Next connect the grid of the vacuum tube to one of the posts of the condenser and then connect the plate of the tube to the _carbon terminal_ of the B or dry cell battery which is the + or _positive pole_ and connect the _zinc terminal_ of the - or _negative_ pole to the binding post _a_, connect the post _b_ to the other side of the variable condenser and then connect the terminals of the head phones to the binding posts _a_ and _b_. Whatever you do be careful not to get the plate connections of the battery reversed.

Now connect one of the posts of the rheostat to one terminal of the filament and the other terminal of the filament to the - or _negative_ terminal of the A or storage battery and the + or _positive_ terminal of the A or storage battery to the other post of the rheostat. Finally connect the + or positive terminal of the A or storage battery with the wire that runs from the head phones to the variable condenser, all of which is shown in the wiring diagram at B in Fig. 41.

Adjusting the Vacuum Tube Detector Receiving Set.--A vacuum tube detector is tuned exactly in the same way as the _Crystal Detector Set No. 2_ described in Chapter III, in-so-far as the tuning coil and variable condenser are concerned. The sensitivity of the vacuum tube detector receiving set and, hence, the distance over which signals and other sounds can be heard depends very largely on the sensitivity of the vacuum tube itself and this in turn depends on: (1) the right amount of heat developed by the filament, or _filament brilliancy_ as it is called, (2) the right amount of voltage applied to the plate, and (3) the extent to which the tube is exhausted where this kind of a tube is used.

To vary the current flowing from the A or storage battery through the filament so that it will be heated to the right degree you adjust the rheostat while you are listening in to the signals or other sounds. By carefully adjusting the rheostat you can easily find the point at which it makes the tube the most sensitive. A rheostat is also useful in that it keeps the filament from burning out when the current from the battery first flows through it. You can very often increase the sensitiveness of a vacuum tube after you have used it for a while by recharging the A or storage battery.

The degree to which a vacuum tube has been exhausted has a very p.r.o.nounced effect on its sensitivity. The longer the tube is used the lower its vacuum gets and generally the less sensitive it becomes.

When this takes place (and you can only guess at it) you can very often make it more sensitive by warming it over the flame of a candle.

Vacuum tubes having a gas content (in which case they are, of course, no longer vacuum tubes in the strict sense) make better detectors than tubes from which the air has been exhausted and which are sealed off in this evacuated condition because their sensitiveness is not dependent on the degree of vacuum as in the latter tubes. Moreover, a tube that is completely exhausted costs more than one that is filled with gas.

CHAPTER IX

VACUUM TUBE AMPLIFIER RECEIVING SETS

The reason a vacuum tube detector is more sensitive than a crystal detector is because while the latter merely _rectifies_ the oscillating current that surges in the receiving circuits, the former acts as an _amplifier_ at the same time. The vacuum tube can be used as a separate amplifier in connection with either: (1) a _crystal detector_ or (2) a _vacuum tube detector_, and (_a_) it will amplify either the _radio frequency currents_, that is the high frequency oscillating currents which are set up in the oscillation circuits or (_b_) it will amplify the _audio frequency currents_, that is, the _low frequency alternating_ currents that flow through the head phone circuit.

To use the amplified radio frequency oscillating currents or amplified audio frequency alternating currents that are set up by an amplifier tube either a high resistance, called a _grid leak_, or an _amplifying transformer_, with or without an iron core, must be connected with the plate circuit of the first amplifier tube and the grid circuit of the next amplifier tube or detector tube, or with the wire point of a crystal detector. Where two or more amplifier tubes are coupled together in this way the scheme is known as _cascade amplification._

Where either a _radio frequency transformer_, that is one without the iron core, or an _audio frequency transformer_, that is one with the iron core, is used to couple the amplifier tube circuits together better results are obtained than where a high resistance grid leak is used, but the amplifying tubes have to be more carefully shielded from each other or they will react and set up a _howling_ noise in the head phones. On the other hand grid leaks cost less but they are more troublesome to use as you have to find out for yourself the exact resistance value they must have and this you can do only by testing them out.

A Grid Leak Amplifier Receiving Set. With Crystal Detector.--The apparatus you need for this set includes: (1) a _loose coupled tuning coil_, (2) a _variable condenser_, (3) _two fixed condensers_, (4) a _crystal detector_, or better a _vacuum tube detector_, (5) an A or _6 volt storage battery_, (6) a _rheostat_, (7) a B or 22-1/2 _volt dry cell battery_, (8) a fixed resistance unit, or _leak grid_ as it is called, and (9) a pair of _head-phones_. The tuning coil, variable condenser, fixed condensers, crystal detectors and head-phones are exactly the same as those described in _Set No. 2_ in Chapter III.

The A and B batteries are exactly the same as those described in Chapter VIII. The _vacuum tube amplifier_ and the _grid leak_ are the only new pieces of apparatus you need and not described before.

The Vacuum Tube Amplifier.--This consists of a three electrode vacuum tube exactly like the vacuum tube detector described in Chapter VIII and pictured in Fig. 38, except that instead of being filled with a non-combustible gas it is evacuated, that is, the air has been completely pumped out of it. The gas filled tube, however, can be used as an amplifier and either kind of tube can be used for either radio frequency or audio frequency amplification, though with the exhausted tube it is easier to obtain the right plate and filament voltages for good working.

The Fixed Resistance Unit, or Grid Leak.--Grid leaks are made in different ways but all of them have an enormously high resistance.

One way of making them consists of depositing a thin film of gold on a sheet of mica and placing another sheet of mica on top to protect it the whole being enclosed in a gla.s.s tube as shown at A in Fig. 42.

These grid leaks are made in units of from 50,000 ohms (.05 megohm) to 5,000,000 ohms (5 megohms) and cost from $1 to $2.

[Ill.u.s.tration: Fig. 42.--Grid Leaks and How to Connect Them up.]

As the _value_ of the grid leak you will need depends very largely upon the construction of the different parts of your receiving set and on the kind of aerial wire system you use with it you will have to try out various resistances until you hit the right one. The resistance that will give the best results, however, lies somewhere between 500,000 ohms (1/2 a megohm) and 3,000,000 ohms (3 megohms) and the only way for you to find this out is to buy 1/2, 1 and 2 megohm grid leak resistances and connect them up in different ways, as shown at B, until you find the right value.

a.s.sembling the Parts for a Crystal Detector Set.--Begin by laying the various parts out on a base or a panel with the loose coupled tuning coil on the left hand side, but with the adjustable switch of the secondary coil on the right hand end or in front according to the way it is made. Then place the variable condenser, the rheostat, the crystal detector and the binding posts for the head phones in front of and in a line with each other. Set the vacuum tube amplifier back of the rheostat and the A and B batteries back of the parts or in any other place that may be convenient. The fixed condensers and the grid leak can be placed anywhere so that it will be easy to connect them in and you are ready to wire up the set.

Connecting Up the Parts for a Crystal Detector.--First connect the sliding contact of the primary of the tuning coil to the leading-in wire and one of the end wires of the primary to the water pipe, as shown in Fig. 43. Now connect the adjustable arm that makes contact with one end of the secondary of the tuning coil to one of the posts of the variable condenser; then connect the other post of the latter with a post of the fixed condenser and the other post of this with the grid of the amplifying tube.

[Ill.u.s.tration: Fig. 43.--Crystal Detector Receiving Set with Vacuum Tube Amplifier (Resistance Coupled).]

Connect the first post of the variable condenser to the + or _positive electrode_ of the A battery and its - or _negative electrode_ with the rotating contact arm of the rheostat. Next connect one end of the resistance coil of the rheostat to one of the posts of the amplifier tube that leads to the filament and the other filament post to the + or _positive electrode_ of the A battery. This done connect the _negative_, that is, the _zinc pole_ of the B battery to the positive electrode of the A battery and connect the _positive_, or _carbon pole_ of the former with one end of the grid leak and connect the other end of this to the plate of the amplifier tube.

To the end of the grid leak connected with the plate of the amplifier tube connect the metal point of your crystal detector, the crystal of the latter with one post of the head phones and the other post of them with the other end of the grid leak and, finally, connect a fixed condenser in _parallel_ with--that is across the ends of the grid leak, all of which is shown in the wiring diagram in Fig. 43.

A Grid Leak Amplifying Receiving Set With Vacuum Tube Detector.--A better amplifying receiving set can be made than the one just described by using a vacuum tube detector instead of the crystal detector. This set is built up exactly like the crystal detector described above and shown in Fig. 43 up to and including the grid leak resistance, but shunted across the latter is a vacuum tube detector, which is made and wired up precisely like the one shown at A in Fig.

41 in the chapter ahead of this one. The way a grid leak and vacuum tube detector with a one-step amplifier are connected up is shown at A in Fig. 44. Where you have a vacuum tube detector and one or more amplifying tubes connected up, or in _cascade_ as it is called, you can use an A, or storage battery of 6 volts for all of them as shown at B in Fig. 44, but for every vacuum tube you use you must have a B or 22-1/2 volt dry battery to charge the plate with.

[Ill.u.s.tration: (A) Fig. 44--Vacuum Tube Detector Set with One Step Amplifier (Resistance Coupled).]

[Ill.u.s.tration: (B) Fig. 44.--Wiring Diagram for Using One A or Storage Battery with an Amplifier and a Detector Tube.]

A Radio Frequency Transformer Amplifying Receiving Set.--Instead of using a grid leak resistance to couple up the amplifier and detector tube circuits you can use a _radio frequency transformer_, that is, a transformer made like a loose coupled tuning coil, and without an iron core, as shown in the wiring diagram at A in Fig. 45. In this set, which gives better results than where a grid leak is used, the amplifier tube is placed in the first oscillation circuit and the detector tube in the second circuit.

[Ill.u.s.tration: (A) Fig. 45.--Wiring Diagram for a Radio Frequency Transformer Amplifying Receiving Set.]

[Ill.u.s.tration: (B) Fig. 45.--Radio Frequency Transformer.]

Since the radio frequency transformer has no iron core the high frequency, or _radio frequency_ oscillating currents, as they are called, surge through it and are not changed into low frequency, or _audio frequency_ pulsating currents, until they flow through the detector. Since the diagram shows only one amplifier and one radio frequency transformer, it is consequently a _one step amplifier_; however, two, three or more, amplifying tubes can be connected up by means of an equal number of radio frequency transformers when you will get wonderful results. Where a six step amplifier, that is, where six amplifying tubes are connected together, or in _cascade_, the first three are usually coupled up with radio frequency transformers and the last three with audio frequency transformers. A radio frequency transformer is shown at B and costs $6 to $7.

An Audio Frequency Transformer Amplifying Receiving Set.--Where audio frequency transformers are used for stepping up the voltage of the current of the detector and amplifier tubes, the radio frequency current does not get into the plate circuit of the detector at all for the reason that the iron core of the transformer chokes them off, hence, the succeeding amplifiers operate at audio frequencies. An audio frequency transformer is shown at A in Fig. 46 and a wiring diagram showing how the tubes are connected in _cascade_ with the transformers is shown at B; it is therefore a two-step audio frequency receiving set.

[Ill.u.s.tration: (A) Fig. 46.--Audio Frequency Transformer.]

[Ill.u.s.tration: (B) Fig. 46--Wiring Diagram for an Audio Frequency Transformer Amplifying Receiving Set. (With Vacuum Tube Detector and Two Step Amplifier Tubes.)]

A Six Step Amplifier Receiving Set With a Loop Aerial.--By using a receiving set having a three step radio frequency and a three step audio frequency, that is, a set in which there are coupled three amplifying tubes with radio frequency transformers and three amplifying tubes with audio frequency transformers as described under the caption _A Radio Frequency Transformer Receiving Set_, you can use a _loop aerial_ in your room thus getting around the difficulties--if such there be--in erecting an out-door aerial. You can easily make a loop aerial by winding 10 turns of _No. 14_ or _16_ copper wire about 1/16 inch apart on a wooden frame two feet on the side as shown in Fig. 47. With this six step amplifier set and loop aerial you can receive wave lengths of 150 to 600 meters from various high power stations which are at considerable distances away.

[Ill.u.s.tration: (A) Fig. 47.--Six Step Amplifier with Loop Aerial.]

[Ill.u.s.tration: (B) Fig. 47.--Efficient Regenerative Receiving Set.

(With Three Coil Loose Coupler Tuner.)]

How to Prevent Howling.--Where radio frequency or audio frequency amplifiers are used to couple your amplifier tubes in cascade you must take particular pains to shield them from one another in order to prevent the _feed back_ of the currents through them, which makes the head phones or loud speaker _howl_. To shield them from each other the tubes should be enclosed in metal boxes and placed at least 6 inches apart while the transformers should be set so that their cores are at right angles to each other and these also should be not less than six inches apart.

CHAPTER X

REGENERATIVE AMPLIFICATION RECEIVING SETS

While a vacuum tube detector has an amplifying action of its own, and this accounts for its great sensitiveness, its amplifying action can be further increased to an enormous extent by making the radio frequency currents that are set up in the oscillation circuits react on the detector.

Such currents are called _feed-back_ or _regenerative_ currents and when circuits are so arranged as to cause the currents to flow back through the detector tube the amplification keeps on increasing until the capacity of the tube itself is reached. It is like using steam over and over again in a steam turbine until there is no more energy left in it. A system of circuits which will cause this regenerative action to take place is known as the _Armstrong circuits_ and is so called after the young man who discovered it.

Since the regenerative action of the radio frequency currents is produced by the detector tube itself and which sets up an amplifying effect without the addition of an amplifying tube, this type of receiving set has found great favor with amateurs, while in combination with amplifying tubes it multiplies their power proportionately and it is in consequence used in one form or another in all the better sets.

There are many different kinds of circuits which can be used to produce the regenerative amplification effect while the various kinds of tuning coils will serve for coupling them; for instance a two or three slide single tuning coil will answer the purpose but as it does not give good results it is not advisable to spend either time or money on it. A better scheme is to use a loose coupler formed of two or three honeycomb or other compact coils, while a _variocoupler_ or a _variometer_ or two will produce the maximum regenerative action.

The Simplest Type of Regenerative Receiving Set. With Loose Coupled Tuning Coil.--While this regenerative set is the simplest that will give anything like fair results it is here described not on account of its desirability, but because it will serve to give you the fundamental idea of how the _feed-back_ circuit is formed.

For this set you need: (1) a _loose-coupled tuning coil_ such as described in Chapter III, (2) a _variable condenser_ of _.001 mfd._ (microfarad) capacitance; (3) one _fixed condenser_ of _.001 mfd._; (4) one _fixed condenser_ for the grid leak circuit of _.00025 mfd._; (5) a _grid leak_ of 1/2 to 2 megohms resistance; (6) a _vacuum tube detector_; (7) an _A 6 volt battery_; (8) a _rheostat_; (9) a _B 22 1/2 volt battery_; and (10) a pair of _2000 ohm head phones_.