I have transcribed the text and pictures as accurately as possible, and I must stress that I can take NO credit for any part of the design. I have built both the receiver and transmitter and they are very impressive performers.
A receiver counterpart to the small one-watt transmitter described in the March issue. By T. W. Dresser.
In a recent issue of Practical Wireless the writer described the "Cigar Box Transmitter," an extremely small 1 watt transmitter suitable for the amateur without mains electrical supplies or who is so restricted for space that anything in the nature of a normal sized shack and transmitter is entirely out of the question. In the present article it is proposed to describe the receiver counterpart to that transmitter, an equally small unit which, despite its small size and simple circuit arrangement, will nevertheless put up quite a good show.
To begin with, those who think only in terms of double conversion, crystal filters, phasing controls and the other more elaborate gagets of the high-priced commercial receivers, will not be interested in this receiver. None of those adornments can be built into a TRF amplifier, leaky-grid detector, with reaction and audio amplifier. All the valves are of the same type, 1T4's, just as are those in the transmitter. The excellent performance arises principally from good layout, adequate screening and the very smooth regeneration provided by potentiometer control on the screen grid of the detector. It has been called a cigar box receiver, and can easily be built into such a box, as was the original, but there is no reason why it should not be assembled in a metal case or on a small metal chassis, and, in some cases, this may be an advantage in that it helps to keep down hand-capacity effects. These were not noticeable in the prototype, as the cigar box was completely lined with aluminium foil, which made a most effective screen.
The form of construction used at the RF end may be rather unusual but it has many advantages, noticeably in keeping grid and anode leads down to a minimum length and in isolating the RF stage from the detector as far as unwanted interaction was concerned. Nor is it difficult to build as the screen can be removed from the box, the RF valveholder and the detector grid coil holder fitted and wired to some extent before it is permanently fastened into position. This greatly facilitates construction.
As originally built, the receiver covered the five main amateur bands 3.5 7, 14, 20 and 30 Mc/s, using a ganged 15pf funing condenser. With some small changes it can be made to cover 3.5-30 Mc/s continuously and the coil data and all necessary information is given for both coverages.
The theoretical diagram is shown above and it will be noted that the reaction is controlled by varying the screen-grid voltage, a method which has a lot in its favour in that it does not affect the tuning to any noticeable degree. The detector anode load is a high-inductance LF choke of some hundreds of Henries and that used by the writer was a pre-war component. It is questionable whether such a choke can be obtained today, but the primary winding of an ordinary inter-valve transformer will serve just as well (I used a 28v PCB relay - Harry). In that case, the secondary can be left disconnected or placed in series with the primary, but care must be taken to see that it is truly in series and not connected in such a way that it reduces the inductance of the primary, which can happen. As with its companion transmitter, all components should be as small as possible consistent with good quality.
The drawing above shows the internal lay-out and front panel of the receiver. The cigar box is lined with thin completely flat and unwrinkled aluminium foil, pressed well down at all corners and fastened round the edge of the box with drawing pins or strips of aluminium screwed down. The screens are then cut and bent from sheet aluminium as shown in Fig. 2 (CIGAR-TM.GIF - Harry), and the RF valveholder and detector grid coil-holder mounted as shown. The screen can then be screwed inside the box with very small brass wood screws of no more than 3/16in. or 1/4in. in length. The remainder of the construction and the wiring are quite straightforward and if the diagrams are followed carefully, there is little likelihood of error. As with any other short-wave receiver all wiring should be as short as possible, and a common earthing point should be used for each stage. Coil formers should be of good quality material, such as polystyrene, and the tuning gang (or coupled condensers) should preferably have ceramic insulation.
The only difference between the amateur band version, and that for 3.5-30 Mc/s continuous coverage are in the size of the coil formers and the sockets for them, and in the capacity of the tuning condensers. In the first mentioned the coil formers are Maxi-Q dust cored 5-pin types, with holders to suit, and the tuning condenser is a 2-gang 15pf variable or two separate 15pf coupled together. In the continuous coverage version the tuning condenser is a two-gang 100 pf variable, and the coil formers are standard 1-1/4in. 5-pintypes with sockets to match. The change in the mechanical layout is so small that it needs no detailed description.
As the interior of the cigar box is completely covered with aluminium fail, it is advisable to keep a close eye on connections and wiring generally in order that they may not earth to the foil. Where there is a danger of this a small piece of cardboard or fibre under valveholders and at strategic points will eliminate the danger. As with all TRF receivers, a good aerial is half the battle. If the receiver is to be used with the cigar box transmitter the same aerial will serve both; a simple form of switching will change over the batteries and the aerial from transmitter to receiver simultaneously. If the receiver is to be used alone, an aerial as high and as long as possible should be erected.
In conclusion, the quiet background and signal getting qualities of this little set will prove a pleasant surprise to those accustomed to elaborate AC types, and, at the same time, its cheapness is such that even the beginner need not remain without a short-wave receiver if he has the pound or two necessary to build it.
Coil data for 3.5 to 30 Mc/s continuous coverage
Range 1, L1=6turns, L2/L3=25 turns, L3tap/L4= 4 turns
Range 2, L1=5turns, L2/L3=14 turns, L3tap/L4= 3 turns
Range 3, L1=3turns, L2/L3= 6 turns, L3tap/L4=2.5turns
Coil data for amateur bands
3.5-4.0MHz, L1=7turns, L2/L3=60 turns, L3tap/L4=10 turns
6.9-7.4MHz, L1=5turns, L2/L3=30 turns, L3tap/L4= 5 turns
14-14.5MHz, L1=3turns, L2/L3=20 turns, L3tap/L4=3.5turns
20.5-22MHz, L1=3turns, L2/L3=13 turns, L3tap/L4= 3 turns
27 - 30MHz, L1=2turns, L2/L3=10 turns, L3tap/L4= 3 turns
Have fun, de HARRY, Lunda, Sweden.