The receiver shown in Figs. 5-24, 5-26, and 5-27 represents close to the minimum requirements of a useful short-wave receiver. Under suitable conditions, it is capable of receiving signals from many foreign countries. It is a good receiver for the beginner, because it is easy to build and the components are not expensive.
With this receiver it is possible to hear amateur and commercial stations in the 2- to 20-Mc. range. This tuning range will enable the builder to listen to the two low-frequency Novice bands. Also, if one is interested in obtaining code practice, W1AW, the ARRL Hq. station, can be tuned in for its nightly code-practice sessions.
While the title indicates that the receiver has one tube, actually it uses two tubes in one envelope - envelope meaning the glass enclosure. The 6U8 is a triode-pentode, and in this receiver the pentode section is used as a regenerative detector and the triode as an audio amplifier.
Referring to Fig. 5-25, the antenna coil, L1, couples the signal to the detector tuned circuit L2C2C3. The capacitor, C2 is larger than C3 and is used as the "bandset" capacitor - once C2 is set for a particular frequency range, C3 is used as the "bandspread" tuning control. To facilitate using manufactured coils, the coil L2 is tapped to obtain a feedback or "tickler" winding. Regeneration in the detector is controlled by changing the screen voltage obtained at the potentiometer R1. An r.f. filter, using two capacitors and an r.f. choke, is placed in the plate circuit of the pentode detector to reduce r.f. appearing at the grid of the triode audio amplifier. Still further attenuation of r.f. at the grid is obtained through the use of a series resistor and a shunt capacitor right at the grid of the audio stage. The audio coupling choke, L3, is made from an interstage audio transformer with the two windings connected in series. A high-inductance choke could be used here, but the series-connected transformer is less expensive.
The headphones are connected directly in the plate circuit of the audio stage, and consequently the plate voltage appears at the terminals - you can get an electrical shock here if you aren't careful. Some receivers eliminate this hazard by feeding the plate through an audio choke and coupling to the headphones through a capacitor, but in the interest of saving a few dollars this protective feature was not included. Be sure to use "high-impedance" headphones with this receiver - the low-impedance headphones that have been available in surplus will not work well in this particular circuit.
The receiver is built on a 7 X 7 X 2-inch aluminum chassis, with the power supply mounted on a separate chassis. In order to minimize hum pickup and vibration from the power transformer, it is not advisable to mount the power supply on the same chassis as the receiver. An aluminum chassis is easy to work; a 1/8- and 1/4- inch drill, plus a small rat tail file and hack-saw blade are all the tools needed for the job, although two socket punches will save some work.
The first step is to mount the coil and tube sockets. They are spaced 2 inches from the sides at the center of the chassis. Ground lugs should be mounted under the nuts that hold the tube socket and also under the rear nut holding the coil socket. Next, the panel holes are drilled.
Looking at Fig. 5-24, front, the knob at the lower left is the regeneration control, lower center is the antenna trimmer, and the headphone tips are at the lower right. The know at the upper left is for the general-coverage capacitor, and the one at the right the band spread tuning. The dial shown in the photograph is the National type K.
After the holes are drilled in the panel, it is held in place against the chassis and the four holes along the bottom are used as a template for the chassis holes. A small right-angle bracket to hold the antenna-trimmer capacitor is made from a piece of aluminum. The hole in the bracket should be large enough to clear the rotor of the capacitor, since both the rotor and stator are insulated from the chassis. The trimmer is mounted tot he bracket by screws and the insulated nuts on the capacitor frame. The bracket, tie points, and audio choke L3 can now be mounted in place.
The two capacitors, C2 and C3, should then be installed on the panel. When the potentiometer R1 and the pin jacks are mounted in place, they will hold the panel to the chassis. Be sure to insulate the pin jacks from the panel and chassis with fiber washers. The through-shaft bushing is then measured and cut to size, making allowances for the insulated coupler.
If this is your first construction project, see the chapter on Construction Practices for tips on wiring and soldering before starting this job.
It is important that a separate ground lead be connected to the rotors of C2 and C3 and the lead brought below the chassis to a common grounding point at the tube socket. This will help make the receiver stable and reduce hand capacity.
There are a few leads coming from the interstage transformer: red, blue, black and two green. The red lead and green leads that are directly opposite each other are connected together. After the leads are soldered and taped, the end of the black lead is also taped. These leads are then rolled up and tucked in the corner of the chassis. The remaining blue and green leads then become those used for wiring the series-connected transformer into the circuit. One is connected to the junction of the 0.01 uF. disk capacitor and the 1 mH. r.f. choke and the other lead is connected to the B+ voltage terminal.
The Barker & Williamson coils are mounted on five-prong plugs, although only four of the contacts are used. The link mounted at one end of the coil is L1 and the coil proper is L2. To make the tickler tap, a short piece of hook-up wire approximately 3 inches long is soldered to the fifth prong on the plug. The piece of wire is then run through the middle turns of the coil and soldered to the tap point. For the 80 meter coil, the tap is connected to the 8th turn to make the tap connection. Be sure that none of the bent turns touches adjacent turns.
For maximum bandspread on 40 meters, it is necessary to remove nine turns from the 40-meter coil. The turns are taken from the end opposite the link end of the coil. The tickler tap is made on the 4th turn from the link end. In all three coils, the tap lead should be insulated where it passes through the coil turns.
The power-supply components can now be wired. There are two important points that beginners should keep in mind when wiring the supply. The first is that electrolytic capacitors should be wired with the leads marked with a minus sign, or negative, connected to the chassis. The plus sign, or positive, connects to the choke leads. Likewise, the selenium rectifier is marked with a plus sign, and this lead is connected to the choke lead. Four leads are brought out from the power supply to connect to the receiver; the two heater leads, the B+ lead, and the B- lead.
When the power supply is wired and the leads connected to the receiver, the unit is ready to test.
If you already have an antenna strung up, connect the end of it to Terminal 2 - the one connected to the rotor of C1. If you don't have an antenna, any wire, 20 to 40 feet long or longer, can be strung up. An outside antenna will perform better than one indoors, although you'll hear many signals with just a wire in the room.
Connect your headphones to the tip jacks and plug in the 80-meter coil. Plug the power cord into the 115-volt a.c. line and watch the 6U8 to see if the heater lights up. If it doesn't, turn off the power and check wiring from the power supply to the heater pins on the 6U8 socket.
The receiver will only take a minute to warm up. Turn the regeneration control and, at one point, you should hear a change in the characteristic of the noise. This is the point where the receiver starts to oscillate. Tune the general-coverage capacitor slowly and you should hear signals. Leave the capacitor set at or near one of the signals and then tune the band-spread capacitor. This capacitor gives a slower tuning rate, making it much easier to tune in signals.
With a signal tuned in, rotate the antenna-trimmer control and the signal should get louder at one point. If it doesn't, change the antenna to terminal number 1 and short terminals 2 and 3 together with a short piece of wire. Try the antenna trimmer again, and you should find that the signal will peak-up. The regeneration control setting may have to be changed to maintain oscillation.
Locating the amateur Novice bands is simple. Tune the receiver until you find an amateur phone station. The Novice band on both 80 and 40 meters is immediately below the phone bands. To tune lower in frequency than the phone bands, the band-spread capacitor is turned so that the plates mesh more.