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.
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.