Connecting Up the Parts.--Begin by connecting the leading-in wire of the aerial with the binding post end of the primary coil of the loose coupler as shown in the wiring diagram Fig. 48 and then connect the sliding contact with the water pipe or other ground. Connect the binding post end of the primary coil with one post of the variable condenser, connect the other post of this with one of the posts of the _.00025 mfd._ condenser and the other end of this with the grid of the detector tube; then around this condenser shunt the grid leak resistance.

[Ill.u.s.tration: Fig. 48.--Simple Regenerative Receiving Set. (With Loose Coupler Tuner.)]

Next connect the sliding contact of the primary coil with the other post of the variable condenser and from this lead a wire on over to one of the terminals of the filament of the vacuum tube; to the other terminal of the filament connect one of the posts of the rheostat and connect the other post to the - or negative electrode of the A battery and then connect the + or positive electrode of it to the other terminal of the filament.

Connect the + or positive electrode of the A battery with one post of the .001 mfd. fixed condenser and connect the other post of this to one of the ends of the secondary coil of the tuning coil and which is now known as the _tickler coil_; then connect the other end of the secondary, or tickler coil to the plate of the vacuum tube. In the wiring diagram the secondary, or tickler coil is shown above and in a line with the primary coil but this is only for the sake of making the connections clear; in reality the secondary, or tickler coil slides to and fro in the primary coil as shown and described in Chapter III.

Finally connect the _negative_, or zinc pole of the _B battery_ to one side of the fixed condenser, the _positive_, or carbon, pole to one of the terminals of the head phones and the other terminal of this to the other post of the fixed condenser when your regenerative set is complete.

An Efficient Regenerative Receiving Set. With Three Coil Loose Coupler.--To construct a really good regenerative set you must use a loose coupled tuner that has three coils, namely a _primary_, a _secondary_ and a _tickler coil_. A tuner of this kind is made like an ordinary loose coupled tuning coil but it has a _third_ coil as shown at A and B in Fig. 49. The middle coil, which is the _secondary_, is fixed to the base, and the large outside coil, which is the _primary_, is movable, that is it slides to and fro over the middle coil, while the small inside coil, which is the _tickler_, is also movable and can slide in or out of the middle _coil_. None of these coils is variable; all are wound to receive waves up to 360 meters in length when used with a variable condenser of _.001 mfd_. capacitance. In other words you slide the coils in and out to get the right amount of coupling and you tune by adjusting the variable condenser to get the exact wave length you want.

[Ill.u.s.tration: (A) Fig. 49.--Diagram of a Three Coil Coupler.]

[Ill.u.s.tration: (B) Fig. 49.--Three Coil Loose Coupler Tuner.]

With Compact Coils.--Compact coil tuners are formed of three fixed inductances wound in flat coils, and these are pivoted in a mounting so that the distance between them and, therefore, the coupling, can be varied, as shown at A in Fig. 50. These coils are wound up by the makers for various wave lengths ranging from a small one that will receive waves of any length up to 360 meters to a large one that has a maximum of 24,000 meters. For an amateur set get three of the smallest coils when you can not only hear amateur stations that send on a 200 meter wave but broadcasting stations that send on a 360 meter wave.

[Ill.u.s.tration: Fig. 50.--Honeycomb Inductance Coil.]

These three coils are mounted with panel plugs which latter fit into a stand, or mounting, so that the middle coil is fixed, that is, stationary, while the two outside coils can be swung to and fro like a door; this scheme permits small variations of coupling to be had between the coils and this can be done either by handles or by means of k.n.o.bs on a panel board. While I have suggested the use of the smallest size coils, you can get and use those wound for any wave length you want to receive and when those are connected with variometers and variable condensers, and with a proper aerial, you will have a highly efficient receptor that will work over all ranges of wave lengths. The smallest size coils cost about $1.50 apiece and the mounting costs about $6 or $7 each.

The A Battery Potentiometer.--This device is simply a resistance like the rheostat described in connection with the preceding vacuum tube receiving sets but it is wound to 200 or 300 ohms resistance as against 1-1/2 to 6 ohms of the rheostat. It is, however, used as well as the rheostat. With a vacuum tube detector, and especially with one having a gas-content, a potentiometer is very necessary as it is only by means of it that the potential of the plate of the detector can be accurately regulated. The result of proper regulation is that when the critical potential value is reached there is a marked increase in the loudness of the sounds that are emitted by the head phones.

As you will see from A in Fig. 51 it has three taps. The two taps which are connected with the ends of the resistance coil are shunted around the A battery and the third tap, which is attached to the movable contact arm, is connected with the B battery tap, see B, at which this battery gives 18 volts. Since the A battery gives 6 volts you can vary the potential of the plate from 18 to 24 volts. The potentiometer must never be shunted around the B battery or the latter will soon run down. A potentiometer costs a couple of dollars.

[Ill.u.s.tration: (A) Fig. 51.--The Use of the Potentiometer.]

The Parts and How to Connect Them Up.--For this regenerative set you will need: (1) a _honeycomb_ or other compact _three-coil tuner_, (2) two _variable_ (_.001_ and _.0005 mfd_.) _condensers_; (3) a _.00025 mfd. fixed condenser_; (4) a _1/2 to 2 megohm grid leak_; (5) a _tube detector_; (6) a _6 volt A battery_; (7) _a rheostat_; (8) a _potentiometer_; (9) an _18_ or _20 volt B battery_; (10) a _fixed condenser_ of _.001 mfd. fixed condenser_; and (11) a _pair of 2000 ohm head phones_.

To wire up the parts connect the leading-in wire of the aerial with the primary coil, which is the middle one of the tuner, and connect the other terminal with the ground. Connect the ends of the secondary coil, which is the middle one, with the posts of the variable condenser and connect one of the posts of the latter with one post of the fixed .00025 mfd. condenser and the other post of this with the grid; then shunt the grid leak around it. Next connect the other post of the variable condenser to the - or _negative_ electrode of the _A battery_; the + or _positive_ electrode of this to one terminal of the detector filament and the other end of the latter to the electrode of the A battery.

Now connect one end of the tickler coil with the detector plate and the other post to the fixed .001 mfd. condenser, then the other end of this to the positive or carbon pole of the B battery.

This done shunt the potentiometer around the A battery and run a wire from the movable contact of it (the potentiometer) over to the 18 volt tap, (see B, Fig. 51), of the B battery.

Finally, shunt the head phones and the .001 mfd. fixed condenser and you are ready to try out conclusions.

A Regenerative Audio Frequency Amplifier Receiving Set.--The use of amateur regenerative cascade audio frequency receiving sets is getting to be quite common. To get the greatest amplification possible with amplifying tubes you have to keep a negative potential on the grids.

You can, however, get very good results without any special charging arrangement by simply connecting one post of the rheostat with the negative terminal of the filament and connecting the _low potential_ end of the secondary of the tuning coil with the - or negative electrode of the A battery. This scheme will give the grids a negative bias of about 1 volt. You do not need to bother about these added factors that make for high efficiency until after you have got your receiving set in working order and understand all about it.

The Parts and How to Connect Them Up.--Exactly the same parts are needed for this set as the one described above, but in addition you will want: (1) two more _rheostats_; (2) _two_ more sets of B 22-1/2 _volt batteries_; (3) _two amplifier tubes_, and (4) _two audio frequency transformers_ as described in Chapter IX and pictured at A in Fig. 46.

To wire up the parts begin by connecting the leading-in wire to one end of the primary of the tuning coil and then connect the other end of the coil with the ground. A variable condenser of .001 mfd.

capacitance can be connected in the ground wire, as shown in Fig. 52, to good advantage although it is not absolutely needed. Now connect one end of the secondary coil to one post of a _.001 mfd._ variable condenser and the other end of the secondary to the other post of the condenser.

[Ill.u.s.tration: Fig. 52.--Regenerative Audio Frequency Amplifier Receiving Set.]

Next bring a lead (wire) from the first post of the variable condenser over to the post of the first fixed condenser and connect the other post of the latter with the grid of the detector tube. Shunt 1/2 to 2 megohm grid leak resistance around the fixed condenser and then connect the second post of the variable condenser to one terminal of the detector tube filament. Run this wire on over and connect it with the first post of the second rheostat, the second post of which is connected with one terminal of the filament of the first amplifying tube; then connect the first post of the rheostat with one end of the secondary coil of the first audio frequency transformer, and the other end of this coil with the grid of the first amplifier tube.

Connect the lead that runs from the second post of variable condenser to the first post of the third rheostat, the second post of which is connected with one terminal of the second amplifying tube; then connect the first post of the rheostat with one end of the secondary coil of the second audio frequency transformer and the other end of this coil with the grid of the second amplifier tube.

This done connect the - or negative electrode of the A battery with the second post of the variable condenser and connect the + or positive electrode with the free post of the first rheostat, the other post of which connects with the free terminal of the filament of the detector. From this lead tap off a wire and connect it to the free terminal of the filament of the first amplifier tube, and finally connect the end of the lead with the free terminal of the filament of the second amplifier tube.

Next shunt a potentiometer around the A battery and connect the third post, which connects with the sliding contact, to the negative or zinc pole of a B battery, then connect the positive or carbon pole of it to the negative or zinc pole of a second B battery and the positive or carbon pole of the latter with one end of the primary coil of the second audio frequency transformer and the other end of it to the plate of the first amplifying tube. Run the lead on over and connect it to one of the terminals of the second fixed condenser and the other terminal of this with the plate of the second amplifying tube. Then shunt the headphones around the condenser.

Finally connect one end of the tickler coil of the tuner with the plate of the detector tube and connect the other end of the tickler to one end of the primary coil of the first audio frequency transformer and the other end of it to the wire that connects the two B batteries together.

CHAPTER XI

SHORT WAVE REGENERATIVE RECEIVING SETS

A _short wave receiving set_ is one that will receive a range of wave lengths of from 150 to 600 meters while the distance over which the waves can be received as well as the intensity of the sounds reproduced by the headphones depends on: (1) whether it is a regenerative set and (2) whether it is provided with amplifying tubes.

High-grade regenerative sets designed especially for receiving amateur sending stations that must use a short wave length are built on the regenerative principle just like those described in the last chapter and further amplification can be had by the use of amplifier tubes as explained in Chapter IX, but the new feature of these sets is the use of the _variocoupler_ and one or more _variometers_. These tuning devices can be connected up in different ways and are very popular with amateurs at the present time.

Differing from the ordinary loose coupler the variometer has no movable contacts while the variometer is provided with taps so that you can connect it up for the wave length you want to receive. All you have to do is to tune the oscillation circuits to each other is to turn the _rotor_, which is the secondary coil, around in the _stator_, as the primary coil is called in order to get a very fine variation of the wave length. It is this construction that makes _sharp tuning_ with these sets possible, by which is meant that all wave lengths are tuned out except the one which the receiving set is tuned for.

A Short Wave Regenerative Receiver--With One Variometer and Three Variable Condensers.--This set also includes a variocoupler and a _grid coil_. The way that the parts are connected together makes it a simple and at the same time a very efficient regenerative receiver for short waves. While this set can be used without shielding the parts from each other the best results are had when shields are used.

The parts you need for this set include: (1) one _variocoupler_; (2) one _.001 microfarad variable condenser_; (3) one _.0005 microfarad variable condenser_; (4) one _.0007 microfarad variable condenser_; (5) _one 2 megohm grid leak_; (6) one _vacuum tube detector_; (7) one _6 volt A battery_; (8) one _6 ohm_, 1-1/2 _ampere rheostat_; (9) one _200 ohm potentiometer_; (10) one 22-1/2 _volt B battery_; (11) one _.001 microfarad fixed condenser_, (12) one pair of _2,000 ohm headphones_, and (13) a _variometer_.

The Variocoupler.--A variocoupler consists of a primary coil wound on the outside of a tube of insulating material and to certain turns of this taps are connected so that you can fix the wave length which your aerial system is to receive from the shortest wave; i.e., 150 meters on up by steps to the longest wave, i.e., 600 meters, which is the range of most amateur variocouplers that are sold in the open market.

This is the part of the variocoupler that is called the _stator_.

The secondary coil is wound on the section of a ball mounted on a shaft and this is swung in bearings on the stator so that it can turn in it. This part of the variocoupler is called the _rotor_ and is arranged so that it can be mounted on a panel and adjusted by means of a k.n.o.b or a dial. A diagram of a variocoupler is shown at A in Fig.

53, and the coupler itself at B. There are various makes and modifications of variocouplers on the market but all of them are about the same price which is $6.00 or $8.00.

[Ill.u.s.tration: Fig. 53.--How the Variocoupler is Made and Works.]

The Variometer.--This device is quite like the variocoupler, but with these differences: (1) the rotor turns in the stator, which is also the section of a ball, and (2) one end of the primary is connected with one end of the secondary coil. To be really efficient a variometer must have a small resistance and a large inductance as well as a small dielectric loss. To secure the first two of these factors the wire should be formed of a number of fine, pure copper wires each of which is insulated and the whole strand then covered with silk.

This kind of wire is the best that has yet been devised for the purpose and is sold under the trade name of _litzendraht_.

A new type of variometer has what is known as a _basket weave_, or _wavy wound_ stator and rotor. There is no wood, insulating compound or other dielectric materials in large enough quant.i.ties to absorb the weak currents that flow between them, hence weaker sounds can be heard when this kind of a variometer is used. With it you can tune sharply to waves under 200 meters in length and up to and including wave lengths of 360 meters. When amateur stations of small power are sending on these short waves this style of variometer keeps the electric oscillations at their greatest strength and, hence, the reproduced sounds will be of maximum intensity. A wiring diagram of a variometer is shown at A in Fig. 54 and a _basketball_ variometer is shown complete at B.

[Ill.u.s.tration: Fig. 54.--How the Variometer is Made and Works.]

Connecting Up the Parts.--To hook-up the set connect the leading-in wire to one end of the primary coil, or stator, of the variocoupler and solder a wire to one of the taps that gives the longest wave length you want to receive. Connect the other end of this wire with one post of a .001 microfarad variable condenser and connect the other post with the ground as shown in Fig. 55. Now connect one end of the secondary coil, or rotor, to one post of a .0007 mfd. variable condenser, the other post of this to one end of the grid coil and the other end of this with the remaining end of the rotor of the variocoupler.

[Ill.u.s.tration: Fig. 55.--Short Wave Regenerative Receiving Set (one Variometer and three Variable Condensers.)]

Next connect one post of the .0007 mfd. condenser with one of the terminals of the detector filament; then connect the other post of this condenser with one post of the .0005 mfd. variable condenser and the other post of this with the grid of the detector, then shunt the megohm grid leak around the latter condenser. This done connect the other terminal of the filament to one post of the rheostat, the other post of this to the - or negative electrode of the 6 volt A battery and the + or positive electrode of the latter to the other terminal of the filament.

Shunt the potentiometer around the A battery and connect the sliding contact with the - or zinc pole of the B battery and the + or carbon pole with one terminal of the headphone; connect the other terminal to one of the posts of the variometer and the other post of the variometer to the plate of the detector. Finally shunt a .001 mfd.

fixed condenser around the headphones. If you want to amplify the current with a vacuum tube amplifier connect in the terminals of the amplifier circuit shown at A in Figs. 44 or 45 at the point where they are connected with the secondary coil of the loose coupled tuning coil, in those diagrams with the binding posts of Fig. 55 where the phones are usually connected in.

Short Wave Regenerative Receiver. With Two Variometers and Two Variable Condensers.--This type of regenerative receptor is very popular with amateurs who are using high-grade short-wave sets. When you connect up this receptor you must keep the various parts well separated. Screw the variocoupler to the middle of the base board or panel, and secure the variometers on either side of it so that the distance between them will be 9 or 10 inches. By so placing them the coupling will be the same on both sides and besides you can shield them from each other easier.