For Simplicity, Cheapness and Power, Build... The "Eagle Five"

Radio Record, Volume V, Issue 19, 20 November 1931, Page 15

For Simplicity, Cheapness and Power, Build... The "Eagle Five"

(By the

Assistant Technical Editor

VE of the first screengrid valves to appear on the market some four years ago-when this type of valve made its debut-was the 8625. This is a cylindrically-shaped , | valve with a base at each end + and with a special holder for

mounting. Constructors of those days will remember what an excellent valve the 8625 was, and its marked superior‘ity over the triode as a radio frequency amplifier. ven to-day, when used in a suitable circuit, this valve is almost capable of holding its own with more modern types. : On learning that there was still quite a number of these valves obtainable in this country at a greatly reduced price, we conceived the idea of designing a powerful receiver incorporating them, and, while still giving first-rate performance, to run out as cheaply as possible.’ We have achieved this in the Eagle Five-a powerful and easily constructed receiver of low cost. Briefly the set employs five valvestwo stages of screen-grid r.f., detector, and a stage of resistance capacity coupled audio feeding into a transform‘ar coupled output stage. Resistance capacity coupling has been used for two reasons. JTirstly, cheapness-it eliminates an audio transformer-and, secondly, tone. Two stages of t.¢.¢c., however, would not give enough "lift," and, what is more, would tend-to accentuate the treble. Again, with two transformers, amplification on near-by stations is often excessive, with resultant overloading of the output valve, and, in addition, tone often has a.tendency toward prominence of bass. Thus.a combination of the two methods was finaily selected. The Cireuit. Now for a few words about the circuit, Which is really quite conventional. The two r.f. stages are autocoupled, or, in other words, the lead from the plate of the first r.f. valve is tagen through a condenser to a tapping ormthe next secondary-in this case, halfway down the coil. By auto-coupling in this way the S625 is, to a certain extent,. modernised, for the effect of inter-electrode capacity is

reduced by one-quarter of that it would have in the normal "tuned grid circuit," in which the anode of the first valve is taken to the top of the next secondary. Though a certain amount of amplification is lost with auto-coup-ling, yet improved stability is achieved. The loss of amplification is to a certain extent offset by the design of the coils The first coupling condenser has a capacity of .00025 imfd., and the second .001 mfd. These values especially the second, should be strictly adhered to. The r.f. chokes in the plate leads of the two screen-grid valves block the r.f. current and pass plate current, and in order to ensure stability should be of high quality. The two imfd. fixed condensers and the two 1000 ohm resistances in the screening-grid circuits of the first two valves are included for de-coupling purposes. The specified capacity, i.e., imfd., of these two fixed condensers should, within limits, be adhered to, but the value of the resistances chosen: is not so important. However, a very low value of resistance hardly provides a sufficiently effective barrier to rf. current, but the .choice of too high a value will do no more than lower the screen voltage by an_ insignificant

amount, Resistances with non-induc-tive windings should be chosen. It will be noticed that the second stage of r.f., like the first, is wellscreened from the remainder of the set. This is vitally necessary, for the r.f. currents have been once amplified and insufficiency of screening would be certain to cause instability. The sec-

ond r.f. stage is not auto-coupled t¢ the detector because in that circuit com ditions different from those obtaining in the r.f. stages exist. Leaky-grid detection is employed as being the most sensitive and because it requires the least plate voltage-an important factor in a battery-operated se’ ‘Reaction is controlled by a differen reaction condenser. As with the "Outspan Five," we debated whether to include reaction of omit it and gang all tuning condensers. Reaction won. A great majority af our constructors, we believe, desire to get the last ounce out of a receiver and to do this they have not the slightest objection to manipulating several controls. In fact, some set-builders we have met aver that the more cies there are on a set the happier th are! Of course, reaction can be us quite successfully with ganged condensers, but only if the coils are scientifically matched-a difficult task for the amateur. Even in this case, however, there is a slight loss in sensi+ tivity. It will be noticed from the photographs and layout diagram (to be published next week) that all, de coupling resistances, chokes, and by

es "pass condensers, together with the gridbias battery, are located underneath the. aluminium baseboard. Thus, shortness of ‘wiring and neatness of layout have been secured, while the above baseboard appearance,of the set is considerably enhanced. Building the Set. Now for the set itself. The first thing to do is to procure the aluminium baseboard and shielding, the panel and the necessary parts. These should all be obtained and checked up against the list of parts given on this Page before, construction is commenced. The baseboard and vertical shield compartment may be bought ready made or can: be shaped by the constructor from sheet aluminium from the dimensions given. © Unless suitable tools are to hand, however, we strongly advise the former. To avoid cutting gaps in thie aluminium and thus reducing the ffectiveness of the screening, the valve olders themselves are cut in half and mounted, on.each side of the shield, ir the manner shown. ‘The leads running to the. ends of the valve holders marked "A" and "G2," should be oz insulated flex to allow this portion of the valve holder to be withdrawn about 4in. from the shielding and the valve thus inserted or withdrawn. The Coils. FTHR securing all the parts, the next task is to wind the coils. These are quite straightforward, and should present no difficulty. All are wound on a 2in. former, which may be ‘of ebonite or glazed cardboard. One 4in, and two 3hin. lengths of this will be required. The three secondaries are wound with 26- d.ec. wire, while the primary of the first r.f. coil and the reaction coil are wound with No. 82 d.s.e. About ib. of the former and a few yards of the latter will be ample. Now for the first r.f. coil. This will have three soldering lugs at its base, for connections to aerial, earth, and the grid of the first valve. Care should be taken when commencing and finishing off windings that these are taken to points nearest to those to which they are to connéct. For example, in the first coil, the soldering lug to which the top of the primary is connected should be directly opposite the aerial terminal. Again, the bottom of the primary and the bottom of the secondary, which are taken to.a.common tap, should be as close as possible to the moving vanes of the first variable condenser. Finally the top of the secondary should be taken out to.a point which will give te shortest connection with the grid the first valve. It is in watching Litle points like these that shortness in

wiring. and increased efficiency is obtained. The constructor may get 4 general idea of where the soldering lugs should be mounted on the coil from the photograph reproduced above, but next week a layout diagram showing clearly the disposition and wiring of every component will be published. Now for the coil itself. About a in. from the end of one of the 34in. lengths

of former make two small holes parallel to the edge of the formér and at right angles to its axis. The end of the 26 d.e.c. wire is doubled through these and attached to a soldering lug mounted on the base, and’85. turns are wound on. The end is doubled through two similar holes to prevent the coil unwinding, and then taken down the in-

side of the coil to the soldering lug designed for it. We are now ready to wind on the primary. Ten thin ebonite spacers, preferably of semi-circular cross section and about #in. long, are then stuck with seccotine, or glue, at equal intervals round the bottom of the secondary. The coil of No. 30 d.s.c. is now taken and the end attached to the soldering lug to which is soldered

the bottom of the secondary. T'wentyfive turns, slightly spaced, are then wound on. ‘ The end may be anchored at the top by two methods, In the first, the turns of the secondary are pushed: slightly aside and a small hole made in the former underneath. The end of the primary is then passed through and down

inside the coil to the soldering lug intended for it. Alternatively it will be found that the wire. will be held quite securely if a turn is taken round one of the ebonite spacers and then taken down outside the coil to the soldering lug. Two points about the winding of this primary. A strip of celluloid or of brown paper may be substituted for the ebonite spacers. Correct spacing of the primary, i, the thickness of one turn, is rather difficult to achieve unless the following procedure is adopted: Secure a piece of thread or thin twine, of about the thickness of the wire employed for the primary, and wind it on together with the primary, alternating the windings. At the completion of the coil the thread is then unwound and the primary will be correctly spaced. When the two angle brackets for mounting the coil on the baseboard have been screwed on, the coil is complete. The next coil, which consists of 85 turns of No. 26 d.c.c., centre tapped, 13 the simplest of all, The same procedure regarding the placing of the soldering lugs should be followed with this coil. and the winding commenced. After 424 turns have been wound on, a hole is made in the former, and the wire doubled and threaded through. It is then taken to the soldering lug, the double end being bared for attachment. In this way the soldering on of a tapping is avoided. The winding of the coil is now continued until another 42 and a fraction of a turn are wound on. The necessary holes are pierced and the ends taken down in the ordinary way. ; The secondary of the third coll is wound on in exactly the same way as the first, but the number of turns is reduced to 77, owing to interaction of the secondary with the reaction coil just above it, and the reduction in number of turns is necessary to give approximate matching of the condenser dial readings. Pxactly 1-8in. above the top of the secondary two holes are pierced and the reaction winding. consisting of 28 turns of close-wound 32 d.s.c. is placed on. The end of this windine ts taken to a soldering Ing on top of the former er enough wire is left after anchoring «Continued on page 30.)

The "Eagle Five" (Continued from page 17.) to make the required connection to the differential reaction condenser. The coils are now complete, and to gether with the other above-baseboard components, should be placed.in their correct positions and screwed dowa. The disposition of the components is shown in the above-board photograph, but should any uncertainty be felt the constructor should wait for next week’s issue, which will contain both above and under-baseboard layout diagrams. The front panel should now be carefully measured out and drilled to take the shafts of the three tuning condensers and those of the rheostat and reaction condenser. All the necessary dimensions are shown in the sketch. In the next and concluding instalment further hints on the construction and operation of this receiver will be ‘given.

rp eE> v List of Parts for the "Eagle Five" — eee |

Aluminium Baseboard and Bereen. Panel Set cf 3 Coils, as described. 3 .00035-mfd. Variabie Condensers 1 00027 Differential Condenser. 3 Vernier Tuning Dials, 2 Small Dials for vheostat and reaction, 2 1000-ohms Decoupling Resistances, 3 1-mfd. Bypass Condensers. 1 ,00025-mfd. Mica Wixed Coupling Condenser. 1 .000t-usfd. Mica Fixed Coupling Condenser. 1 .i-mfd. Mica Fixed Coupling Condenser. 1 .0003-mfd. Grid Condenser. i 3-meg. Grid Leak, with mount. 1 100,000-chms Grid Leak, with mount.

1 250,000-ohms Grid Leak, with mount. R.E. Chokes. 30-ohms Rheostat. i Audio Transformer, 34-1 or 5-i. 3 UX Valve Bases. 1 On/Off Filament Switch. 2 $625 Type Valves, with special bases. , 3 Valves for Det., ist and 2nd Audio. Battery Cable. Aerial and Earth, and Output Terminals, mounted on blocks. Angle Brackets, Screws, Glazite, ete. "A", "B" and "C" Batteries, with 3 "C" Battery Wanderplugs. mm &