The Why of Wireless

Radio Record, Volume I, Issue 13, 14 October 1927, Page 2

The Why of Wireless

Interesting Series Setting Out Scientific Facts Simply

(By

Electron

PART VI. Low frequency amplification can produce a large increase in the strength of the signals, and if it is required to operate a loudspeaker is almost essential. No tuning is mecessary, and therefore we can have as many stages as we desire without any extra controls. The arrangement is shown in fig. I, from which it will be seen that the general principle is unaltered. The most drastic alteration is the addition of the low frequency transformer. This transformer was suggested in a previous article, when it was stated that a current flowing in a coil produced magnetism which if made to flow through a second coil, would produce 4 similar current in it. The low fregnency transformer merely consists. of two coils of wire placed oue over the other on a common core of ironthe iron being used to magnify the macnetism passing through the coils, If the number of turns in the second coil or secondary is, say, five times that of the primary, the pressure will be also five times that of the primary. Again, the reader must be warned against thinking that he is getting something for nothing, because he will find that, although the pressure of the secondary is five times that of the primary, its current -cannot be more than one-fifth. This reduction in current is no disadvantage for our present purpose, since, as was mentioned last week, the grid is ‘pressure operated"’ and does not require anv current but merely the pressure of the secondary. This therefore shows the advantage of low frequency amplification because, besides the multiplving power of the valve itself, we can increase it another five-fold by means of this transformer. Tf we wanted to carry ont still further amplification we would place the primary of another transformer avhere the ‘phones are and repeat the connections, placing the "phones or loudspeaker in the plate circuit of the last valve. POWER VALVES. Each valve causey the signals to increase in strength, so that we must be certain that the valves themselves are not being overloaded. Tf small valves are being used it is quite possible that the plate current will reach its saturation value, especially during loud passages. This will mean that the plate current will not be proportional to the grid voltage, and therefore the sienals will. be distorted and of bad quality. For this reason if loud signals are required for a large loudspeaker it will be advisable to use one of the so-called power valves, which are specially constructed to | have a large plate current withont | becoming saturated. Because of thie larce variations in grid voltage these last valves shonld have a high plate voltave (about 100 volts) and a large negative grid bias (from 5 to 7 volts). The actual values will be given with the tubes themselves. The second Jast valve should, however, not be a power valve because the heavy current in its plate circuit might (and frequently does) cause the trans-

----~ former primary to be damaged by excessive heat. Sometimes this heayy current will damage the londspeaker windings, and this can be safeguarded in two ways, | (1) by having plenty of negative bias. Negative bias reduces the plate current, as well as improving the quality of the reception. (2) The second method of relieving the loudspeaker windings from the steady plate current is to use a combination of a condenser and choke coil. In an earlier article it was stated that a coil of many turns would iet a steady direct current flow through it, but would effectually prevent an alternating current to traverse it, while on the other hand a condenser will allow the alternating current to flow through it easily, but will prevent a direct current from flowing. ‘he arrangement is shown in fig. 2. Recently another method of doing this — las been introduced. This method ntilises a special transformer whose pri-. mary is connected to the "speaker". terminals, and the speaker is connect- | ed to the transformer secondary. ‘Lhis

method is claimed to reproduce jow notes better than the choke filter method just described. It has the further advantage of eliminating the danger of shock by touching any electric light fittings which may happen to be defective, as the secondary is insulated from the set and from "earth.’? This | transformer is specially constructed so that its primary can carry a heavy current, and as this heavy current is constant in quantity, it will not be induced into the secondary, which will thus receive only the alternating or audio |

currents. Another advantage which both these latter methods have is that the speaker can be connected anyhow to the set, as there is now no fear of the heavy plate current demagnetising the magnets. CRYSTAL VALVE SETS. The case of control of erystal sects, together with the clearness of reception which they invariably give, has kept a peculiar combination called the crystal valve set. In this arrangement a pure crystal set is used first for rectifying, and the primary of a low-frequency transformer is placed where the phones usually go. The secondary of the. transformer is taken to the grid of the. valve just as an ordinary low-frequency amplifier. This arrangement, while being much more powerful than a crystal alone, is not equivalent to two valves, because, although the crystal may act like a yalve in rectifymg, it will not amplify, and since its resistance is always high it will keep the current

‘down. Still, in spite of this, it is a useful type of set for tise near a broadcasting station, especially for those who feel a bit scared to operate a valve set in case they oscillate unwittingly and at» tract the attention of a passing inspector, because a set of this type can never under any circumstances cause trouble with any other listener, THE MAGNETIC BAR AMPLIFIER, _ Still another method may be used to increase signals of phone strength to

that for a loudspeaker. This is a rather curious inversion of the transmitting apparatus, because it consists in principle of an ordinary telephone receiver to the diaphragm, of which is fastened by means of a solid rod the diaphragm of a microphone. Great amplification can be obtained, because the microphone is made to vibrate as it would do if air waves of considerable magnitude were striking it, A small pressure of about 3 to 6 volts connected to the microphone causes a varying current, as has been explained before, This current is very much. larger than the original one in the phor.e receiver and this magnified current is made to flow through the speaker coil, A special speaker winding oi low resistance is required, and several makers make both amplifier and speaker in one unit. This gives a very simple andcheap means of amplifying signals, but for a time the quality was not up to the standard of that given by valves. Prospective customers should, therefore, compare these amplifiers with valve am‘plifiers to be sure that the quality is sufficiently good. | REACTION. The methods of nsing valyes deseribed in the preceding articles are what we might call the legitimate or straight methods. here is a device, due to Armstrong, who has been responsible for many radio advances, notably the super-heterodyne, which adds practically the power of another valve to the set very easily. As has been mentioned several times already, if there is an alternating current fiowing in a coil, and that coil is brought up to another coil, the magnetic effect of the current in the first will pass through the second and induce a similar electrical pressure in it, Tf we, therefore, arrange our set as shown in Fig. 3, the magnetic effect of the plate current will amplify the pressure in the grid, and will therefore still further amplify the plate current. This device, however, cannot be used beyond a certain point, becanse if the two coils are brought too elose together the set will behave as a transmitter, and this is the cause of all the howling which one hears at night when trying to get distant stations, Perhaps the air will he cleared in more senses than one if a little time is spent investigating this matter. OSCILLATION. The receiving set in its normal state is very similar to a clock pendulum, which is set swinging by repeated taps from ontside. When the taps hit the pendulum at the same rate as the pendulum swings naturally, then the pendulum is said to he : _~

‘tnned" to the frequency of the taps. } All the devices we have been considering up till now have been similar to sets of levers attached to the pendulum, in order to magnify the ; movement, Reaction, c~ thie other hand, acts like some form of clockwork to increase the swing of the pendulun? itself, Up to a point this is quite good and harmless, bué if a certain amount of this "feed back," as the Americans call it, ig exceeded, it will be seen that the pendulum will con-

tinue to swing of its owa accord, whether the taps are coming in or not. A receiving set in this state is said to be oscillating, and when this is the case it is not only distorting the received signals to the point of unintelligibility, but is a menace to all listeners within a large radius, perhaps even a mile or so. Nearly everyone has heard the result of an oscillating receiver-it is anything from a highpitched whistle to a low grunt, and even when the actual musical note of the oscillator is not heard, the receiyed speech or music will be very badly mutilated. In order to reduce trouble from oscillation, and also to obtain greater magnification, it is customary nowadays to use high-frequency or "radio"’-frequency transformers for connecting high-frequency valves together, The principles are exactly the same as for the low-frequency transformer, but for various reasons the iron core is not used, so that it consists of two coils wound side by side on a cardboard cylinder. A set of this type is shown in Fig. 4. ‘The reason why the radio-frequency transformer was not used much in the earlier sets was the fact that oscillas tion was almost always present. With modern sets, however, we .can use reaction as well as transformation, and thus get a high degree of amplification in the radio stages. It should be stated, once again, that these notes are not constructional, and several of the sketches show connections which have been declared illegal in New Zealand. ‘This applies particularly to Figs. 1 and 3 of this article. They show, howeger, the general progress made, and may be considered historical, A general tule which all constructors should know is that it is illegal to have the grid of a valve connected directiv ta the serial, unless the set is unable to oscillate.