Radio News for January, 1924 899 A Practical Neutrodne Receiver By ALLAN T. HANSCOM This article explains the action of the neutrodyne circuit in a manner which can be grasped by the novice. Its formation is described stage by stage, followed by complete constructional details for the building of a receiver of this type. on which the secondary coil is wound. It is difficult to buy one tube to fit inside an- other in this manner, but it can be overcome by sawing a piece of tube /" wide length- wise from the same tubing. Then when it is wound, the wire will draw it to a smaller diameter whereupon it may be slid into place inside the full sized tube. It is very im- portant that these transformers be mounted Insulated wire fig. 2 /w/sted together A One -Stage Radio Frequency Amplifier and a Non- Regenerative Detector, the Respective Secondary Circuits of Which Are Tuned by Means of Variable Condensers. IT is unfortunate that so many good cir- the neutrodyne after we add the neutralizing cuits are labeled with such formidable condensers which will be considered later. titles. The name "neutrodyne" is usu- In Fig. 3, there are two stages of radio ally enough to scare the average radio frequency amplification and the detector, fan. A neutrodyne circuit, as commer- each stage being tuned by a variable conden- cially developed, is, however, a great deal ser in the grid circuit. This circuit would more simple to understand than the regen- give wonderful results if it were not for the erative or reflex circuit. internal capacity coupling between the plate and the grid circuit of each radio frequency Bus wire Meta/ frbe tube. This internal capacity is neutralized by the combination of capacity and induct- ance; the inductance being the windings of the air core radio frequency transformers R. fig. d The capacity necessary for the average vacuum tube is very small and is usually Several Ways of Making the Neutralizing Capac- about equal to two circular plates /" in ities. diameter separated by 3/32 of an inch. This is only approximate and the proper value may at least 6" apart and at such an angle that easily be found. Other types of condensers a line drawn at right angles to the axis at for this purpose are shown in Fig. 4. the end of the winding does not intersect The radio frequency transformers can be the winding of the adjoining transformer. made by winding 80 turns of wire on 3" tubes See Fig. 6. Note the relative position of Two -Circuit Non -Regenerative Receiver Having a Tuned Secondary Circuit. with a tap taken off at the 20th turn. It is primary and secondary windings. The average fan is not interested in the higher mathematics of a circuit but he does like to know the size of the parts so that he can put one together to obtain results. Fig. 1 represents a simple type of non -re- generative hook -up. By varying the conden- ser the natural wave -length of the set can be altered. This is exactly the diagram of the first stage of the neutrodyne. In Fig. 2, another stage exactly like the first has been added. In this case the first tube acts as a radio frequency amplifier and the second tube as a detector. fig 3 It will be noticed that the plate circuit of Two Stages of Tuned Radio Frequency Amplification and Detector. These Circuits Are Free to the first tube is not tuned, but that the grid Oscillate Due to Feed Back Through the Internal Capacity of the Tubes. circuit of the second tube, as well as the grid circuit of the first tube, are both tuned and preferable to use cardboard because it is an NEUTRALIZING CONDENSERS in reality make the two stage filter through advantage to have a close coupling between Since the variable condensers used are con- which the incoming signal must pass. this coil and the primary, which is inserted nected between the grid and the filament, it Fig. 3 represents three stages exactly as inside. The primary consists of 20 turns of is possible to connect the grid to the fixed outlined above and this is the scheme of wire on a tube which will just clear the tube plates and then mount the other small mov- able plate on the back insulated portion of .002 M.f the condenser. By this means a very small change of capacity can be made in the neu- Vh0rIe5 tralizing circuit. See Fig. 7. t meg0lim After the proper value of this capacity is I found, it need never be disturbed unless tubes 1.1222 are changed. It is likewise advantageous to + 5in shield the set, although it is absolutely im- M.f possible to make this set oscillate if it is .005 properly constructed. To find the proper value of the neutraliz- ing condensers it is necessary to tune the set -B to a very powerful signal. The Flewelling .0005 A "screamer" makes an excellent oscillator for o this case. A paper may be inserted in the socket to cover the filament connections but fig. 8 leaving the plate and the grid free. Then the tube is inserted and the signal is still Complete Circuit Diagram of a Neutrodyne Receiver Having Two Stages of Tuned Radio Frequency heard. Amplification and Detector. The Circuit is Kept from Oscillating by Means of Small Condensers Which Neutralize the Capacity of the Tubes (Continued on page 914) www.americanradiohistory.com 914 Radio News for January, 1924 1.1.1117-MrMliiillummilnIFIIIIIITIllummIlmmuilnIIIMIM II 111 rimonmovanimmummantme Eliminating Interference __ (Continued from page 898) E .1,,,,11,,,,,,,,,,.1,111,1111111111111,,,,,1,111111,1111,111,,,1,,,,,,1,,,,,1,,,,1,111,111111111,1111,1,111111111,,,,,,,,1 TIC ,,,111° E upreme sharply with the single and multiple turn upreme switches of the tuner. This is in effect an LOUD SPEAKER. air core transformer coupled amplifier and HEAD SETS is equally effective over the entire range of the tuner. The resultant complete circuit is shown in Fig. 2. ADAPTED TO SINGLE CIRCUIT TUNERS For single circuit tuners the amplifier shown in Fig. 3 has proven very effective. As in the first case the output of the am- plifier is connected to the input of the tuner. When a receiver such as an R.C. is used where the tuning elements are in one cabinet and the tubes in another, busses connect the two. The post marked "G" in Fig. 3 con- nects to the post marked "Grid" on the tuner. When the tuner has no outside connections, such as in an Aeriola Senior, a separate wire is run from the grid lead inside the tuner to "G ". On some sets it is necessary to re- move the grid leak when it is connected around the grid condenser instead of to the filament. This arrangement makes a tuned imped- ence coupling between the first two tubes No extra Batteries The Head Set required. that is Always Ahead Finely and as accurately made as a high -priced watch and fig. 4 there you have the whole A Regenerative Circuit Employing One Stage story of Tuned Radio Frequency Amplification to In- crease Selectivity and Distance and to Prevent Re- of BERWICK Headset su- At LAST. radiation. premacy. and is shown below with the detector in Fig. 4. Every BERWICK Headset A Perfect Loud It is not necessary to install this apparatus is perfectly matched for tone Speaker With a in a cabinet, as very excellent table mounted instruments are available. The parts used - TESTED in ACTUAL MICROMETER are : 1 Variometer use for clarity and reproduc- Adjuster and Tuner 1 Potentiometer tivity, and sold with an uncon- 1 Rheostat This remarkably powerful special Unit with the marvelous MICROMETER Adjuster, gives 1 Socket ditional guarantee, or your you -with just the turn of a little knob -maxi- 1 .00025 M.F. Fixed Condenser mum clearness, and a ready ability to meet 1 Amplifier Tube. money back. every individual condition as easily, as simply as you would adjust a pair of opera glasses to I feel confident that any reader who will suit your own vision. spend the few dollars necessary to install 2200 ohms-$5.00 FIBRE HORN, non -resonant, non -breakable, either of the amplifiers will be well paid by damp- proof. TWICE TESTED in actual radio the in his I know 3000 ohms -$6,00 reception before it reaches your hands. improvement set and will SEALED to prevent tampering. UNCON- be blessed by his neighbors. DITIONALLY GUARANTEED FOR A BUY A BERWICK Su- YEAR against any defects. t,.1,,,11,111111111111,111,,,,,,,,,,n,11.,m11M1,,,,,,,,1,1,11,,,,11,M7M,.,1,7M. ,,,M1,11111111, Here indeed is the Loud Speaker that brings preme Headset TODAY you everything you want to hear - clearly, - faithfully -from the most delicate shadings of A Practical Neutrodvne and note the difference. the human voice to the rich, full blare of a great brass band. É Receiver At All Reliable Dealers Convince Yourself- (Continued from page 899) =- Get Yours TODAY. Sold Through Jobbers If your dealer cannot supply you, get in touch with us direct. This is done for the first stage and second DEALERS: Ask your jobbers for information. stage separately, adjusting each condenser TRIANGLE until the signal is no longer heard. I have found it possible to adjust these condensers Electro Trading Co., Inca TRIANGLE without going to this trouble by simply set- Manufacturers Co., Inc. ting them at the greatest capacity which 632 -634 Broadway, New York Electro Trading would not cause the set to go into an oscillat- Manufacturers New York Chicago Minneapolis ing condition.