University of San Francisco - USF Don Yearbook (San Francisco, CA)

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THREE E LEM EXT YACVI'M TIRE 31 action is called “Regeneration.” A great step in the development of the Vacuum Tube was made when Captain Armstrong discovered this means of increasing the strength of the input without adding any extra energy from the outside. The “Armstrong regenerative circuit” is subject to endless modifications but the principle is always the same and may be formulated thus: The incoming oscillations, alternating at radio-frequency, enter some form of coupling device. In this, the secondary or detector circuit is tuned to the length of the incoming wave which it receives by inductance from the primary. This induced wave (in the secondary) is reinforced by high frequency waves fed back from the anode or plate of the Vacuum Tube. One wire from the secondary bears these fed-back oscillations. The other connects to the grid and carries the induced incoming current, reinforced by these fed-back oscillations which have the same form as the incoming waves. At this point by the function of the tube, alternating waves of high frequency are generated and are later taken around to strengthen the incoming wave. Simultaneously the Vacuum Tube converts the message-bearing current from radio frequency to one of direct pulsating low frequency, already strengthened by the fed-back waves. The two currents of varying frequency arc now sent through a variometer which serves to tune the high frequency waves escaping from the tube to the same frequency as the incoming wave. By this method the low frequency currents are strengthened from ten to one hundred times and they enter the phones at about four to ten thousand cycles a second. Of course they are accompanied by the other radiofrequency currents, but since these have no effect on the phones they are passed through a condenser provided for that purpose. These waves, after passing through the condenser, go again to the secondary of the coupling device and the circuit is repeated. This system constitutes one branch of radio amplification. The other is accomplished when the current from the plate of the detector tube is coupled to the

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30 THE J ax ATI AS incoming wave group due to the flow of the rectified current. (2) This is the 44Grid condenser method. ’ With a properly constructed tube it is most sensitive because the plate potential varies at an audio frequency due to the condenser interposed somewhere in the grid circuit. Unidirectional current passage between the filament and the grid rectifies the impressed groups of radio oscillations and the condenser consequently receives an unidirectional charge which, in building up, causes the plate current to decrease. When the incoming oscillations have ceased the condensed charge dissipates through a high-resistance grid-leak and the plate potential returns to normal. The effect of inter-electrode capacities is next to be considered. Remembering the fact that a slight variation of the grid potential greatly changes the strength of the plate current, we can appreciate the fact that this phase of operation has two broad applications with respect to Wireless. Primarily it causes the Tube to be a self-contained amplifier which in no way interferes with its detective qualities, which may even be simultaneous, and secondarily, it is in this respect that the tube is used as an oscillation generator. These characteristics have been fully set forth by their discoverer, (’apt. E. H. Armstrong. In some types of circuits there is a marked difference in the action of the tube than what might have been expected. This is due to inter-electrode capacities. The effect of these is to cause a tube to produce oscillations even if there is no mutual inductance between the input and the output. It is also on account of these that there is an impedence between the cathode and the grid which depends both on these capacities and on the constants of the input circuit. When oscillations are impressed on such a circuit they die out at a rate which varies as the total resistance. If the impedence is negative the total resistance is reduced and there will be a smaller consumption of current in the input circuit and the tube will give an increase of amplification. This increase of amplifi-

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32 THE IG ATI AN input circuit of another tube which functions as an amplifier only. The output of the amplifier may in turn be connected to the input of another and the second amplifier to a third, etc. In the light of facts already explained this system needs no explanation. The action of the Vacuum Tubes is the same in any case. When a method of this kind is employed the Tubes arc said to amplify in cascade. Amplification may be secured by cascade at either audio or radio frequencies, jointly or separately, by a change of hook-up. Tubes are not coupled either directly or conductivcly for cascade amplification but through the agency of amplifying transformers using a primary and a secondary on an iron core for audio frequencies and the same on an air core for radio frequencies. In regeneration coupling is termed electrostatic when amplification is secured in the detector tube. Regenerative circuits are also called reactive or tuned plate circuits, because the high frequency current generated has to be modified so as to “fit” the incoming wave. If the waves do not “fit’' each other no result will be secured, for the waves, instead of synchronizing, will constantly interfere and destroy each other. When the subject of regeneration is discussed it leads logically to another most important function of the tube. Regeneration essentially combines this other distinctive function; for the generation of those waves which are fed back demonstrates the fact that it is possible to use the Vacuum Tube for the sole purpose of generation. The Vacuum Tube as an oscillation generator has made possible the modern wireless telephones and the C. W. methods of transmission. These latter methods are slowly supplanting the older method of either arc or spark. In other words, a radiated C. W. when intercepted by the receiving station, is so sharp and constant in character that the receiver must be tuned exactly to its wave length, otherwise it will not affect the instrument. When we consider that there are upwards of fourteen hundred amateur transmitting stations in the United States and