Show edited by 0 douglas war drap editor of radio claas by PAUL 0 in most radio frequency circuits in use at present uio the tuning la Is done by varying the capacity in parallel with a coll coil by a variable condenser this method la Is ideal from n manufacturing standpoint the variable condenser of today Is a pretty good piece of uppa apparatus pa ratus in spite of being called a mon alty by some straight colls aro are easy to wind and can be nindo made with accuracy if tho the tuning Is dono by varying inductances duc the Tarl ometer Is practically the only means of accomplishing this the varl ometer was in great favor with the old time fan but now has pretty nearly disappeared this ts la very unjust while the varl ometer to Is a rather undesirable piece of w work rk for tho the manufacturer it has few d drawbacks ra w for the far fan in multistage radio frequency receivers capacity tuning has one drawback it if the receiver Is balanced for long waves it will have a tendency to oscillate at short waves it it Is 18 ba balanced I 1 for short waves it will lose much ot of its at long waves the receiver described in tills this article P provides capacity and inductance tuning in the tha aerial circuit and inductance duc duct tance lInce tuning between n R F P tube and detector this permits an excellent balance in the ding diagram Tam figure 1 we have fl a 23 plato variable condenser in the aerial find and a small varl ometer in the grid circuit tho the grid Is connected in approved fashion between the conden condenser ser and the ground and Illum filament ent are connected connect ed to tho the other terminal of tho the varl ometer for tuning purposes a alone would be sufficient clent As too much inductance would moke make the set os chiate the varl yarl ablo abla condenser la Is provided dy by balancing one against thid aho tho efel may be prevented from oscillating quite effectively the coupling device Is a choke coll coil combined with a varl ometer in a n very unusual way follow instructions carefully for this part of the receiver the most convenient choke Is 1 the be secondary condary of 0 some small and inexpensive audio transformer itsou want to use a very efficient choke use a 1000 turn honeycomb coll coil or wind a spider web with 1000 turns of no 80 30 silk covered wire if you have the patience necessary for such work the choke Is connected as indicated dica ted between the plate of the IL II F tube and the positive terminal of the B battery which should bo be of approximately DO 00 volts for best results the varl ometer should be one of the largest types available to cover the entire broadcasting range title this varl ometer Is connected between the plate pinto of thu tho it II r F tube and the grid condenser of tile detector tube tile grid rid condenser Is connected in the usual way the grid leak however Is connected between grid and positive filament for a hard kinrd tube which we prefer to it n soft detector we may then use the full voltage on tile the detector plate A midget variable condenser Is connected across neresa the tills this condenser Is not used for tuning but la Is set permanently it la Is therefore mounted inside the cabinet and not en ca the panel in mounting the parts leave sufficient distance stanco dl between the two vart vari ometery to avoid back coupling it will bo be well to mount them nt at opposite ends of the panel and to leave the middle for the audio amp amplifier lIfler in selecting tho the parts for your sour receiver keep in mind that the tion in this circuit to Is extremely high therefore select nil all component parts from the best available and do not lot bo be too niggardly as to space midget radio frequency rec receivers elOrs do not work well to tune this receiver begin by setting the aerial condenser nt at its minimum then tuce the it F varl ometer until you hear either cither the broadcast of a station or its beat note tune the aerial varl ometer until signals or whistles come in loudest if the set Is still oscillating turn the aerial condenser a few degrees and turn the varl ometer back baci until the aerial circuit Is again in tune if the set Is still oscillating turn tit the aerial condenser ahead a ME bit more and ulo the varl ometer back a corresponding amount this until you ou find h i setting that will keep keel the receiver from oscillating you will then have tile the maximum of sensitivity at its best one stage in this circuit will be equivalent to two in many others so far we have said nothing about tile the midget midge t condenser in parallel with II 11 r F varl ometer I 1 if the receiver covers the entire wave length range with the midget condenser all out leave it there if you flud find however that you ou cannot tuno tune in WH or for instance turn the midget until these stations como come in oat i at near vic the maximum setting of the it II r P it will bo be well to lock the ho midget midget in that position then yo oi it may calibrate call brato the it P varl ometer for stations you will find that this calib calibration pation holds true regardless of changes jn in the aerial this receiver will give you vou coast to coast reception on oil an outside norial in a good location in cramped quarters you may use a 20 toot foot wire w tro and eipert loud speaker reception of all local stations and also some powerful distant stations new york sun bun |