Show Dy A DINSDALE In n New Mew York Her Her- Tribune ald-Tribune frequency aid S RadIo frequency amplification has engaged the attention of ot experimenters both hero and on the tb other tide side of ot the Atlantic for some same time past but with 1 the exception of ot the tir all the systems BO so fur far devised have I proved unsatisfactory when applied to I more than at most three stages The The reason ren for tor this difficulty with multistage It R F FF amplification lies In inthe Inthe I Ithe the fact that self-oscillation self of ot the R n F tubes Is difficult or Impossible to toI I control Many different methods of preventing this un unwanted anted o have hare been tried by Introducing Into the circuit different stabilizing devices John Scott Scott the well known designer ner set out to find means ot of overcoming the difficulty without in- in introducing In Introducing more or less Inefficient stab stab- stabilizers stabilizers He lie alternated tuned circuits with circuits the grid Id cir cir- circuit circuit cult being tuned and the plate circuit This method Immediately proved prove successful for tor the stability Is s perfect regeneration being nece necessary nry In fact fact to make the set oscillate A tube seven-tube circuit giving five I IA es of ot It R 1 F amplification In which re regeneration was employed was con constructed con con- hut but was found to be lacking J JIn In selectivity although very stable and sensitive I I The circuit was wn therefore modIfied by the Introduction of ot wave e traps Into I all the tuned circuits which modification tion Immediately elevated ele the system Into the ne class as RS regards re- re regards I reI gards selectivity while reducing the sensitivity b by only a small amount The method Is Illustrated In the dia- dia diagram dia dla diagram I IThe gram There are arc two kinds of ot wave traps In In one method the Interfering station Is la cut out len lu leaving the desired signals sl nals and In the other the selectivity of ot th the e cn W a aH H B w N H C kI kIR I R pl S SA A Seven Tube Circuit receiver Is Increased to such Mich uch a pitch that the tir Interfering sl signals cannot get ge t through Tile The latter method Is the more seten and Is the one adopted The Th explanation of ot the principle Is as tol- tol follows oh lows It If the trap circuit L Cl were not no t present the tir coupling between the firs first t find and second tubes would be by means mean s ot of the Inductance LI Ll only only-I only 1 e en en- en entirely entirely Thus all nil interferences present In the tir aerial cIrcuIt would be parsed on to the next stage By ny Introducing the trap circuit and tunIng It exactly to the tho frequency ot of the desired signals the coil coll LI Ll no n longer remains entirely Since LI Ll Is tightly coupled to It comes under the tho Influence of ot the trap circuit and ond Is Inclined to respond more mor morto to the frequency of ot that circuit If It on the other hand la le not accurately tuned to the Incoming tre- tre frequency Quency LI Lt reverts to Its original condition Now a choke coil coll to produce an R appreciable amount of ot must consist of a considerable number ot of turns otherwise there will be up no build up or Increase In amplitude ot of the oscillations flowing through It That Is to say If It LI Ll consists of ot rela rela- relatively few turns It will act as a vIrtual short circuit to the oscillations transferred trans trans- transferred to the plato plate circuit by the grid Applying Wave Traps Trap In practice the circuit L may consist of ot any of the usual forms ot of loss ollow-loss low loss Inductance say soy CO 00 turns tuned by n a low loss condenser of ot capacity LI Lt which Is tightly coupled to L mo may consist of ot a n coil coll of ot 2 25 turns or less As low n number as os eight turns has been used with success the determinIng lag Ing factor the lie degree of ot volume voluBle volu i required as against the degree of ot selectivity se- se se selectivity necessary Eight ht turns will give Ive very much greater selectivity sele than 25 23 turns and the loss of ot volume will only be he slight alight The point Is la one which the Individual experimenter can with advantage decide tor for himself Except eDt for tor tho colis coils colls the th components are all standard and null the number and arrangement of ot the vari varl- van van- various one ous stages are given Ilven just Ju t ai us a n suggestion Uon tion to the experimenter As a n matter of ot fact the writer strongly recommends the reader to commence experiments with only three stages of ot R It It F as aa the Ule selectivity will willbo willbe willbo bo be found to be so M great reat In n a well laid well out out receiver using good low loss well laid components com com- components components that two condensers will be found to be bo quite enough to handle handlo It matters little whether the tickler coil Is coupled to the aerial coil or 01 to the w wave e trap Inductance Those who prefer preter tolse to use lse plu plug plug-In ln tolls colis can use an ordinary coll toll holder In this position Similarly plug-In plug coils colla mounted In- In Inside inside In Inside side the cabinet may be used for tor the It R It F chokes which are shown variable In the diagram This variability Is desirable because In a sense a n choke chole coil coll of ot this sort Is not absolutely A coil which wIll present sufficient Impedance to a certaIn frequency to cause the necessary necessary sary up build-up of ot oscillation amplitude wIll present a negligible Impedance to another much lower frequency Thus If It f plug-In plug coils colls are used they cnn can bo he changed to suit the tho wa wave ve length lenth being received Nos or 01 wIll cover covel the American broadcast band satisfactorily Alternative Choke Coll An alternative choke con coil which can he be made up by those who desire maxi mail maximum I mum possible efficiency on any given wave vate length consists of ot l O turns ot of otI I No 10 40 s s s c resistance wire wound on a former about 2 2 Inches In diameter dIameter I eter fire are taken to a ten ten- ten point point rotary switch from the ends of 01 the coil coll and from the fiftieth turn From the fiftieth turn to the one one hun bunn hundred dred and twentieth are made at every ery tenth turn The switch for this toll coll may be mounted on the face tace of ot the panel or more convenient conveniently on the end of or the coil coli which can con be bo mounted on the baseboard This Is n a point for Tor the experImenter himself to settle The n R F I F wave wate trap coils colis may be wound as cylindrical coils colls and the few turns of ot the plate coils colis wound over the central part or of orthe the winding In tills this way turns can easily be put on or taken off oft during the course of ot experiments The aerial cons coils can be made mode up In the same man man- manner manner ner ner If It plug plug-In coils colls are not used There Is of ot course no reason why special low loss coils colls of ot the various wound basket wound types should not be made up the few tew turns of ot the coils colls being being- wound on the same former to Insure tight coupling Having familiarized himself with the method of ot tuning the experimenter may proceed to add as many stages ot of R n F as he pleases but It if more than three are employed It Is practically eS essential that a multiple condenser be used In conjunction with carefully matched coils colls Suggestions of Out Lay-Out The main thing to bear In mind when arranging the out lay Is to avoid overcrowding In the R B F circuits and to so arrange all coils colls that no interact Interaction Interaction tion can cnn occur between them Using only three stages tn es of ot R fi F and no A F man many report loud loud- loudspeaker speaker loud speaker signals from American stas sta stations lions almo almost t every night with an an- anun anun an un- un unusual un unusual usual absence of ot fading and freedom from distortion This alone speaks volumes for tor the lie efficiency of ot the re- re receiver re receiver for tor although many different types of ot receivers In n use In England today will bring In n American stations regularly fading fallIng swinging and night E effect distortion are almost always present As regards regard selectivity It has hai been found possible within a mile or so or of the the local station to completely complete tune It out and receive uninterruptedly upon a wave length len within a few tew meters of ot It |