Chapter 11

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 glass 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.

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.

Assembling 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.

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.

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.

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.

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.

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.