"THE DIFFERENTIAL FOUR"

Radio Record, Volume IV, Issue 38, 3 April 1931, Page 16

"THE DIFFERENTIAL FOUR"

Successful Series Completed

By

CATHODE

‘ EFORD going on with the de- _ .Séription of the: final stage of this’ receiver, it. may .be : well to touch briefly. upon the addition of a volume con‘trol: which, when the final stage is ‘added, will be found . Very. necessary. : The form of volume ‘which has: been fouiid Best for most circumstances consists. of a ‘potentiometer shunted ‘ across the’ first .audio’ transformer, the connection from the‘ moying. arm of. the potentiometer -being then :taken to :the grid of the first audio, valve.. This is, as:a matter of fact, one’.of: the very few forms of volume control which: do ‘not’ Have an adverse effect on the frequency response of the. audio system. ‘The potentiometer should have a value of one megohm, or thereabouts, and must, of course, be mounted on the panel. A symmetrical .appeafance will be lent the front of the panel if the volume control is mounted alongside the réaction’ conderiser and on a level with it. tion condenser and immediately underneath it. It will be noticed that a part of the panel at.the extreme right end. has: been left. unoccupied with, a view to the addition of this ' "The Differential Four." PART from the volume control, all that:is required to add ‘a second audio amplifier. to the receiver is an audio transformer of good quality and a valve socket. Space will be found for these at the rear of the newly-added volume control, their precise location not being a matter of any great importance. The lead which runs from the negative speaker terminal to the plate terminal of the first audio valve holder should now be changed over to the plate terminal of the second audio

valve’ holder (that just added). The now vacant plate terminal of the first. audio -.alye holder should be connected to one end of.the primary of the new audio transformer, the other’ end of the primary being connected to the B+ max. terminal. The secondary of the audio transformer is, of course, connected one end to the grid terminal of

the second audio valve holder and the other end to the empty C negative terminal on the terminal strip The wiring is completed by running wires from the filament terminals on the existing first audio valve holder to the new second valve holder, so that the power

valve will receive its proper filament supply. A valve having a good power handling capacity should be chosen for the last stage as otherwise strong signals will result in distortion, A valve with an impedance of 2000 to 4000 ohms will be found very suitable, and the recommendations of the valve manufacturer

as to the grid bias to be used with the particular plate voltage employed should be scrupulously followed. ‘The plate voltage. should be raised to as high a figure as the purse will allow, provided that the maximum voltage recommended by the valve maker is not exceeded. A high plate voltage is always.of assistance in obtaining the minimum of distortion on powerful signals. © [2 will be noticed from the diagram that slight alterations have been made in placing the audio stages of the Diff. Four. These are suggestions to get’a more compact layout than is possible in building the set up stage by stage. Ata later date a compact fourvalve set will be described. It will be noticed, too, that the coil. used in the radio stage employs an English base. The five-valve bases are difficult to obtain, and in this circuit are unnecessary. We have resorted to an English base to prevent the valve being placed in the coil socket, or vice-versa. This would have disastrous results both to the valve and the battery. The rheostat has been shown in the detector circuit on the layout diagram and in the radio circuit in the theoretical stage. This has been purposely done to help those who, wishing: to use the set primarily for short-wave, wish to control the voltage of the det. valve. . Clearing Away Difficulties, T appears as though a slight misapprehension occurred when the radio stage was described. The writer specified a .0005 condenser, thinking that

constructors would build the special coil for that condenser described earlier in the series. Some builders followed the same specifications as those for their .00085 condensers in the detector stage, This has, of course, thrown the dials right out of alignment, and it means that the whole broadcast band cannot be. tuned in.. To make matters quite clear, a table was published recently in the Questions and Answers columns. On trying out different types of. valves, the writer encountered a diffiy culty which has been met by many cox respondents, that is, with regard to reaction. Unless one happens’ to be using a valve which is a particularly -ready oscillator, it. is necessary to increase the number of turns on the reaction coil. ‘The small.diameter of the coil and its consequent inefficiency on the broadcast band contributes to this to some extent, but the real reason lies in the smallness of some of the differential condensers on the market. The insertion of a small fixed condenser across one side of the reaction condenser completely eliminates the difficulty, but renders the condenser less suited for short-wave work. What is really required is a differential condenser having a capacity of about .00025 on either side. Although reaction coils should be kept as small as possible, 60 turns is not an unreasonable size for the broadcast band, everything: being e-nsidered. Some difficulty has been encountered on the h.f. side, and this is most likely due to the use of an unsatisfactory type of choke for parallel feed. This is a key component in this type of circuit and a careful choice must .be made. The .001 condenser originally specified has been superseded by a .0001, which

oe Nmany correspondents have found bet_ter. ‘We, however, had no difficulty With the 001 typ. = So far as short-wave work is concerned, it was explained in the first article of the series that the h.f, stage had but little, if any, influence on the amplification of short-wave, and readers were advised if they wished to use the set primarily for short-wave reception to leave out the hf, stage. The s.g. valve gives a considerable lift on the broadcast wavelength. The efficiency of the broadcast coils can be appreciably improved by increasing their diameter to 2in. and reducing. the number of ‘turns and increasing the diameter of the wire. The construc-, tion, then, is rather difficult for those m0 are not accustomed to construction... However, a turned wooden or ebonite ring to slide over the valve. base of an outside diameter to fit with- : ‘in the 2in, former must be prepared, and this requires the’use of a lathe. When using the set for short-wave only, a .0001 double-spaced variable condenser should be used for the tun-’ ing and a special h.f, choke must replace. the one in the detector circuit. An excellent set may be had by using the detector and two stages of audio. Another useful addition. to ,the set is a 400. ohms potentiometer. . This must be shunted across the filament terminals, andthe variable tap taken to the grid return. This will be found to -make regeneration very smooth. The detector voltage should be as low as possible consistent with mooth regeneration control. The potentiometer can quite conveniently be mounted on the baseboard, as once adjusted for a valve and detector voltage it remains unaltered.