Show coo I 1 I 1 of B ra ad fig 3 showing the scheme devised by glenn browning for neutralizing a radio frequency receiver by L W HATRY 1 in new york 11 herald erald tribune many home radio engineers using the term engineer with extreme generality era lity design excellent little sets that Is these home designers subtract ideas out of 0 various articles they it have ave 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 electriss but much mon important por tant they understand und estand a deal of the practical application of things tills this la Is written for these men it la Is hoped that it will help in their rectifying rectify ing a thing often done haphazardly haz ardly first at the risk of being uninteresting let us go into the theory of the Il Hazelt azeltine lne or any neutralizing scheme the theory fits most ot of them but the specific application discussed will be hazel tines the neutralizing arrangement there la Is a feedback feed back from plate to grid in a radio frequency tube that results in the oscillation so annoying without souie some ann annulling or neutralizing scheme the feedback feed back may be reduced by sacrifice of effectiveness by introduction of losses or by a neutralizing so called arrangement the ne neutralizing 11 idea works on what Is known as the bridge principle notice 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 0 and the inductance of alij L or by the capacity of OP and the inductance of I 1 11 la 11 LIA because 1 L Is tapped in the middle now if the current divided 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 gill so that they are equal and thus pass equal but opposing forces that being the lie case caie ca ie it becomes become apparent that if lah L and ll 11 li are equal the condenser C must have the lie same capacity as between GP that sounds hounds all right but the capacity from 0 to P lg Is very small and it Is quite a trick to provide so batall a capacity at 0 with out having the lie wires running front froni L and 0 overdo the job now the exact arrangement of F fig ig I 1 Is ie being used at present in many sets employing a single stage of radio frequency amplification the detector coupler primary L Is a coll coil of double winding two wires in parallel or a C fig 1 showing the bridge principle of neutralizing single winding tapped in the center as L indicates yet practical set conditions often make it impossible to put the proper capacity at 0 so BO that the process of neutralizing li Is practically impossible successfully this trouble la Is simple to rectify it if we express the relation mentioned in the foregoing paragraph in a mathematical formula it will be LC LOP to represent capacity between grid and plate it Is apparent immediately that it if any one member of the equation to la reduced an increase of its partner will balance things again it if L Is reduced then C must be increased or vice versa verga that this lg Is true offers a solution to the neutralizing trouble it Is general practice to use a small adall midget var variable table condenser whose capacity Is mad fd or greater generally under the circumstances wi could just as well become 41 on the C side of the equation in this case L to 0 would be a half of L to P 11 in turns since the turns ra ratio tj 0 make ke the voltage present at C speaking concretely where L would ordinarily be a total of 30 turl turns s tapped in the center it should be in stead 27 turns tapped to put 18 turns between P and B plus and 9 turns on w the neutralizing side to the conden ser C the neutralizing current the method of obtaining the necessary neutralizing current as used in the neut odyne Is well known krilow it I 1 la shown schematically in fig 2 what the foregoing should have imparted in part Is that the balancing or neutralizing side of the bridge formed did not need so BO much to be actually part of the primary inductance i L lis merely a control circuit allowing the passage dauage of 0 the correct opposing current to the tb stid rid of the tube to be neutralized so if we arrange such a control circuit from a proper source of appo opposing ong current wo we can effect the same game thing tills this was done in the commercial by tapping oft off the lie inductance LI 14 which was waa secondary to 12 for the voltages or currents in a secondary circuit are generally completely out rf et phase with those in the primary tl stift primary L wa ans a almost always stuck at the filament end of ll LI as fig 2 indicates so 80 that the lie voltage transfer front roni L to ll LI at the lower portions would be made with small loss thus the top tap at LI generally included the sanie same of turns as were present ent in L giving an all oppositional current whose voltage was practically equal to t that t put on the plate by L control of t alio I 1 neutralizing capacity 0 would then put the exact neutralizing voltage on G since L and ll LI were equal the LIM condenser eon denser C had to have the same ca opacity as that between grid arid and plate a very hard condition to meet at as was said in the homemade home made and in many cases the factory made set L impal I 1 fig 2 method of obtaining necessary neutralizing current the remedy Is again as before have a small neutralizing inductance and a large neutralizing capacity using tho the midget variable condenser tap off the secondary of the neutro former LI 11 to include the number dumber of turns to the filament equal to about one half of L the primary then the feuti allying eon condenser dener Is safely the lie usual midget but not very satisfactorily vie tie so ao culled called neut rodon of the sliding tube type to obviate the troubles fig 3 illustrates the simply scheme devised by glenn browning of browning drake fame which obviates the troubles we have just men mentioned tinned the little disk of metal coming coining near to the inductance coll coil makes of itself itsel f and the coll 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 and thus fall sets gets employing a neutralized s stage ol of radio frequency regenerative detector and whatever necessary audio are easy to neutralize by means meana of a simple procedure starting off with the neutralizing capacity at its maximum adjust the tickler for oscillation with the two tuned circuits in tune it probably will be found that the tickler rotates through a greater are arc than the proper 00 90 degrees for the 5 teady steady oscillating state this sig signifies that the first tube to la oscillating a as well and it should not make blake a slight alight adjustment of the neutralizing conden ser see if the result to Is a necessary greater increase of the tickler continue this until not having changed the tuning the adjustment of the tickler arrives lit at its greatest proportion of non oscillAting to oscillating portion of the are arc of its swing in othir words until it Is necessary to give the greatest amount of tickler to start oscillation cil lation when this condition has been reached the first tube la Is probably not oscillating and the behavior of oscillation cil lation and regeneration will be lots lota less leas erratic and more regular this la is a method of neutralizing that may be done any time of day or bigia t independent of both a strong signal or a buzzer and battery |