The Band-Pass Four

Radio Record, Volume III, Issue 42, 2 May 1930, Page 29

The Band-Pass Four

Utilises Band-Pass Filter and Sereen-Grid Valve —

HE batid-pass filter citeuit is not réally anything new,- for it has beén in use abroad for some time, and is consequenti ly embodied in many factorybuilt sets in use in New Zealand. There has been‘a need for a. circuit that is selective yet does not lack in quality because of this, and so the band-pass filter has been designed to meet this need.. We will not enter here into the theorythat. would take an article of its own,

but to the construction of a receiver embodying such a unit. The circuit, "The Screen Grid Four," is a simplé one. It has been con"sAstructed by a regular reader, who states that it-is working excellently. fe.) Its construction may be taken on by anyone who has little if any previous ‘expérience in construction. First take the accompanying list to your dealer and obtain the necessary components. In using components other than those specified the experimentér is safe but the novice decidedly unsafe. For those commencing this, their first set, follow the componénts set out, or if your dealer has not one ’ exactly the same ask him to substitute. The Panel. HH first job is to drill the panel. Lay it flat on a smooth surface (a sheet or two of newspaper on a firm bench) glazed side down, and mark out as follows. The condensers, small holes are three ase ,- > --________ __, ___ _--_-_-

inches from the top of the panel (or higher according to the type of dial to be used, but be careful that. the radio condenser will fit in the screen) and at these intervals from the end: 4in., 12in., 20in. . The rheostat, switch and the jack,

4}in. from the top and at these intervals from the end: The switch 8in., the rheostat 15}in., the jack 23in. Secure the base board to the panel by raised headed screws or by angle brackets and mount the components on the panel as shown in the diagram, The condensers open upwards and the tandem condenser (two condensers on the same spindle) at the end near the jack. Making the Coils. [THE coils are the next task. The band-pass filter is composed of two

separate solonoid coils wound on 2in. formers. The first (primary or lefthand coil) is composed of: For a .0005 condenser 70 turns of 28 D.C.C. or D.S.C. s.w.g. wire and for a .00035, 85 turns of the same wire. The righthand coil or secondary is really the more. elaborate. Starting at the. top, the réaction coil is wound on.. This will vary with the value of the reaction cond., viz., .0005 cond. 20 turns: .00085 cond. 25 turns; .0001 cond. 40 turns; all of 30 D.C.C. or D.S.C. 8.Ww.g. wire. For the .0001 cond. it will be found advisable to wind the 40 turns in two layers to save space and keep the reaction winding as small as possible. After the winding, leave tin. space and wind on the secondary winding exactly as the primary.’ In making the coils it will be found advisable to bring the ends of the various windings to nuts and bolts as shown in the diagram. , When putting the coils together they should be arranged so that the bottom turn of the reaction winding comes opposite the top turn of the primary winding. The coils are all wound in the same direction, that is, either clockwise or counter-clockwise. Looking down on the coils, and starting from the top, the wire is wound toward the constructor. . The radio frequency coil is made by winding one similar to the primary of the band-pass filter, but with two tappings, one at the 15th turn and one at the 80th, these tappings are brought out to two small terminals, so that a flex lead from the aerial can be clipped on to one or the other. Now es

fasten the. components to thé baseboard. This will not present great ditliculty, but it is impossible to give. definite measurements as the sizes of ditferent makes are not the same. | The Screening Box. __ ‘HIS can quite converiiently be .the standard "Radio Record" screening box, obtainable from most dealers, Those who. have the facilities:-to make one néed not adhere to the standard size, but’ might utilise the one shown in the diagram Tin, x 10in. x 6in. The amateur is advised not to attorapt to make a screen. . It will be found the: easiest way of mounting this item is to remové the first condenser from the panel and arrange the front of the screen until the spindle hole of the condénser is in the

All windings in same direction looking down on coils (anti-clockwise). _- —

horizontal centre, then place the front on the bench and drill it. Two large (1-8 or 3-16-inch) holes are to be drilled in the back to allow the aerial and the screen battery cable to enter. These will be very low, about sin, from the bottom of the screen. Half-way along the other side and half-way up another such "hole is drilled. -In mounting the components, small »holes will have to be drilled in the base to allow the components to be bolted to the screen,

General Notes. Little is to be gained by giving a point to. point wiring descriptio:, This diagram shows the connection clearly, joined wires being indicated by a square mark and under baseboard wiring by a dotted line. Use tinned wire

like "Glazite," which is already covered. . Square busbar is difficult to work, Solder all connections, Connecting up the batteries is not a difficult task, and the voltages are shown in the theoretical diagram. -If a power valve is. used in the last stage, it is advisable to incorporate an output filter choke. This consists of a low frequency: choke between the speaker terminals and a one or a half microfarad condenser between a _ speaker, terminal and the speaker itself. This unit can be housed in the set if there

is room. A one to one transformer can also be employed with a horn, cone or inductor dynamic speaker. A moying coil requires a step down, usually 25 to 1. Because the screen grid valve is utilised the’ set will not need neutralising, and if the specifications are followed, it will oscillate easily. All the BX leads are shown bypassed with condensérs. They are preferable when expense will permit. The connections on the B.P. filter are marked A. B. C. D. E. and F. This was the only way which they worked satisfactorily in the original. In the wiring layout a fuse has been placed in the screen lead. This is done for a good reason, as our contributor burnt out a screen grid valve through trying to do without, Hspecially if the wire to the plate of the 8S.G. valve is long, one is very necessary. A volume control consisting of a 0/500,000 ohm potentiometer should be -=

connected between aerial and the screen. In carrying connections through the; screen slip a short length oft insulated ‘Steering or insulated tape over the existing wiring. Tandem condensers are out of date, though they function well in this circuit. Separate dials may, of -course, be used if the layout is followed. All three condensers may be gauged if a balancing condenser (shown) is used. It is hoped to publish the A.C, version of the circuit shortly.

The R.F. Choke may be purchased or made, according to directions previously published in the "Radio Record."

Primary .0005 70 turns 28 d.e.c. and Secondary .00085 85 turns 28 d.c.c. Reaction.0005 20 turns 30 d.c.c .00085 25 turns 80 4d.c.c. .0001 40 turns (2 layers) 30 d.c.c.

TABLE 1.