Show spitted Scitt ed by G douglas editor of aldio 7 A t four tube circuit by THOMAS W BENSON the principle of neutralized tuned radio frequency amplification Is utilized in the circuit given in the attached illustration to form a receiver that la Is tion radiating selective has baa good range and volume and Is easy to build and operate the most interesting part ot of the circuit Is the method of constructing the inductances and the source of the current used to prevent oscillations in the circuit it will be noted that two inductances are shown in the diagram shunted by 5 mal condensers tor for tuning these inductances are termed and are made from 75 turn honeycomb colls coils in the following manner take a 75 turn tum coll coil and strip off 2 turns then count back 15 turns on the coil and carefully raise the wire and take a tap this tap Is lettered C now count back 15 more turns and make another tap B it Is not necessary to unwind the colls coils to take these taps the calls are then clamped between two pieces piece of bakelite 2 inches square in which abich have been drilled holes boles to mount terminal posts if 1 0 f 11 1 IJA L S I 1 c 0 J a I 1 moun ranc a gl X y deral m p F M P ll 11 F 0 R M E R the inside end of the coll coil goes BOSS to post posta A the second top tap made to post B and the first tap to post C and the outside end of the coll coil to tap D the first coll coil does not need tap B taken out and this coll coil has but three terminals A 0 and D for convenience in mounting a bole Is drilled in the corner of the bakelite end pieces and a small brass ansle anale bolted to the coil so the colls can be mounted on one od of the terminals of the condensers in mounting the colls coils they should be mounted nt at right angles to reduce any coupling between them to zero the connection to the rest ot of the apparatus Is made as shown in the diagram the neutralizing condenser Is a small midget variable condenser mounted on the panel the rest ot of the circuit presents nothing really new I 1 As to the theory of its operation the 15 turns of the first coll between 0 and D form an primary circuit and by reason of the inverse aerial connection grounds the negative filament reducing body capacity the condenser tunes the remainder of the first arst inductance which Is the grid input to the radio frequency tube the plate circuit of the radio frequency tube includes 15 in the second coll coil and forms an plate circuit closely coupled to the tuned input circuit of the detector it win will be noted however that the portion ot of the coll between 0 and D li Is neither in the plate or grid circuit but I 1 Is closely coupled to both the end of the coll coil to it connected through the ing condenser to the grid rid of 0 the radio frequency tube now should there be any tendency of the plate circuit to feed back to the grid through the tube capacity it I 1 Is neutralized by an equal and opposite feed back obtained from the th extra winding on the coll coil per feet stability can be obtained by the adjustment of the stabilizing con denser denier but this condenser la mol mounted anted on the panel for a very good reason we could mount it fixed behind the panel but by making it adjustable a gain in volume and selectivity Is 13 obtained it Is well known that the greater the tendency of a radio frequency amplifier to oscillate the greater the amplification but the instant oscillations oscillation start the amplification stops therefore when a signal Is picked up a slight blight adjustment of the stabilizing condenser Is made to slightly unbalance the circuit when the signals will be greatly strengthened and the circuit can be put right on edge so to speak it would not be practical to keep the circuits in this critical condition while tuning tor for the set would go into oscillation without warning but when a station Is picked up it Is entirely practical to push the circuit up to the edge of oscillating but not beyond the receiver Is remarkably easy to handle and gives results equal to any four tube set built it possesses all the stability of the as well as the selectivity and in addition permits the slight unbalancing of the sup erdyne and teledyne circuits that accounts for their efficiency new york world |