The "Home-made" Three

Radio Record, Volume III, Issue 14, 18 October 1929, Page 28

The "Home-made" Three

Refinements for Smoother Operation

By

CATHODE

HB improvement most urgent- : ly called for is the fitting of ®)} a proper panel and the inclu3 sion of a filament switch pro viding greater convenience than the practice of connecting or disconnecting one of the battery clips. The measurements of the nanel are: Leneth 1% inches, width 7 inches. The materia! used is of no particular importance, since the only component which is not at earth potential will be the reaction condenser when this is finally inecorporated. A very useful and service able panel which, although being very much less expensive than the conventional ebonite; is. nevertheless almost as efficient, is one of three-ply coated with duco finish. This, when dry, is rubbed with fine glasspaper, reducoed and polished with any suitable polish The result is a handsome efficient panel, Where possible, the duco should be sprayed on. The baseboard being of a fair thickness, panel brackets. should not be necessary. It is assumed _ that the variable condenser used will be of the single hole fixing variety, and where condensers having. several mounting screws are used, the drilling template supplied with the condenser must be resorted to, the shaft of the condenser occupying the positior shown in the diagram for the single hole mounting. Only one tuning condenser (that already in use) and the filament switch will be mounted on the panel at this stage, but it is wise to do all the drilling before the panel is screwed to the baseboard to avoid the difficulty of doing it later. The tuning condenser may be removed from its temporary mounting and mounted on the panel proper. The filament switch should also be mounted on the pane] about 34 inches from the end. In making a choice of a filament

switch avoid those in which a small lever is rotated through an angle of 90 degrees to switch on or off; switches of this type are quite satisfactory in most circumstances but, mounted below two projecting knobs, as will be the case ‘here, they are distinctly inconvenient -a tumbler or toggle switch, or 4a

switeh of the pull-out type, will be more suitable. ' The connections to, the tuning condenser will, of course, be unchanged. It is quite possible, if the condenser has previously been mounted right down on the baseboard, that one or both of the wires connected to it will be too short to reacl it in its new position. Filament Switch. ‘THE inclusion of the filament switch. will necessitate certain changes in the wiring. As things.are, a wire runs from the positive filament terminal of the detector valve-holder to the "A"+terminal on the terminal strip and thence to the low potential end of the detector gridleak. Disconnect this wire from the "A’+ terminal and in: ae a —

stead take it direct from the detector valve-holder to the gridleak; it will be advisable to leave the wire a little longer than is absolutely necessary, in order to avoid havin, it too short to make the necessary connection when the temporary gridleak is replaced by a commercial. product.

ESTSSSSSSSSS SSSR eeeesAseeeeeeeeeees Next connect a wire from the "A’+ terminal on the terminal strip under the baseboard to one side of the filament switch; the whole length of this wire had best be encased in insulating sleeving. Finally connect the other side ot the filament switch to the positive filament terminal of the last valve-

holder (where‘ there is, of course, one wire: already connected). This completes the wiring of the filament switch. To put the receiver in working order again all that igs necessary is to connect the wires left at a loose end when > the little terminal strip carrying the loudspeaker term‘ .als was removed, to the same terminals, now mounted on the panel in the position indicated. If the wires are too short, they must be_ replaced by ‘longer ones.

Transformer Coupling. AVING arrived at this stage, the receiver may be put into commission again until its constructor feels disposed to indulge in further expenditure to bring it nearer ‘its final form. It ig suggested that, as the next step, the resistance-capacity coupling between the first and second audio valves be replaced by a transformer, thus substantially increasing the volume. Before describing this change, a word of explanation is desirable. The lay-out has been so nged that the constructor is offere.. xf choice between a single output valve, equipped with a choke-capacity output filter, and a pair of output valves arranged in ash-pull, The merits of push-pull amplification have been discussed before in these columns; for the present it will be sufficient to say that the volume obtainable from the two valves will be substantially greater; on the other hand, the drain on the "B" battery will be several milliamps greater, thux shortening its life, and it will be necessary to purchase both input and output push-pull trans-. formers at once instead of, as can be done in the instance of the single output. valve, purchasing and installing the intervalve transformer first, leaving the output filter to be added when convenient. Both arrangements are illustrated, the push-pull system in Fig. 2, and the

single valve arrangement in the wiring diagram showing the result of making all the improvements suggested in this article. The necessary alterations/ in wiring are rendered so clear by; the illustrations-particularly if these are compared with the original wiring diagram in the issue of September 6-that a verbal description seems unnecessary. The coupling condenser left over when the resistance capacity coupling ig replaced by a transformer may be

N our issue dated September 6 "Cathode" described a three-valve resistance- — eoupled set that could be assembled for a very modest sum. The plan adopted has been to describe a number of improvements in each article, -but to arrange matters:so that only one such improvement need be carried out at any one tinie. Readers will have recognised that the aim of the "Homemade Three" is to cater for those of us whose pocket books are unburdened with undue adipose deposit and, if it were made necessary to carry out.a number of alterations at once, financial embarrassment might be the result.

nountea alongside and connected in parallel with the existing coupling condenser for the first stage, its effect being to accentuate the bass notes slightly. Where’ the mounting bases illustrated are used, the- two con densers may be mounted one on each side; it will perhaps be necessary to replace the existing fixing bolts by slightly longer ones. Output Filter. IN regard to the 2mfd. condenser in series with.the speaker in the output filter, a word of caution is desirable. It-will be observed that this

eondenser is-unless the speaker is dis-eonnected-connected directly across the B battery, whether the set is . switched on or not. A leaky condenser in this position would obviously have the effect of running down the B battery, possibly with considerable rapidity, so be sure that the condenser used is one of first-class quality. In view of this objection, some constructors may wonder why this particular design of output filter is preferred té one of those in which the condenser is not connected across the B supply. The "object of adopting this design is to keap the audio frequency currents out -of the B supply, which, were it of appreciable impedence to these currents, mig™ be the cause of distortion, or, worse still, an audio-frequency howl or "motor-boating." A further refinement having the same object is that of incorporating a filter circuit in the B supply to the detector valve. This filter circuit comprises a 2 or 4 mfd. condenser (which must be of the highest quality for the same reasons as are outlined above), and a wire-wound resistance; the precise value of this resistance is not important, and if the constructor has one on hand or can readily obtain one of any value between 10,000 and 50,000 ohms, it may be used-about 20,000 , ohms is the usual value. The wiring diagram depicts the manner of connection véry clearly, and it is not considered necessary to give any additional directions. © When incorporating this filter cireuit for the detector, it is by no means a bad plan to take the opportunity of

replacing the home-made resistance of 150,000 ohms by a commercial wirewound resistance having the same value, or perhaps a little lower, say, 100,000 ohms. Or, of course, if the constructor prefers; the change in the plate resistance may be made first, leaving the filter circuit to be incor porated at some future time. In referring to the wiring diagram, the costructor must clearly understand that only the portions in which changes have been made are depicted. This has been done not only in order to avoid unnecessary work, but also because the changes are rendered clearer thereby. The new diagram should be studied in conjunction with the original wiring diagram published in. the issue of September 6, when the precise nature and extent of the changes will be immediately apparent. No negative filament circuits are shown, since the only changes in the filament circuits are those necessary for the incorpora-_ tion of the switch and these affect only the positive circuit and have been verbally described in detail. Other Refinements. [t is not anticipated that even the slightest trouble will be encountered in making any of these changes, particularly if, as has been suggested, they are made one at a time. When the second resistance-capacity coupling has been replaced by a transformer, try the effect of increasing the 14-volts negative bias on the first audio valve to 3 volts negative. It will depend principally on the particular -valve in use whether results are the same or worse; if they are no worse, leave the bias at 3 volts negative, as_this will economise in B current; if, however, quality is impaired, return to the 1}volts negative. The. improvements described have been dehlt. with more or less"in the order.in which they should be made. In order to dispel any shadow of doubt, however, the following suggestions as to order may be followed, particularly where it is desired to spread the expenditure over,as long a period as possible and have the receiver out of commission for only a day or so ata time :- (1) Fit panel and wire filament switch, then put receiver into commission again until funds for next improvement are in-hand. (2) Fit inter-valve transfor mer, or, if push-pull output is adopted, both input and output push-pull transformers. Put receiver into commission again until output choke and condenser are:acquired. (3) Fit’ output filter (assuming that a single output valve is used) and again introduce the receiver to the family circle. (4) Replace home-made plate resis‘tance for detector valve by commercial wire-wound resistance, the sef then being again available for use, (5) Incorporate filter circuit (wirewound resistance and condenser) for detector valve. All these changes having been completed, the receiver should now give really excellent volume and quality on the local station, providing components and construction are both sound. It is still, however, unsuitable for distant reception, although some patient listeners may be able to achieve fair results in this direction.