Constructional details of the

Radio Record, Volume IV, Issue 20, 28 November 1930, Page 16

Constructional details of the

L oftin White

Three

HIS week we are intending to combine two weeks’ articles and give readers in concise form the details of the adapted Loftin-Three and of the tuner that can be used in conL junction with the existing outfit without any alterations to the latter, except a slight change in the wiring of the input circuit. We recommend this week’s description of the Loftin-Three because the screen and r.f. plate current ‘is drawn from taps in the main resistance chain, thus eliminating the high resistance in the tuner. Furthermore, the altered resistances make for slightly louder signals than those used in previous articles. Another point about this version of the receiver is that the pick-up jack is inserted in the grid return of the detector valve. We have explored both systems and believe that this alteration is not only more simple but it gives better results than when the jack is inserted in the. cathode of the detector. Before proceeding any further, let us make the point clear that apart from the bias and detector plate circuit the resistances of the amplifier are not extremely

critical. Their purpose is closely allied to that of the ordinary voltage divider and need not be correct to a few ohms. As far as possible we have simplified out’ the resistances and presented them in round figures and a slight deviation from these will not be of any account. When a high potential is delivered from the power pack a greater resistance must be placed in the voltage divider, Probably the most satisfactory arrangement is to have resistances R3 and R4 continuous, say, 2500 or 3000 ohms. and a variable tap leading off to the screen for the voltage on ‘this is more or less critical, So are the bias resistances of the detector R8 and R1. So long as R4 and R5 jointly amount to about 6000 ohms the point where the tap leaves to the r.f. valve is not particularly critical. It can likewise be varied to suit individual purposes The values R6 and R7 are not of great importance, Their function is merely to divide the potential between the main source of high voltage current and the current after it has passed through the valve. The values of the resistances must be reason- ably high so as to allow a minimum of current to pass. The tap is arranged so

that it will lead off 50 volts or so to R9, so that it will cancel out part of the high voltage in that chain to provide a working bias voltage for the 245. The resistances of the main chain R’s. 1-5 must be capable of passing the total current passed by the valve-30 milliamperes. The other resistances have to pass only a1 m. amp. or so with the exception of R11 and R13, which pass 4 or 5. There is now little that remains for the constructor to do but make the set. The layout was shown last week and the alteration in the resistances need make but little difference to this If constructors have difficulty, a further picture diagram will be published, but it is felt that it is unnecessary. Follow the above panel layout published here, and _ the resistances underneath will fall into place without any trouble. The coils have already been described. There is no alteration for this version of the set. It should be pointed out that the numbers of turns on the primaries are fairly high to make possible the greatest transfer of energy. But this has certain disadvantages when it comes to considering the question of stability, and if any trouble is experienced in this direction the constructor will know that he must remove some of the wire from the primary. It is probably hardly necessary to remark that the pick-up jack must be such that when the jack is removed the springs will short-circuit and complete the grid circuit of the detector. An alternate arrangement is to connect the pick-up permanently and wire a switch across the jack. When this is closed the receiver is in the position to receive wireless signals, and when it is open the pick-up is in position, thus providing

a gramo-radio switch, which is in accordance with the most modern design. When the Amplifier is Not to be Rebuilt ‘p HE above description has been slightly modified to allow constructors who already have a Loftin-White amplifier to add a radio stage without rebuilding or going to unnecessary, expense in the provision of further resistances. The system needs little explana-

. tion for th tory. Exar it will be se the same 2 the o iy dif of the "B" current is d B-+- from tl from the set If one unte there may that might operation. increase in

r the diagrams are self explanaExamining the theoretical diagram be seen that the circuit is virtually me as that in the Loftin-Three, y difference lying in the derivatior "B" or high voltage current. The -is drawn through the tap marked om the high voltage source-that is 1e set side of the smoothing choke. untapped choke is provided then may be some elements of hum ight affect the quietness of the on. If such is the case, and an e in the capacity of the smoothing

system will not materially reduce the ripple it will be necessary to incorporate a further smoothing choke.: It is unlikely, however, that this will be necessary. The properly designed tapped choke is, of course, the proper expedient in a circuit of this nature. The value of R3 is 3000 ohms or so, and it must pass at least four milliamps. If the current is drawn from the centre tap of the last stage valve this resistance can be omitted, its value being of little importance so long as it is about 3000. If a fixed resistance is to be used with current drawn directly from the power pack its resistance should be 100,000 ohms. R2, which controls the screen voltage of the r.f. valve, is placed on the panel and acts as an excellent volume control. The valve must be fairly high because the current flow through it will be very small indeed, and it will be found that 500,000 ohms. is sufficient. The biasing resistance must pass the four or five m. amps. taken by the valve and raise the potential cathode some 3 volts above the grid which is at absolute negative potential. For this a fixed resistance of 600 ohms. should suffice. The coils and the condensers are the same as for the complete Loftin-Three, and were given in last week’s issue. The fixed condensers C2 and C6 are .5 mfd. condensers, while C4 and C5 are 1 mfd. They should have a working voltage of 180 volts. The Layout Diagram O much for the theoretical considerations. Let us look for a moment at the layout diagram and the sketch of the completed set. There is an aluminjum sub-panel 7in. x 84in. and this is turned down 2in. The size of the subpanel must, however, somewhat accord with that of the amplifier, especially as regards the turned down portion and the depth. The shield cans are the size adopted as standard for the Loftin-Three.

The two condensers are ganged as shown in the sketch diagram, and between them is a shield, but this is not necessary under ordinary circumstances, and should be

used only if the system is unstable, The diagrams have not been drawn to scale, but the components are well separated in order to show all the connections as clearly as possible. All but the r.f. grid wires are under the shield along with the condensers and the two fixed resistances. The dotted wiring is shown thus only to simplify matters, and does not indicate any special feature of the plan, The arrangement of the circuit is in« (Concluded on page 29.)