"Tetrode" Crustal and Amplifier

Radio Record, Volume III, Issue 4, 9 August 1929, Page 28

"Tetrode" Crustal and Amplifier

Very Low Maintenance Cost

(By

GALENA

-IN the Special Orystal and hi Portable Number, December 14, 1928, the writer described a crystal set and amplifier to work from 223 volts B battery and dry cells for the A. Numérous satisfied correspondents and constructors have testified to the efficiency of this receiver, and this encouragement has prompted the description of the improved set with condenser controlled reaction... The original was in no way a freak _ circuit, the secret of its success lay in the type of ‘valve used as amplifierthe tetrode or double grid. With the combination the reception of overseas stations has been reported in more than one case, but this ean not be held as a feature of the set. It has been designed to give good speaker reception of the local station at a low running cost. Generally speaking, the amplification is the same as a good general purpose valve with a high amplification factor operated with 90 to 135 volts on the plate. Some have stated that the set is the equivalent of a two-valve amplifier, but this must-be questioned. The set. to be described should equal a twovalve amplifier. A feature of the "Tetrode" Crystal Set and Amplifier is that it can be made from an existing one valve amplifier at a minimum of cost and trouble-and if carefully made will give satisfaction. _Use of Tetrode Valves. XAGGERATED ideas exist with regard to the use of these valves. The writer’s experience is that as single stage amplifiers for crystal sets they are difficult to excel, but in multivalve sets they are not satisfactory. They give the amplification but can be overloaded very easily with the consequent ruination of the tone. mn ae reas aemmeaned a --- Oe

Double grid valves have been used widely in France, and in glancing through French magazines one. is struck with the very large number. of circuits still employing them. They are termed Bigrilles (two grids), and this name has been seen in this country in the Micro Bigril valve. This was the first valve of its type used by the writer, and was on the "Radio Record" stall at the exhibition. The base was unconventional, a special socket had to be constructed, but the Philips A441 may now be used with the ordinary holder. An extra terminal is provided at the side for the second grid. In both the set to be described and in the original set a Pentode valve may be used to advantage, but a high voltage in the order of 135 volts will have to be applied. The Circuit. XAMINATION of the circuit will show that there is nothing in the way of "freak" combination. The crystal set is the usual with an extra winding on the aerial coil for reaction. The amplifier is slightly complicated by the addition of reaction devices, but is nevertheless simple. The aerial coil ig tapped in preference to a separate primary. This, while not conducive to selectivity is the best arrangement for sensitivity and this is the main attribute of this circuit. x’ The constructor is hdvised not to attempt to dispense with any of the components, all have been selected only after a careful test and all are necessary, with perhaps the exception of the R.F. choke.

The Components. HE selection of the components is left to the constructor who will adapt the quality of these to his pocket. "Nothing but the best" is a good motto, but first-grade components are very expensive dnd as the object is to keep expense to a minimum, this rule will no doubt be deviated from. Secondgrade parts may be used with a slight sacrifice to efficiency, but providing the transformer and the fixed condenser are of good quality little will be lost in cutting the price on the others. A reace-*hly good transformer may be

had for 15s, Cheap ones soon burn out, besides their tone is poor. A list is appended. A Suitable Cabinet. "THE usual procedure in construction is to build a set comprising a baseboard panel and components, and connect these by long leads to batteries. either concealed or lying about the set. A cabinet may be constructed, but usually not till some time afterwards. if then. This time procedure is reversed and the cabinet is constructed first, and the amateur will not find this difficult. Diagram 1 will give an idea how this ‘should look when finished. The timber is half-inch, well-season-ed heart: 8 x 4 inch dressed lining is very suitable. and is readily obtainable from a timber yard for a very ‘small sum. About % feet will be ample. Space will not permit full construetional details of the cabinet. Measurements are fully given, and no difficulty should be encountered. Outside measurements are given in each case. The front portions should be joined as in Digram. 2. The edges can then be rounded off and polished. It is advisable to fit the bottom and back into the front and sides: that is, like the end of a fruit box. The top forms a lid and is provided with hinges. A small gverlap, say half-

inch, is preferable in ‘the front. This . should be bevelled off to give a neat appearance. When complete, smooth with sandpaper, stain, varnish, and allow to dry. Not until thoroughly dry is it advisable to earry on with the assembly. — The Assembly Layout. THIS aspect of the construction is of paramount importance and unless there are very. definite reasons that described should be followed. The only component that can be constructed satisfactorily and economically is the coil. Coil construction was dealt with by "Megohm" a few weeks ago ang the prospéctive constructor: can re ex to this if he desires to construct a type of coil other than that described here. The simplest and most efficient eoil is wound with unspaced D.C.C,

wire on a three-inch former. Obtain six inches of cardboard former from a dealer. Now make a sma" aperture in this about one inch from the end. Through this pass about six inches of wire and, making this secure, com- ' mence Winding so that each succeeding layer lays snugly against its neighbour. At every ten turns pause in the winding, pierce the former with a penknife, serape the wire clear of insulation and pass a loop through the slit. From the inside pass a thicker wire through the loop and double it over to pygfent slipping. Now draw the. coillyire tight and proceed with the wil ‘ng. In this way complete 40 turns. ‘The reaction coil has now to be wound, — Without cutting the wire, make the join already described and wind on 25, turns. Pass the end-through the former and secure by means of a small. nickel bolt through the former. With the soldering iron run a little solder into the joints in the inside of the former. . The fate of these tappings rests with the constructor. If the local station only is required all/that is necessaty on completion of the set is.to try éach connected with the aerial to ascertain which gives the best result, and connecting this with the aerial terminal, disregard the others. Those wishing to search for other stations should

connect the aerial lead to a pin socket © and solder each lead to a pin. The t aerial tapping can then be varied at will. The top terminal, the terminal common to the reaction and the aerial coil, and the reaction coil terminal are not connected with pins. Mount the components in the cabinet as indicated. The two condensers and

The diagram illustrates the cabinet for the original set. The same dimensions apply, but a reaction dial replaces the yheostat which is to be centred. the rheostat are mounted on the sloping front, the remainder on the bottom eq with the exception of the crystal, ‘which is mounted on the side. A permanent crystal is to be preferred to any other for this-set.

_The coil is best mounted vertically. If it is secured to a small mount of wood by Meccano angle brackets its manipulation within. the cabinet is facilitated. Likewise the transformer and the valve base might be secured to a movable piece of wood.

\ Wiring the Set. Wit the aid of the diagrams the | wiring is simple. Connect the aerial terminal, to a tapping on the » coil, usually number 4. From the top of the coil (1) run to the crystal. This end of the erystal is connected with the fixed places of the condenser, ""B" of the transformer. The other side of the crystal is connected with "P" of the valve, and with one side of the fixed condenser. The lower end of the coil (5) is connected with "B" of the transformer, the earth, and the moving ‘plates of the condenser. "Q" of the transformer passes to one end of the fixed condenser and to "G" of the valve socket. "GB" of the transformer goes to the zinc covering of the torch cell. The brass cap of this is joined with the negative terminal of the "A" battery. From this point a wire goes to the rheostat, which in turn is connected with "H"’ of the valve socket. The other "F" terminal is to be joined to "A+." A connection is now made between this point and "B" negative. "B" positive runs direct to the output jack or terminal. The lower end of the reaction coil connects with the moving plates of the reaction condenser, the fixed plates of which are joined with the "P" terminal of the valve and with one side of the R.F.C. if it is used. The free side of this passes to the jack. If this component is not used connect directly the plate to the jack. One connection remains. "B+" is to be joined to the terminal on the side of the valve, and now all is ready to try out. : If, on trying out, it is found that the loudest point is at "100" more

turns should be added to the aerial coil. Remove the aerial coil and solder a little more wire to this point and wind on a few more turns. If the condenser reads at "0" turns will have to be removed. ‘Try first by connecting the lead to the erystal to tapping No. 2, and if this brings the local station in more to the centre of the dial make a permanent connection and remove the surplus wire. It will be found that when the batteries are new the maximum strength will be obtained with the rheostat only partly on. Do not pass this point or the valve will be impaired. To shut off the set merely turn off the rheostat

If the constructor wishes he may connect wires from P and B+ of the transformer, to another jack. Warphones may: thus be worked from the crystal alone. a may be an improvement to earth sé a.List of Components. Permanent crystal. alb. 20 gauge D.C.C. Variable condenser, .0005 mfd. Variable condenser, .00035 mfd. Fixed condenser, .0001 mfd.

Rheostat, 30 ohms. Transformer. Valve base. Valve A441, "B" battery, 22% volts. Three "A" cells. Torch battery. Jack, terminals,. clips.