The "Akarana" Band-Pass Four

Radio Record, Volume IV, Issue 6, 22 August 1930, Page 22

The "Akarana" Band-Pass Four

Screen-Grid R.F . Stage Contributed by an Experimenter

" ‘HIS is a four-valve battery-operated ‘" yeceiver with screen-grid radio valvye.and a band-pass filter. Working drawings and particulars of construction have been contributed by Mr. H. i’. Bashford, of Blake Street, Avondale, Auckland, who is an experimenter of considerable experience, dnd claims that after trial of many components a highly efficient four-valve receiver is the result, Situated in Auckland, this receiver will give, it is stated, full loudspeaker reproduction of 2YA in the daytime. It is recommended that constructors should adhere closely to all measurements, which can be ascertained from the lay-out plan, and a most important point is that the variable condensers should all be of the same make, because the products of different makers, nominally the same, have not the same eharacteristics. An aluminium base is specified, and to eliminate possibility of a "short," a dise of insulating material should be placed under each valve socket.. All wiring is carried out with "Glazite," and where necessary may be covered with spaghetti as an extra precaution, especially. where passing through the shielding. The aperture in the partition through which the screen-grid valve passes should be made at. a height suited to the arrangement used for holding the valve socket. All components are bolted to the base with 1-8in. screws and nuts. A good method of bending aluminium ds to place the sheet between two pieces of board with the proposed flange projecting. This may then be tapped lightly with a wood or hide mallet and gradually brought to a right-angle. Four pieces of }in. vulcanite in. long are tapped and screwed to the four screws that fix the side panels to the base. These act as legs for the reeeiver, and also as insulators. Drill the front and rear panels lin. from the top to take 1-8in. bolts and nuts to support sides when in position. Note that all dotted lines shown in the wiring plan indicate wires, mostly to filaments, passing underneath the base. ‘Band-Pass Filter Coils. FA VALVE-BASBH is used for convenience of plugging in, a round block of wood itin. high being turned to two diameters, as shown in the diagram.

The lower portion fits into. the valve base, and is'secured with seccotine, after winding coils and threading leads through holes in block and valve base. The after portion fits into the lower end of coil former, which is secured in position with three small brass screws. Note that the finish of the left-hand coil ie exactly in line with the start of the right-hand coil. The 30-gauge wire is spaced 1-8in. from the 28 gauge.

A strip of formica 3-16 by #in. is recessed 1-16in. as shown, so that it stands clear of the windings, while acting as a support for the side coil. Washers are placed between the strip and the coil formers on both sides, so that the windings are jin. apart. In order that the coils may be readily interchangeable, an insulated terminal may be put into the screen immediately above the screen-grid valve. A flexible wire is taken from the .0005 condenser to this terminal, and also one direct

from the valve terminal to the insulated terminal. ‘The flexible wire from the 1 mfd. condenser has a small "crocodile" clip attached to it, to clip to the "start" terminal of the right-hand coil. The "finish" terminal is then connected by a flexible wire to the insulated terminal on the screen, Always switch off the "B" battery before changing coils, otherwis sparks will fly if the clip should tetich the shielding. ° a A sheet of glass in a wood frame isspecified as a cover for the top of the receiver to exclude dust, but there does not appear to be any reason for not completing the shielding with an aluminium top. The screen-grid valve used in the original was the A442, with A415 as detector and first audio. When a 6-voli accumulator is used with UX type valves in later stages, a total resistance of 34 ohms is required in the filament circuit of the A442,‘or 22 ohms for the UX222, but the use of the latter would entail slight alterations in the lay-out owing to the grid connection being on the.top of the valve. In the original a 4-v6lt pentode was used as the output valve, but in this stage the substitution of a diffe¢ent valve is a simpler: matter than in the rf. stage. A trimmer condenser of 16 mmfds. is shown in parallel with one of the gauged .0005’s and is situated on the. right-hand end of the receiver. A diagram of the front panel is given, showing a front dimension of. 18 x 7 inches, but at the two ends and lower edge there is a Hin. flange turned at right-angles. Distances between drilling’ centres for holes are given at the foot. Suitable openings must be cut in the position of the dial apertures.