The Why of Wireless

Radio Record, Volume I, Issue 12, 7 October 1927, Unnumbered Page

The Why of Wireless

interesting Series Setting Out Scientific Facts Simpl y

(By

Electron

PART V. The plate current, although dependent.on the plate and grid voltage, cannot be increased indefinitely, With a given filament brilliancy the number of electrons shot off per second is limited, and when ali of these reach the plate no increase of plate pressure. will‘increase it. The vaiue is then said to be delivering its saturation current. The saturation current can be. increased by passing more current through the filament, thus increasing its brilliance, but this is obtained at the expense of the life of the valve. THREE ELECTRODE VALVE AS AMPLIFIER. This piece of apparatus has so many mses that the name ‘‘yalve" is too limited, and the American appelation, ‘‘tube,’? is better. The term "valve," however, is still used in England, When the voltage between the grid and filament is gradually varied, the repulsive action of the grid on the electrons is gradually varied in the same way, so that a feeble current in the aerial circuit will produce a much larger current of exactly the same. kind im the plate circuit. } When one considers the exceedingly complex form of sound wave coming from, say, an orchestra, and that the current from the microphone is amplified by valves several times in succession at the studio, sent up to the transmitting station by telephone wires, amplified there by more yalves, and then broadcast, amplified, perhaps. another six times in the receiving set, the" wouder is, not that the signal is slightly distorted, but that it should be recognisable! This will show that the modern amplifying tube is wellnigh perfect. It has uses other than im wireless telephony It is used frequently in Jong telephone lines overland, where the resistance of the wires would cause the messages to become _ too faint. In this way very much smaller and thinner wires .can be used, and the saving in copper is many times greater than the cost of the valves. DULL EMITTERS. The ordinary valve filanrent has to be raised to a high state of incandescence before it will emit electrons in sufficient quantity. The filament, as a matter of fact, burns more brightly as a valve than does the ordinary electric lamp for illuminating furposes. It was found that if the filament were coated with the metal thorium (a metal closely allied to radium) this thin coating would give off electrons. in as great a quantity when it was oun

J operating at a dull red heat. A valve of this type is known as a "dull emitter," and has the advantage of requiring much less power to operate it, as well as having a longer "‘life.’’ The dull emitter must not be run higher than its rated voltage, as the coating will be burned off, and it will, thereafter have the characteristics of a bright emitter. If a dull emitter of 1 volt is given the full pressure of. a 2-volt accumulator, the result would be tle same as trying to run a 100volt lamp off the 200-volt mains. The dull emitters have been condemned as being "microphonic." What is meant is that if the valve is tapped or jarred when alight, it will cause a peculiar singing or twanging sound in the phones. The reason for this is that the filament core js made of a very’ hard metal, which at the dull red heat is still considerably "springy." When the tube is jarred the filament vibrates to and fro, and the relative movement of filament and grid causes a yariation of plate current, with the aforementioned effect. The filament of the bright emitter is quite soft at its working temperature, and does not vibrate when jarred. This tendency of dull emitters can be nulliled by using some feet of rubber for the set, ‘f these noises are prevalent, but in the vast majority of cases no trouble will be experienced. Dull emitters will be found to give off a small plate current, even when cold, so that it is advisable to disconnect the "B’" battery when the set is not in use, in order to prevent it being run down needlessly. Later advances by the manufacturers have produced a valve which operates at a temperature so low that _it is not even red. These valves are the true "dry cell" valves, and no accumulators are . necessary at "all These valves are particularly useful for owners who are an inconvenient distance from battery charging stations, and who have no ‘‘battery charger" of their own. THE VACUUM TUBE AS DETECTOR. In the preliminary discttssion on three electrode tubes it was stated that when the grid is made positive it will cause a large increase of plate current, and if made negative it will decrease the plate current, oe

F When the tube is required to act as a detector (that is, as a non-return valve) the characteristics required are that normally no current should flow in the plate circuit and when the grid becomes positive a current will flow, the value of this current being of course proportional to the voltage of the grid. When the grid becomes negative it must suddenly stop all plate current from flowing. Tubes can be manufactured quite easily with these characteristics, but they must be used in a special way, Firstly, the plate voltage should be low-just about 25 to 80 volts, otherwise the attraction of the plate for the electrons will be greater than the | repulsive action of the grid when it is a negative, and the tube will cease to rectify. Secondly, the grid for this purpose should be slightly positive, and this is obtained by means of a small condenser called the grid antes

‘condenser. The capacity of this condenser is exceedingly low, about .0002 of a micro farad, and its action is that while it allows the high frequency currents to flow (as explained in a previous article), it will not allow the electrons which have collected on the grid to escape. The action of this grid condenser in allowing the high. frequency currents to pass "through" it is very similar to a thin membrane of rubber in a water pipe, figure 1. If the water is forced from the left the membrane will stretch or yield to the right, but as soon as the pressure is released it will come back to its vertical position. If the right hand side is compressed the membrane will stretch towards the left. If now the water is alternately pushed and pulled from, say, the left, it will cause all the particles of water to oscillate to and fro along the pipe, and to this motion the membrane will offer practically no resistance. It will, how- | ever, effectually prevent a steady flow of water from traversing the ‘pipe. | The grid condenser therefore will allow the grid to have its pressure raised and lowered just as if it were not there, but will keep the electrons on the grid and thus have it always slightly positive, | In order to prevent the grid from. becoming too positive (due to fresh electrons alighting on it every time it rises in pressure) they are allowed | to leak away slowly through a very high resistance connected across the lates. The value of this resistance is in the vicinity of one million ohms_ which, for convenience, is called one megohm. ‘The actual value of this | ‘grid leak’? must be adjusted to suit the particular valve and sometimes rises to as much as 5 megohms. | The arrangement of a tube fitted with grid condeuser and grid leak to form a single valve detector is shown in figure II, which should now be selfexplanatory. THE HIGH FREQUENCY AMPLIFIER. The detector valve described in the receding paragraph is perfectly satis- | if the signals in the aerial are loud enough to be heard in the ’phones after detection. If the distance from the broadcasting station is too great, then the signals must be amplified before being applied to the detector and ---~ --

Is the tube arranged for this purpose is called a high frequency amplifier, since it amplifies the high frequency waves just as they are received from the aerial. The arrangement is shown in figure WI. The feeble high frequency currents in this case go straight to the tid of the first valve and corresponding, but much magnified currents flow inthe plate circuit. Note here that these plute currents are still of high frequency, and therefore must still be tuned so that another coil and condenser are required as shown at Y. Vor certain reasons to be explained later it is illegal in New Zealand to connect the grid to the aerial direct, but these diagrams are explanatory only These magnified high frequency currents flow through the grid condenser to the grid of the sccond valve, where they are rectified as pteviously described. — Several new points are of note. First the pressure of the plates of these two valves should not be the same, so that separate contacts have to be supplied. Secondly, if the grid leak were connected straight across the grid conGenser it would raise the grid of the seccend yalve vp to that of the plate of the first. This would. obviously render the second valve useless, and thefefore it is connected straight on to the filament. Thirdly, it will be seen that the same "A" and ‘'B" batteries cat be used for both valves, although the actual pressures and currents are not the same, The brilliancies of the filaments are regulated by means of the variable resistance shown in the negative side of the filaments. The two-valve set shown in figure 3 will bring in distant stations quite clearly, but perhaps somewhat faintly. We could strengthen the signals further by adding another high-frequency valve before the detector, but this is not ad¥isable for several reasons. First, if’ we add more H.F. stages of amplification we must necessarily increase the number of controls, since each H.F. valve must be tuned Secondly, the amount of amplification thus obtained is not so great as can be obtained by amplifying the signals after they have been detected, becatise all the wires in the HF. side act like little condensers it they run parallel to any other wires, with the result that the electrons are tempted to stay at all. corners or where the wires are in close proximity to others. This property is so pronounced that it has been found that a considerable increase of signal strength can be ebtained by wiuding the tuning coils as an open helix with a space between each turn. In fact, the main difference’ between the popular Browning-Drake and many other sets is merely in the design of the coils. This is what is meant by the phrase "low loss coils." ~~