Fitting Headphones to an All-Electric Receiver

Radio Record, Volume V, Issue 4, 7 August 1931, Page 18

Fitting Headphones to an All-Electric

Receiver

[Contributed]

---_- } OST radio enthusiasts have at m)i some time or other wished to | connect headphones. or other ui, loudspeakers to their. allé|| electric radio set, but they 24 have not done so, as they j have thought that the task was too complicated. This article describes three ways of fitting headphones to an all-electric radio .set; the first method is for sets whose last audio stage is push-pull and requires no alteration to the set or speaker; the second method, for sets whose audio stage is push-pull, makes a more permanent job than the first method; but entails -altering. .the dynamic speaker. This is not a difficult task, but it is only practical if the output transformer is in the base of the speaker. The third method is for sets whose last audio stage is not pushpul. In these instructions take no notice of the field coil wires froin the set to the speaker except when counting the number of wires in the third method. First Method. TPHIS method uses two adaptors to make contact with the plate con nections of the two valves in the pushpull audio stage. The first job is to make the adaptors. . Cut two strips, each 2in. x fin., of very thin sheet copepr. Then take the valves out of the last audio stage. Drill a hole in each strip not quite a quarter of an inch from the end. The holes

should be of such size that they make a tight contact when slipped over the plate prong of each valve. The best way to make the holes is to drill them on the small side and then to ream them (the points of an old pair of

scissors will do for a reamer if you do not have one) to the exact size, To ‘find the plate prong on the valve base, see fig. 1. Then slip one of the strips over the plate prong of cach of the valves, take from the last audio stage. Point each

strip parallel with the pin on the side of each valve (Fig 1), bend them up the sides of the valve bases, and then bend about a quarter of an inch outwards at the end of the strips (Fig. 2). Then take the strips off and insulate them by glueing one thickness of ordinary writing paper over them from within half an inch of the tops to within an eighth of an inch of the holes. Solder a piece of flexible wire of suitable length to each of the bare ends of the strips (Fig. 3). Then connect one of the wires to a terminal of one Imfd. condenser, and the other wire to another 1Imfd. condenser, and the other wire to another imfd. condenser. The headphones. or other speakers are then connected to the other terminal on the condensers (Fig. 4). Slip the strips over the plate prongs on each valve, and be sure that the corners of the ends of the strips cannot touch the other prongs. If there is any doubt about it, clip the corners off, but be careful not to clip away too much round the hole, as this would weaken the hole and the adapters might work loose. Put the valves back in their sockets, and the installation is complete. The condensers may be mounted in a small box witha phone jack in the side of it, if desired. When the valves are mounted on metal shields, be very careful that the bare

strip does not touch the metal. ‘Insulate it carefully. Second Method. OUNT the number of. wires going into the dynamic speaker, or if the spenker is connected by a plug or by plugs, count the total number of prongs on the plugs. (Do not forget this applies only to sets whose last audio stage is push-pull.) If there are only four wires or prongs the output transformer must be in the set, and you should use the first method of connecting phones. If the speaker is mounted on the audio amplifier or power-pack, it is wiser to adopt the first method of connecting the phones. Disconnect the wires going to the set from the speaker by pulling out the plugs or unscrewing the terminals and unfasten the screws and bolts securing the speaker to the cabinet. Gently lift the speaker out of the cabinet and turn. it upside down so that you can see right (lown the base. In some speakers: the connections to the output transformer are covered by a metal cover, and this must be remoyed by unfastening the

nuts that hold it down. You will see three wires coming in from the set and two going up to the voice coil of the speaker, ee On some" speakers the wires from the set go into the transformer on one side and the voice coil wires come out on the same side, but generally they come out on the opposite side. The wires going to the voice coil go right up into the speaker and are generally riveted to the frame by insulated rivets. Solder, or screw, as the case may be, pieces of flexible wire to the two outside connections from which the wires go to the set (Fig. 5). Next drill a hole in the base of thalil speaker large enough to take an Igranic insulated jack, or, alternatively, an

ordinary jack and an insulated bushing. * Be careful that the drill does not go through the metal suddenly at the :end and damage something on the other side. Mount the jack, and if it is an ordinary jack with a bushing be careful that no part of it is making contact with the frame of the speaker. , Mount the condensers in the base of the speaker by drilling four holes in suitable positions and bolt them in position. Run the flexible wires to a terminal on each condenser, and connect the other terminals to the jack (Fig. 6). If desired, a switch can be inserted in series with one of the voice coil leads, so that the dynamic speaker may be turned off and yet leave the headphones or other speakers on. The best switch for this purpose is the panel mounting A.C, snap-switch that is used for all-electric sets, as in this switch, the knob that operates it. is insnlaty from the electrical contacts. a Mailing an A.C. snap-switch an ordinary switch with insulated bushings ean be used. Third Method. FOLLOW the same directions given in’ the second method, remembering that

PARTS LIST. Virst method: 2 1 mfd. condensers. _ 2 strips 2in, x fin. thin copper. . : Flexible wire. Second method: 2 1 mfd. condensers, Plexible wire.: 1 Igranie: insulated jack, or 1. ordinary jack with insulated bushings. Third Method: 2 1 mfd. condensers. Wiexible wire.

(Coneluded on pzge 30.)

Fitting 'Phones

. (Continued from page 16.) two connections going into the output transformer from the set. Disregard the number of wires going into the speaker. The jack on the speaker can be dispensed with (in the case of midget sets a jack cannot be used, as there is no room to mount it) and the phones are connected as shown in Fig. 4, taking no notice of the adaptors. If a jack is used connect as shown in Fig. 6, the two flexible wires go to the two connections on the transformer from which the wirest go to the set. When you connect headphones to a powerful all-electric radio set, you should be very careful with the volume

control, because if you turn it up too far a sudden loud burst of static might not be too good for your ear-drums. Also it is bad for the ears to pass quickly over a local station when tuning in with the earphones. A separate volume control could be employed by connecting .a 100,000 ohm variable resistance across the phone terminals.