S.W.G. copper wire, D.C.C. 2b. 30 S.W.B. copper wire, D.C.C. 3 Variable condensers, .0005 m.f.d. 1 x .0025 fixed condenser. 1 x .001 fixed condenser. 2x .5 m.f.d, fixed condenser.

5 Valve sockets. 1 Rheostat 6-15 ohms. 2 Neutralising condensers. 1 R.F. choke. 2 Audio transformers. 12 Terminals. Panel, 2lin. x Vin. x 3-16in. 1 Single circuit filament jack. Connecting wire and sleeving, ete. Baseboard, 22in. x 10in, x din.