AMPLIFYING THE CRYSTAL OUTPUT

Radio Record, Volume I, Issue 6, 26 August 1927, Page 14

AMPLIFYING THE CRYSTAL OUTPUT

DETAILS FOR ADDING ONE VALVE

Hloweyer wonderful reeeption may at first seem with the aid of a simple crystal, there soon comes a desire for something that will put a little more "ginger" into the concerts, and from *phoues to a loudspeaker is the natural Step. This necessitates, if a crystal is still to be used, the addition of one or perhaps two amplifying valves, according to the distance from the station. The addition of one amplifier will now be described to work in conjunction With the ‘Record crystal set. Those Who wish to build the set complete may increase the width of panel to 9 or 10 inches, keeping the height the same, 7 inches. A depth of six inches from back io front will be ample. If the crystal set has already been made, tTie Size of the panel mav be increased by adding a strip at the right-hand side

of the present panel, extending the baseboard and cover to suit. } The extra components required will be a valve, and holder for same, rheostat, 2-valve sockets, 1 pin for do., large L.F. transformer, A battery, B battery, 4 terminals, ebonite panel, conRiecting wire, etc. Provision is to be made for using *phones on the detector only if desired, cutting out the amplifier. This is arranged for by drilling three holes as shown below ‘the crystal. The two fower holes must be about }-inch apart and drilled to take the sockets, which Should, if procurable, be a type that Will sink flush with the panel. A_ Short piece of flex. is to be put through the top hole, and to its outside end is. connected the pin that fits the two sockets, The wire that connects the back of the crystal to the right hand *phone clip is disconnected from the erystal and the inner end of the flex connected in its place. The free end of the wire from the "phone clip is now connected to the hack of the socket. These alterations are all that

are required to au existing detector. When it is desired to use the amplifier, the pin on the end of flex is plugged into the right-hand socket, and for the detector only with ’phones, the pin is put in the left-hand socket, looking at the front of panel. The positive or marked tag of the speaker or ’phones must always be placed in the clip connected with the positive of B battery, that is, the right-hand clip or terminal. The baseboard and panel having been completed, the transformer and valvyeholder may be fixed in position as shown, and the rheostat and terminals fixed in place on the panel. Four holes may be drilled in the panel as shown so that the filament of the valve may be seen, unless it is a dull filament valve such as the P.M.3, in which case the "ports"? would hardly be necessary.

At the lower part of the panel two clips or terminals are provided to connect the loudspeaker. In wiring-up, one primary terminal of the transformer is connected to the left-hand ‘phone clip of the earth terminal, which amounts to the same thing, as both are connected. The other primary terminal of the transformer is connected to the right-hand socket on the panel. One secondary termina! of the transformer is connected to the gtid connection of the valve holder On an American holder the terminals will probably be marked, but a diagram is given of an Iinglish holder so’ that no difficulty is presented. Tt will be noticed that the four sockets are atranged in the shape of a kite.. The plate connection is where the tail would be attached, the grid the top, and the two filament contiections are at the "ears." The other secondaty is connected to the end of the rheostat winding and then continues to the top terminal of the four on the panel front. The plate connection of valve holder is wired to one of the speaker connec-

Amplifying the Crystal (Continued),

tors on panel, and the other connector to the lowest terminal, B positive. Now connect the tw» centre terminals on the panel, A positive and B negative together, aud the same wire is to continue to one of the filament connections ou valve holder. ‘he other filament is connected to the arm of the rheostat. If there is 20’s enamelled wire left from making the tuning coil, it may be used for conuections, but the enamel must be very carefully scraped away wherever contact is to be made with the wire. No thinner wire than this should be used. The Batteries, A very snitable accumulator for the filament of one valve is a couple. of small Exide cells in series to give up to'four volts. A small carrier could be made to contain these for convenience in taking to a charging station or garage ‘These small accumulator cells can be obtained for 7s. 6d. each, and if a two-volt valve is to be used, only one of these cells would be required. A P.M.1 .F. Mullard -valve would be suitable as a two-volt amplifier valve, or a P.M.8 as a four-volt, requiring two Exide cells. A dry B battery of 45 or 60 volts will be most suitable, . Anybody not used to handling batteries must be very careful not to allow the positive and negative ends to become connected, even for an instant, except in the specified way provided for in the set. In the case of the A accumulator an accidental connection of the two terminals results in an inStant discharge of all the stored-up energy, and only many hours of recharging can restore its former condition, In the case of "grid plate" accumulators, such accidental "shorting’’ may cause permanent ruin of tlic plates, Short-circuiting of the B -battery must also be carefully avoided, and great care ‘must also be taken in contiecting the B battery to the set, as if connected across the top two termin- |

als the filament of the valye will be destroyed when the rheostat is turned on. Sonie constructors might prefer to extend the baseboard back a few inches beyond the cover, so that the B battery could be placed there and kept permanently connected to the set and lifted about with it. The battery should on no account be kept under the cover, as fumes emitted by it are injurious to metal work. ‘Io be more in keeping with general practice, the aerial terminal could be placed where a cross is marked at top of back of panel, and all connections that now go down to the A terminal would go to that point instead, but as space is limited care would have to be taken that the wires were kept clear of the condens2r vanes in all positions. A large sized IF. transformer of 5 to 1 ratio should be purchased, as it will be ts. ful in case of going in for a yalve set later on. ‘There are plenty of suitable valves to be obtained besides those mentioned, but a valve suitable for amplification must be chosen, and preferably one that. does not use more than about half an ampere on the filament, as this combined with low voltage means less frequent charging for the A battery, which, by the way, should

always be recharged whet not more than about three-quarters of its full capacity kas been used. ; Grid-bias is not usually provided on a single L.F. amplifier, but. 1s an assistance towards greater volume and clarity. It can be added by. breaking the wire running from rheostat to transformer secondary and inserting one or two small dry cells to give 1} or 8 volts. The negative side connects to the transformer. ma If dry batteries are preferred to the small A accumulator, then a valve suitable for dry battery operation must be selected, If the charging of an accumulator can be conveniently managed by taking it to a garage, it will be found more satisfactory ‘o use one rather than dry cells, which use up much more quickly on the filaments than is the case with dry B batteries. Charging at home is, of course, the ideal, but hardly worth the expense of installing for one valve. The chemical charger described elsewhere is for charging B batteries only, and is not suitable to charge an A accumulator. .