The "Eagle Five"

Radio Record, Volume V, Issue 20, 27 November 1931, Page 15

The "Eagle Five"

(Part II)

(By the Assistant Technical Editor.)

| AST week we concluded" our deseription of this receiver at the point where all components, including coils, the two r.f. valves, and the vertical screen were mounted on the aluminium baseboard ready for wiring up. The panel components should also be mounted, but to facilitate wiring, the panel itself should not be screwed to the baseboard until absolutely necessary for connection purposes. \ Before commencing wiring operations carefully check up by means of the two layout diagrams given this week, to ensure that both the above and below baseboard components are positioned correctly. The position of the under-baseboard components, relative to those above, should be carefully checked to ensure that the wiring running through the aluminium baseboard is kept as short as possible. The following two small discrepancies between the photographs and the layout diagrams should be noted. In the latter a filament off-on switch and a series aerial condenser have been included. ‘The latter was not built into the original model because in the experimental test to determine a suitable capacity, it was found more convenient to include the aerial condenser actually in the aerial lead. The value finally chosen aS one giving good. selectivity while not reducing strength on distant stations was of .0001 mfd. Any value up to .00025 or .0003 mfds., however,. may be used if selectivity is not important. The off-on switch should be of the compact type illustrated, and because it is mounted on the aluminium baseboard, a switch in which the — mounting is insulated from the internal portion should be chosen,

Glazite, or No. 22 bared copper wire covered with spaghetti, is most suitable for wiring the receiver. All wires should be run as directly as possible, and those leading to the under-base-board components should be taken through small holes in the baseboard drilled in the most convenient’ positions. As mentioned before, the leads from the two left-hand portions of the screen-grid valve holders should be of flex and enough: wire should be used to allow these portions of: the. valve holders to be withdrawn about din. for

the insertion and withdrawal of the valves. In the experimental model the series aerial condenser and the two coupling condensers in the plate leads of the sereen-grid valves are of the new flat compact type, measuring only about lin, long x 4in. wide and 1-8in. deep. Because of their small size, these are particularly suitable here, and their high break-down voltage ensures an ample margin of safety. Incidentally, in last week’s instalment of the deseription of this receiver, the value of the second s.g.r.f. plate coupling con-

denser was wrongly given in the text aa .001 mfd, The value of .0001 mfd. as given in the parts list, is correct, and this value, together with the .00025 mfd. used in the preceding stage, should be strictly adhered to. A seven-wire battery cable will be needed. To save terminals and extra wiring, this ‘is run through a #in. hole in the back of the baseboard to- the ‘underneath portion of the set, and the various wires taken directly to their flesignated positions. All the abovebaseboard components, with the exception of the two r.f. chokes, which are mounted by means of long screws passing through their centres, are attached to ‘the baseboard by means of angle brackets. The mounting of the two decoupling resistances in this way is clearly shown. ._ The rheostat regulating the filaments ‘of the two screen-grid valves is ineluding in the — lead. This is so because the wire connecting the screen-grid valves with one side of the rheostat runs the whole length of the set, and in order to minimise the danger of undesirable coupling, it was made the negative lead, which is earthed through the other side of the rheostat. The Sereen as a Return. ‘WORD of caution about using the screening as a convenient means of making earth returns. Though it is apparent that by such means the length of wiring is materially shortened, in practice it is often found that the indiscriminate use of screening for earth returns produces serious instability. However, there are one or two general rules which, if followed, will prevent such undesirable results. First of all, the screening system may be safely used for the completion of the negative side of the filament cir-,

ruit. As in the "Bagle Five,’ however. the negative side of the s.g. valve filaments are not earthed, through the rheostat, it was not considered worth while to earth "F-" of the three remaining valves by means of direct connection to the screen. If, however. the constructor desires to do this, all that is necessary is to take wires from ‘Y-" of the detector, 1st, and 2nd audio valve holders, to the bolts which hold the valve holders in place. The "H-" negative lead in the battery enable would then, of course, have to be connected to the screening by means of some convenient nut on the underside of the baseboard.

The question of the earthing of one side of the de-coupling condensers appears to be a question rather difficult ta decide. In the case of the ‘Eagle Five" it has been found quite safe to run leads to the nearest points on the screening system, but in other receiyers we have built it was found necessary from the point of view of stability to take the connections straight to the negative filament terminal : ° the valv« eoncerned. It is undoubtedly in ‘the tuned circuit, however, that it is most important to use discretion in employing screen returns. In general it is a wise rule not to use such returns at all, but it is often possible to use them

to some limited extent without resulting harm. The Batteries. WORD about the "A," "B,"’ and "CO" batteries. The valves recommended for use in the detector an: audio stages are:-Detector, an R.C.C. valve, type A630; Ist audio, a valve of the A609 type; and. 2nd audio, a smail power valve of type B609. The bias required for these valves is not more than 9 volts, so we have been able to mount the small bias battery under the baseboard: If valves taking a larger piate current than those mentioned are used, however, a larger bias batters,

ta give correct bias values, should be used. ‘If possible a six volts accumulator of generous capacity should be used for the "A" supply. As an alternative, however, eight 14° volt dry cells connected‘in series-parallel will be found to give quite satisfactory results, . Regarding the "B" supply. Only heavy‘. duty: batteries’ should ‘be used. . Two- 45-volts , blocks will be found to give results, but for best results ' at least tliree should be used. Of course,’ a suitable "B" eliminator ig ideal. Operating. the Set. B® will now assume that the wiring is completed and the set is ready for the initial try-out. First of all connect up the "A" battery only, turn on the filament switch, and rotate the rheostat. The screen-grid valve filaments should light up brightly, or glow’~ a dull red according to the position of ) the rheostat. If this. does not happen,’ then the mistake in the filament wiring should be immediately located and rectified before proceeding further. No glow will be seen from the last three valves, however, and their filament circuits should be tested. in the manner outlined in the article on the "Night Owl Three," on page 29 of last week’s issue, ‘If the constructor is doubtful of his wiring capabilities, both screen-grid valves should be removed while the "BY" voltage is applied to the holders to determine whether it has accidentally been connected across the filament circuit or not. In any case, however, the circuit should be carefrlly re-checked before tests are carried out. Incidentally, if the speaker is not connected up, there will be no plate voltage on

the last valve, and hence any wiring mistakes in this circuit will not be revealed in the preliminary tests. .. Assuming everything is O.K., the contructor should insert all the valves nd connect up the batteries, The aerjal and earth are then connected to the set, and it is ready for operation. By the way, because resistance capacity coupling has been used, it will be found necessary to place at least 100 volts on the detector plate because of the large voltage drop across the resistance in the plate circuit of this valve. The three tuning condenser dials

approximately in step, and the reaction condenser dial advanced until a hissing sound, or plop is heard. If you cannot obtain this, turn reaction full on and ‘manipulate the tuning dials, listening all the while for the set to sound "alive." If this does not happen, reverse the connections of the reaction coil to the reaction condenser and try again. Jf it does happen, the tuning dials should then be slowly rotated, keeping them in such a relation with one another that the "live" sound persists. A ; carrier wave should then be picked up, . and by careful operation of reaction

and of the tuning dials the station should be brought in. If the set cannot be made to oscillate, however, the following procedure should be adopted. First of all take the aerial to the terminal of the grid condenser farthest away from the grid-

leak, and turn out the filaments of the screen grid valves by means of the rheostat. The set is now functioning as a three-valve. Now touch the gridleak with one finger. If a "squawk" is produced, the detector and audio systenis have been moved up correctly, See also if the set will oscillate, or bring in any nearby station. If it will do neither there is a mistake in the wiring, and this should be rectified before proceeding further. If it does oscillate, take the aerial to the plate of the first sereen-grid valve, and turn on the filaments. ‘The set should still oscillate and signal strength should be greatly improved. If no improvement is noticed, however, or no signals are received, disconnect the first coupling condenser in the plate lead of the first rf. valve and attach to it a six to geven-inch length of flex. The other end of this should be taken to the terminal on the grid condenser most remote from the detector valve, and the second coupling condenser running to the same terminal should be disconnected. The set is now still functioning as a four-valve receiver with one stage of sg.r.f., but this time only the first s.g. valve is in operation.

If signal strength is greatly improved then there is some mistake in the wiring of the second s.g.r.f. valve, and this should be located and rectified. When first tuning the receiver it may be noted that the rotating of one of the condenser dials makes no difference to the strength of signals received. In this ease there is a short between the fixed and moving vanes of the condenser in question. This is particularly likely to occur in the second tuning condenser, which is mounted through the vertical screening. If the shaft is not adequately insulated from the screen, the mov(Concinded on page 80.)

The "Eagle Five" (Continued from page 17.)

ing and fixed vanes are certain to short through it. Conclusion. 7" ¥ the constructor carefully and wires up this receiver-and there is nothing very difficult in doing this-we are confident that it will give him no trouble whatever. With regard to its capabilities. We first tested this set between 11.0 and 12.0 p.m. one evening, and in that time no less than 24 Australians were heard on the loudspeaker at strengths varying from R4 to R9. This, and other tests, convinced us that for dx work there is very little to choose between this receiver and the. "Outspan = Five’one of the most popular _ sets we have described. This statement, combined with the fact that the "Hagle Five’ may be built, including the valves, at a cost not exceeding £8/10/--an unnusually low price for such a set-makes this receiver one which every constructor will want to build.