Radio Broadcast - 1924, January - 84 Pages, 8.2 ... - VacuumTubeEra

The Second of a Series of Three Articles on Building, Neutralizing,and Operating a Five-Tube Hazeltine Circuit ReceiverBy KIMBALL HOUTON STARKChief Engineer, F. A. D. Andrea, Inc.ANY people who have constructedneutrodyne receivers have possiblyfound the adjusting or "balancingout" of the set beforeit isready for use, a difficulttask. In most cases, it has been difficult, notbecause the process itself iscomplicated butbecause of a misunderstanding of the resultsto be obtained and the methods of obtainingthem.It may be well at this time to consider thefunction of the neutrodyne circuit in order tounderstand just what isaccomplished when webalance out or adjust it for capacity neutralization.Professor Hazeltine devised a method ofneutralizing both the stray circuit capacitiesand the inherent capacity of the vacuumtube, i.e., the capacity existing between thefilament or cathode and plate or anode of thetube. It is the presence of these couplingcapacities in an ordinary tuned radio-frequencyamplifier circuit that causes the circuit toregenerate and oscillate when the tuned amplifiercircuits are in resonance, unless somemeans isprovided for controlling the gridpotential of the tubes, which usually results ina loss of efficiency.Such parasitic oscillations make itimpossibleto secure pure radio-frequency amplificationand in addition cause very decided signal andspeech distortion. Every listener-in knowsthe disadvantages of tuning-in to a local concertand hearing all sorts of "birdies."In Professor Hazeltine's arrangement, however,coupling capacities are made to counteracteach other and each individual tube circuitis balanced against another tube circuit so asto reach a condition where no signals can beheard in the phones because of passing throughcoupling capacities existing in the circuit.In the neutrodyne circuit, the electromagneticcoupling between the neutroformers isapproximately zero. The effect of this is tomake one portion of the circuit capacitybalance against another practically equalportion of circuit and tube capacity, so as toeliminate or neutralize the effect of the parasiticfeed-back capacities of the entire circuit.HOW TO NEUTRALIZE THE CIRCUITIN PRACTICE, the balancing-out processconsists of exciting the coupled receivercircuits with a comparatively strong signaland then adjusting the special neutralizingcapacities or Neutrodons, as they are called,to a point where that signal becomes inaudible.The fact that this adjustment of the neutralizingcapacities is made toward a minimum orinaudible signal and can accordingly be madevery exact, gives us proof that the method ofneutralization is a true process of actual circuitFIG.Top view of the five-tube neutrodyne receiver. This is the set described in the December, 1923, RADIO BROADCASTI