A Good Three-Valve Set

Radio Record, Volume II, Issue 27, 18 January 1929, Page 26

A Good Three-Valve Set

The ‘"Pentode’’ Three

By

Pentode

LANCING through the pages of an English or American yadio magazine, one may come across a circuit that is considered to be just the thing for local conditions, and the cost of components = just suit one’s pocket, only to find, when on the shopping expedition, that quite a number of the components are unobtainable in New Zealand. Very often the specifications of these standard parts are not given, and the builder is forced to turn his attention to the assembly of a machine more or less out of date. As a result of numerous inquiries, this receiver has been designed and built to suit the man who wants utmost efficiency from every stage, and the components to be readily obtainable in this country. There are many who do not care to wind their own coils. In England, standard coils and bases are obtainable already wound, and circuits are given for the use of these. Practically the only standard coils in New Zealand are the well-known Browning-Drake kit and the Neutrodyne. The "Pentode Three" has been designed especially to utilise the Browning-Drake aerial coil and regeneroformer. Hach of their components is readily obtainable here. Tf battery voltage is used as recommended, the tonal qualities are all that can be desired, and although reaction is employed, the receiver is non-radia-ting and cannot annoy neighbours through howling valves. The B.D. Kit. NE or two commercial B.D. kits include two tuning condensers, in which case they may be omitted from the list. Whatever make of components are utilised, the general layout will have to be checked over to make sure that all parts will fit in without undue overcrowding.

To suit the high impedance of the screened grid valve itself, there has to be a high impedance in the anode. The circuit is known as the tuned anode method of coupling, and the small primary winding, inside the regeneroformer is not used. To prevent small losses this can be removed, The writer has in mind one or two commercial B.D. kits in which the detector gcrews directly to the frout

panel with the variable tickler or reaction coil above. If the reader has one of this type, the layout of the detector and audio section will have to be altered to suit. The Sereening Box. HE only part requiring much attention is the screening box. This is made of No. 2A gauge aluminium, and when finished measures 62 inches square wtih removable lid. Some builders may prefer to use sheet copper, in which case a thinner gauge can be used, and all the seams soldered.

The sheet of metal will have to have overall measurements of 193in. by 19% in. Mark and cut to a shape similar to the used for the screening box for the "Rotorua Portable’ described in the special issue, but make each side 64 inch x 6% inch. Half an inch is allowed for overlap, and the holes can be drilled after the box has been made. Four small nuts and bolts will be required to hold the box to-

gether, and four wood screws into the baseboard hold the side and base firmly together. , A lid will have to be made 6zin. square, and the constructor can easily do this without any help. In order that the screening box will not project over the top of ebonite panel, the baseboard will have to be sin. thick. Cut this from a piece of dry wood to measure 18in. x Tin. x 4in., and screw the panel to the front edge. Brackets will hardly be required as the box will hold the whole rigid. On the extreme left-hand side of the panel and one and a half inches from the end, fix one of the 20 ohm. rhéostats. Three

and a quarter inches from the centre hole of the rheostat drill a hole large enough to take the spindle of one of the tuning condensers. Unscrew these two components from the panel, and lay the screening box along the back, flush with the panel. The side of the box on which the nuts and bolts are fastened will have to be at the rear, so that a perfectly flat edge is presented to the panel. Now, with a sharp point mark the box through the holes in the front panel, and drill to the same size. Before fixing the box

finally into place drill four holes oneeighth of an inch diameter along the back and right-hand side and half an inch from the bottom. Reference to the diagram will show where these are placed. To earth the metal screen a nut and bolt with a soldering lug can . be fixed in the centre of the back of the box, half an inch from the bottom. Before fixing the panel, drill holes to take the remaining components to be serewed to the ebonite. The two. megohm grid leak is seen to be close to the panel, and due to the grid coil. peing at a high positive potential the gl leak cannot be fitted directly across the condenser. Unless a special holder is provided, two small metal clips can be screwed to the panel to hold the grid leak. Now fit the box to the front by clamping between the condenser and the holding nut. The rheostat also passes through both the ebonite and the metal. Four small wood screws are screwed through the double thickness of metal at the bottom, into the baseboard. Behind the first rheostat lies the grid coil for the high fre- quency valve. Unless the small .0001 m.f.d condenser is provided with insulated feet, a slip will have to be made so that neither of the connections come in contact with the screen. A valve socket and .5 m.f.d. condenser complete the components inside the box, ; At either end of the rear of the baseboard are fixed terminal strips, each with two terminals. A. longer strip fitted with six terminals is fastened in the middle (see diagram). The remainder of the components will have to be arranged according to their sizes and shapes. As has been stated before, different types of coils have various methods of mounting, and the builder will have to arrange his parts to suit the parts themselves. The Wiring.

HATEVER the arrangement, the point-to-point wiring will be the same, and by following the wiring diagram no difficulty should be experienced. The points marked X in the high frequency section are leads taken direct to the metal screen. Small soldering lugs can be fixed under the wood screws or nuts and bolts that hold the box together, and the wires soldered on direct. If the end plates of the condenser are of metal, then this component is already earthed. If the rheostat of the high frequency

f valve is connected as shown, a negative. bias of nearly 1 volt is applied Lo the first valve, as this rheostat need never be turned full on. Tor wiringup, tinned copper wire, No. 18 or 20, is to be recommended, with lengths of Insulated sleeving slipped over. Wire the filament circuit up first. A negative solders direct to the metal screen. When wiring the coinponents where the wires run through the holes in the screen, care must be exercised. to see that the sharp edge of

the aluminium does not cut into the insulation. ‘ The lead running to the small cap on the top of the screened grid yvilve can be of flexible wire as short as possible, and taken in a direct line to the component to which it is wired. The small terminal found on the side of the cap of the pentode runs direct to B + 135. If this voltage is used then ‘the grid bias will have to be 12 ivolts negative, = Trying Out. AVING finished the wiring all that remains is to try out the receiver. In operation it is controlled in exactly the same way as the Browning Drake. The two condensers are used to tune in the station while the tickler coil increases volume until a point of oscillation is reached.

Both the screened grid and the pentode are four-volt valves, so that the detector valve will have to be the same. If a six-volt battery only is available, a resistance will have to be inserted in either of the two main supply leads from the accumulator. The amount of resistance will vary according to the make of valves used. To calculate this’ resistance refer to the data of ~S.the valves used supplied by the ' makers. Add: together the _ total amount of current used by the three valves. As an example :- Screen. grid ...... .06 amp. Detector peer eces al amp. Pentode occssesone 1D amp.

The total current’ consumption when the three valves are in use is therefore .31 amperes at 4 volts pressure. As we have a six-volt battery we have to find the resistance required to drop two volts with a current of .31 amperes.. ' Divide two by .81 and we bave the answer, Approximately seven ohms are required. This can be obtained by arranging that about seven-tenths of the total resistance of a 10-ohm

rheostat is in series with either the negative or positive lead direct from the six-volt battery. Resistance of 8.G. Valves. A WORD on the amount of resist- " ance required by different types of screened grid valves. One make of valve uses 38.3 volts and .132 amperes. With a four-volt accumnlator six ohms will always have to be in series with the valve and the first rheostat will never have to be turned more than two-thirds on. "B" Battery. REGARDING the B battery. The full voltage of 135 is strongly recommended by the writer. If lower voltages are used then the receiver will be very little better than a threeHSEDHQQUQLQOQYDSEVNSENEUQICOOUASHSESQAD0UOEOSEOQUEGOSTESEGRDASUSHGHIDODS DEE ETEGEGLEEEHES ATTEN

valve set using ordinary valves and less expensive to build. Also, 43 volts C battery is not enough. Use the correct bias recommended by the valve makers. It is false economy to use small B and C voltages, and if the reader is determined to use no more than 90 volts B with 4% volts ©, then use an ordinary valve in place of the pentode. An important point worth noting is to make sure of the construction of the screen grid valve before attempting to build the radio frequency side. The American valve has the cap at the top attached to the control grid

inside the valve. Sereen grid is connected to the usual grid terminal on the base, The English and the valve manufactured by Messrs. Philips, Ltd., arrange the plate connection to the cap on the top screen grid in the place of the usual plate connection on the base. Before attempting to build this side of the receiver, check over the connection by referring to the data supplied by the maker.

Components Required for "Pentode’ Three

In the list of components the reader will see that he has a wide range to choose from, and no difficulty may be expected regarding inability to obtain the specified parts. 1 Browning-Drake kit. ° 1 Sereen grid valve. 1 Pentode valve. 1 General purpose valve. 3 Valve sockets. 1 Intervalve transformer.

2x .5 m.f.d. condensers. 1 Grid condenser and leak with series clips. 1 x .0001 fixed condenser. 1 x .001 fixed condenser. 2 Rheostats, 200hms. 2 Variable condensers, .0005 m.f.d. 1 Ebonite panel, 8in. x Vin. x 3-16in. 10 Terminals. Sheet Aluminium (24 gauge.) Insulated Sleeving, etc.