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Show - To gr,a Fig. 3 Showing the Scheme Devised by Glenn Browning for Neutralizing a Radio Frequency Receiver. By L. W. HATRY In New York-Herald Tribune. Many home radio engineers, using the term engineer with extreme generality, gen-erality, design excellent little sets. That Is, these home designers subtract ideas out of various articles they have read and screw them together to make a component whole including the things that they want. The men that do this sort of thing know a little of the technical and mysterious end of radio-electrics; but, much more important, im-portant, they undestand a deal of the practical application of things. This is written for these men. It is hoped that It will help somewhat in their rectifying a thing often done haphazardly. hap-hazardly. First, at the risk of being uninteresting, uninter-esting, let us go into the theory of the Hazeltine or any neutralizing scheme. The theory fits most of them, but the specific application discussed will be Hazeltlne's. The Neutralizing Arrangement. There is a feed-back from plate to grid in a radio-frequency tube that results In the oscillation so annoying without some annullng or neutralizing scheme. The feed-back may be reduced re-duced by sacrifice of effectiveness, by introduction of losses or by a neutralizing, neutral-izing, so-called, arrangement. The neutralizing idea works on what Is known as the bridge principle. Notice No-tice the diagram in Fig. 1. The path from input to B plus, going backward for the moment, Is a double one. The current can either go by the capacity of C and the inductance of L, or by the capacity of GP and the inductance of L (L, because L is tapped In the middle). Now, if the current divided di-vided equally in both of those branches and reversed itself in one branch to result in opposition, the result at B plus would be nothing, assuming that we control the paths CL and GrL', so that they are equal, and thus pass equal but opposing forces. That being be-ing the case it becomes apparent that if L and L are equal the conden- - ser C must have the same capacity as between GP. That sounds all right, but the capacity from G to P is very small, and it is quite a trick to pro-Tide pro-Tide so small a capacity at C with- , out having the wires running from L and G overdo the job. Now, the exact arrangement of Fig. 1 is being used at present in many sets employing a single stage of radio frequency amplification. The detector coupler primary L Is a coil of double winding, two wires in parallel, or a Mr1 I Fig. 1 Showing the Bridge Principle of Neutralizing. single winding tapped in the center, as L indicates. Yet practical set conditions condi-tions often make it Impossible to put the proper capacity at C, so that the process of neutralizing Is practically impossible successfully. This trouble is simple to rectify. If we express the relation mentioned in the foregoing forego-ing paragraph In a mathematical formula, for-mula, it will he: y2 i.u 1"' ""i""e, "p resent capacity between grid and plate. It Is apparent immediately that if any one member of the equation Is reduced an increase of Its partner will balance things again. If L is reduced, then C must be Increased, or vice versa. That this is true offers a solution to the neutralizing trouble. It is general practice to use a small midget variable condenser whose capacity is .00003 mfd. or greater generally. Under the circumstances L could just as well become L on the C side of the equation. equ-ation. In this case L to C would be a half of L' to P in turns, since the turns ratio make the voltage present at C. Speaking concretely, where L wou'.J ordinarily be a total of 36 turns tapped in the center, it should be instead in-stead 27 turns tapped to put 18 turns between P and B plus and 9 turns on the neutralizing side to the condenser conden-ser C. The Neutralizing Current The method of obtaining the necessary neces-sary neutralizing current, as used in the" neutrodyne, is well known. It is shown schematically In Fig. 2. What the foregoing should have imparted in part is that the balancing or neutrnl-i-ing side of the bridge formed did not need so much to be actually part of the primary inductance L as merely mere-ly a control circuit allowing the passage of the correct opposing current cur-rent to the grid of the tube to be neutralized. So, if we arrange such a control circuit from a proper source of opposing current we can effect the same thing. This was done in the commercial neutrodyne by tapping off the inductance LI, which was secondary sec-ondary to L2, for the voltages or currents cur-rents in a secondary circuit are generally gen-erally completely out of phase with those in the primary. The primary L was almost always stuck at the filament fila-ment end of LI, as Fig. 2 indicates, so that the voltage transfer from L to LI at the lower portions would be made with small loss. Thus the tap at LI generally included the same number of turns as were present in L, giving an oppositional current whose voltage was practically equal to that put on the plate by L. Control of the neutralizing capacity C would then put the exact neutralizing voltage on G. Since L and LI were equal the condenser C had to have the same capacity ca-pacity as that between grid and plate a very hard condition to meet, as was said in the home-made, and In many cases, the factory-made set. Inpui Fig. 2 Method of Obtaining Necessary Neces-sary Neutralizing Current. The remedy is again as before. Have a small neutralizing inductance and a large neutralizing capacity, using the midget variable condenser. Tap off the secondary of the neutroformer LI to include the number of turns to the filament equal to about one-half of L, the primary. Then the neutralizing condenser is safely the usual midget, but not very satisfactorily the so-called so-called neutrodon of the sliding tube type. To Obviate the Troubles. Fig. 3 illustrates the simply obvious scheme devised by Glenn Browning of Browning-Drake fame, which obviates the troubles we have Just mentioned. The little disk of metal coming near to the inductance coil makes of itself and the coil a neutralizing capacity and tap in a unique fashion. Of course, if it is not placed properly in relation to the voltage, at which It meets the secondary, It is possible to overneutralize and thus fall. Sets employing a neutralized stage of radio frequency regenerative detector de-tector and whatever necessary audio are easy to neutralize by means of a simple procedure. Starting off with the neutralizing capacity at its maximum, maxi-mum, adjust the tickler for oscillation with the two-tuned circuits In tune. It probably will be found that the tickler rotates through a greater arc than the proper 90 degrees for the steady oscillating state. This signifies that the first tube Is oscillating as well, and it should not. Make a slight adjustment of the neutralizing conden-. ser. See if the result Is a necessary greater increase of the tickler. Continue Con-tinue this until, not having changed the tuning, the adjustment of the tickler tick-ler arrives at its greatest proportion of nonoscillating to oscillating portion of the arc of its swing. In other words, until It is necessary to give the greatest amount of tickler to start oscillation. os-cillation. When this condition has been reached the first tube is probably J not oscillating and the behavior of oscillation os-cillation and regeneration will be lots less erratic and more regular. This is a method of neutralizing' that maybe may-be done any time of day or night, independent in-dependent of both a strong signal or a buzzer and battery. |