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.
