The 'Sellens' Shortwave Set

Radio Record, Volume IV, Issue 40, 17 April 1931, Page 16

The "Sellens"

Shortwave Set

Four Valves with

h Screen Grid

By

F. W.

Sellens

| We publish here details of set

coveted by many short-

wave enthusiasts. It i is the one om ae a

used by Mr. F. W. Sellens,

_presiaent of tne short-wave Club, eo: a &

who compiles our weekly

10g. Many correspondents have

requested details of his

. elrenit.. There could be few occasion

better than this when

it might be described.

--| HH ‘Technical Editor has yi asked me for a. deseription of my set. Well, really,: that is a difficult task, for an experimenter rarely hag his Set quite the same from one week-end to another, Things are always rough. . One is continually changing components about, trying different things, and generally improving reception:° After he has done this:a ‘few times the set begins to look far from the original neat job it was intended to be. . And this is to some extent true about mine -but it enables me to get good results, and that is everything. — In the first place I do not. believe in cramping a set, but at the same time I strongly decry the use of long leads. They must run direct, . particularty ° those carrying high frequency current. Do not worry about the look of the set. It will certainly not look as. good as nicely-bent stiff wires, but there is a big difference when it comes to results. ~Run your wires direct; take them by the shortest path. Until you know just what you are doing,-do. not be too keen to depart from the layout given. Every competent ex- perimenter, has his*own ideas about this, but.they nearly all come down to the same fundamental rules. The whole cabinet is metal, and the’ screen grid valve is not only shielded from the rest of the set, but above the grid is another shield, reaching to the. top of the cabinct. . ; You will notice that I am using big’ coils. Now ‘some peopie-are going to say they are old-fashioned, but, al-

though they take a little time to make. and are perhaps more bulky than the. neater valve base ‘coil, ‘there is x0 doubt they are more efficient. . I. use a large number of coils-six-but this is to provide ample overlap so that no station need be brought-in with .the condenser. almost full in. To obtain the best results there should be as little. "capacity and as much inductance in the- circuit as possible. This’ means that big coils and small condensers are to be preferred. Now you know why I have so many coils. If I want one station in particular, I make a special coil for it. I have done this for G5SW, so that I can bring him in at about 10 on my dial. I suggest that double spaced condensers be used for the two tuners, and if .you wish to tune very finely, use the ball-bearing type with special 200-1 _ Vernier dials. You will find that the . detector stage is far more eritical than the radio, so if some’of your parts are better than others, put the good ones into that stage. . ANOTHER wrinkle that I have dis-. covered is that the detector filament voltage is very critical and; what is more, it works best a long way below its rated voltage. I find that 60 ohms should be used on the filament of the detector valve. Valves vary, and, what is more, they vary from

_ , time to time, and you will find that for _ Very best. results these two rheostats will have to be altered fairly often. The rheostat controlling the radio valve _.need be adjusted ‘only the | once. ’ ‘Tt then "stays put." | ' . I do not believe in the use of a filament switch. . A far better plan is to use the rheostat outside the set. When "a battery is chargedit ‘may be as high a8 6.5 volts and it gradually falls until it'might be as low as 5.75. I use a rheostat between this battery and the set’ and so maintain the current as ; near as possible to:6 ‘volts. ° When the. -set-is finished with I merely: turn out the rheostat. : By: using a switch the current is cut off sharply and the pro-. ‘cess'is analagous with stopping a car by:-jumping onthe. brakes when it is travelling at top speed. Far better _ to turn off the gas and apply the -brakes gently. If you look at the théoretical circuit you will see very many by-pass condensers. These have been put in because they have been really necessary, particularly because I use an eliminator -an ordinary commercial job-and this accounts for the apparent surfeit of condensers. In the detector lead from my eliminator I use an ordinary Ferranti B3 choke and a 4 mfds. by-pass condenser. I can listen-in on phones and get -only the slightest hiss, which is no greater than R1. In tinkering around the set I found that a .006 condenser between the grid of the first audio and the plate of the second audio would sometimes be beneficial and at other times would reduce Signal. strength and make a quieter background, so I fixed it up with a switch. _I have shown this on the diagrams. OF the sub-panel wiring there is very little, and I have thought it unnecessary to prepare a separate diagram. ‘ The filament wiring and the leads to the batteries or eliminator are all that pass under the base. The filament wires have been indicated by dotted lines, while the other battery

connections are. shown disappearing into holes. From there they go by the shortest route to the power supply. Attached to the "B" leads are the one microfarad condensers. These are joined at convenient places by connecting the battery wire on one side and the other side to the metal sub-panel. Condensers have been shown in all the leads, but if batteries are used most of these are unnecessary. The detector one is the most important, whether balteries or eliminator are used.

It will be noticed that the metal porons are connected to A+ and to earth. nly one side of the moving condensers 8 shown connected to anything, for he moving plates automatically conect with the screen, . Note that the etector condenser has two connecions, each of whom is from the stator lates, The jacking arrangement should not resent any difficulty. The top jack is erely a two-spring affair insulated rom the panel, and one side connects

with P of the second audio transformer and the other to B+ of that transformer. Do not use a complicated arrangement to-eut out the primary of this transformer when using the first stage.: It complicates the wiring too much and is liable to introduce unwanted oscillation. Use a filament break jack for the second stage and arrange it so that when the plug is in position the filament circuit to the last valve is made. One of the diagrams shows this fairly clearly. It is always easier to tune with the ‘phones in the first stage and then if the station is coming in strongly enough connect up the speaker to the last stage. There is usually .a little too much background when four valves are used on the telephones. IN output filter out of the set is almost a necessity. Arrange this in some. convenient place and connect the output side to a ’phone jack. The input side is connected to a plug, which can be connected with whatever ‘jack is to be used. Hither "phones or speaker can be connected with the output side of the filter, but of course the plug connected with the filter must be taken to the appropriate jack. It is perhaps unnecessary to add that the aerial terminal is well insulated from the sub-panel. It is on a piece of ebonite with a hole a little larger thau its shank, and thus cannot connect with the frame. The earth terminal is taken direct to the cabinet The Coils. [TF you can buy the formers for these well and good; the job of winding them will be simple. If they have to be made, the job is not a difficult one, but will need care, for they will best be wound on celluloid. I shall not pause here to describe in detail how the coils are made. That was done fairly fully in the 1930 "Radio Guide," but as I understand this is out of print, I have asked the Technical Editor if he would reprint the portion that is of interest. OIL formers of various kinds | are offered for sale, and many convenient forms can be purchased, but there are still many constructors who prefer to construct their own. coils, and to such there is no material that makes a greater appeal than does cel-

luloid. It is easily worked and joined, and looks neat when finished, and . the amount of labour involved is small. Many constructors are already familiar with this type of coil construction, but for the benefit of many newcomers to the constructional realm we give a resume of the important points of the process. ‘One advantage of this material is that it may be procured in any town possessing a motor-trimming establishment, and its cost is small. Roughly speaking, two thicknesses are usually obtainable, one being heavy and the other light, the latter being most suitable for the construction, of cylindrical formers. If the sheet celluloid is too heavy, it will go out of shape at the joint, Where flat pieces are required, the heavy sheet is more suitable, as it gives strength. Celluloid can be cut with scissors, but the usual method is to scratch the surface heavily with a sharp steel point, when the sheet may be bent smartly, and will separate at the scratch. A pair of punching pliers is very handy for making round holes for screws, or for cutting out special shaped holes or slots, cutting away the celluloid little by little with the punch . The best solvent to use is a mixture of two parts of acetone to one part of amyl acetate. The chemist will put these both into one. bottle, and the solution is there ready for use. To make‘ celluloid cement, waste celluloid is cut into chips, placed in a small bottle, and covered with the liquid to twice the height of the chips. With an occasional stir, the celluloid will be dissolved in about an hour, and should be of creamy consistence for use-running off the stick which is used for application. As the solution evaporates quickly, keep the bottle wellcorked when not in use, and keep away from an exposed flame. The solution, if applied thinly and liberally, sets in a few minutes. This mixture, if made extra thin, is suitable for "doping" linen-diaphragm speakers.

Round bottles are the handiest arttcle upon which to maye cylindrival formers. All that is necessary is to select a bottle just under the required diameter, cut a piece of celluloid the length of the cylinder in one direction, and the opposite dimension just sufficient to-go round the bottle and overlap not more than 3-8 inch. Both surfaces that overlap are covered with a ‘thin layer of cement and then drawn round. the bottle, overlapped, and pressed together. The whole must now be. covered with many turns of twine wrapped tightly round at random, so that.turms are an average of 1-8in, apart, keeping close to the ends particularly. When overlapping the join, a strip of thin paper should be (Concluded on page 31.)

~ \Sellens" Short-wave Set ms (Continued from page 17.)

inserted. between the joln and the Dot tle as a precaution, against sticking. It is well to. give a cylinder not less than two hours to set, owing to the tendency to:spring- apart. Any winding of wire should be put on with the bottle in place, and if the — wire is ‘cotton-covered, may be ‘doped" all ayer with thin solution, which keeps out damp and helps to keep the coll'a good ‘shape. The first and last turns of enamelled wire may be held in place with solution, or it may be applied all over, ° The ‘constructor may desire to add an end to the cylinder so that it will stand upright, secured by a screw through a‘hole in the centre. In this case € square piece of célluloid, preferably heavy, is cut a little larger than the diameter of the cylinder. A screw-

= g LOS eS Os: een aan | hole is then made in the centre. Now, . when the twine has been placed upon the ‘cylinder to. held it, place. the cylinder on the square. of celluloid, . haying first applied cement.to. the lower edge, afterwards. putting cement all round the joint.and-leave to, set, after which. the end may be cut round with scissors close up to the cylinder. Weaow have a former looking like Fig. 3}, and ready for winding. When an-end has to be put on the cylinder,