Grid Bias for A.C. Receivers

Radio Record, Volume III, Issue 10, 20 September 1929, Page 46

Grid Bias for A.C. Receivers

Three Distinct Methods

BY;

CATHODE

BOO ) EN EyFesithe vi

HE- problem of providing ‘grid-biag for the now popular 'a.c.-eperated receiver is one that at sometime or other has rexercised the minds of most \designers. The simplicity and ithe technical advantages . of deriving bias from dry batter-

ies are largely discounted by the popu: | lar prejudice against what is regard-. ed as a compromise, and indeed ‘there is much to be said for the elimination of a battery whose modest dimensions render it apt to be forgotten, and whose unnoticed failure involves also the probable failure of an expensive set: of valves. , As to the possible methods of biasIng without batteries, there seem to be three methods which may be adopted, one of these methods, as wi be -explained, being capable of a number of modifications, while one provides bias for the last stage only, leaving ‘the previous stages to.b. biased by one of the other two ‘methods or by batteries. rae Separate Rectifier Filter. TPE three methods are :- , (1) Employing a separate rectifierfilter combination similar to that commonly employed for. plate-current, or B supply, but connected in reverse; that is to say, the terminal which would ordinarily be used’ for supplying plate current al maximum potential is connected to the filament circuit of the receiver, while the various negative biases aré ‘tapped off at suitable intervals along a potential divider -connected across the output. Fig. 1 illustrates the arrangement, also showing the method of by-passing with condensers and series resistances ‘to avoid -back coypling. This method of securing gridbias is one that can be thoroughly -recommended, its principal drawback being the expense involved in providing the’ additional rectifier and filter; another slight drawback is that, in the absence of.a very high resistance voltmeter, it is difficult to determine with any accuracy just what negative voltagés are:available at the various tappings. ~ A Variation. METHOD two is really .a variant of ‘the above, but, since its ex:

pense renders it practicable only in high-power work where bias voltages of the order of 100 or more are required for power valves of the UX250. or "LS6A: type, it will not be. treated. in detail. The separate rectifier. and filter of Method 1-is here used to provide :plate current for all stages.but the last, ‘for ‘which it-still provides grid bias. This is accomplished by connecting the filaments of all stages but the last to the negative end of the small separate rectifier-filter, the. transformer winding for feeding the filaments of the last stage, however, being very highly insulated and connected to the. positive terminal of the separate rectifier-filter. It will be clear that. bias for-the last stage

hahahaha nile ieee ae Secevrcvrr"' can be obtained by tapping the resistance a suitable distance’ down from the positive terminal, while ! plate supply for the previous stages: is secured by tapping, the same resistance at suitable distances up ; from the negative terminal ; in certain circumstances the same tapping would

TEE be C-.:for the’ last stage and B+ for the prior stages. ‘ * echnically, this system is ideal for high-power .work, but it suffers from two slight disadvantages: firstly, expense, and secondly, the output voltage of the separate rectifier-filter is added to that of the big rectifier-filter for the last stage, the combined voltage requiring some care in handling. ‘Further, of course, the problem. of. proyiding bias for the prior stages still

requires to be dealt with; the third method might well be made use of for this, however. Bias through Voltage Drop. THE third method, however, whatever may be its claims technically, certainly heads the list so far as popularity is concerned. Practically all the commercial A.C.- receivers. use it in one or other of its modifications. Briefly expressed, it consists in utilising the drop in voltage occasioned by the passage of the "B" current through a suitable resistance to provide negative bias; the "B" voltage is. lowered by whatever maximum negative bias

is required, so that the reetifier-filter combination has to be rated to deliver a voltage that much higher than would otherwise .be necessary. Considering the diagram of a con: ventional unit for supplying B and © current to an A.C. receiver employing a 171 type valve in the power stag¢

(see figure 2), it will be seen that if the centre taps of the filament windings for all valves are -connected to the point marked. --B, the whole of the plate current drawn by the receiver, passes through: that portion of the tapped resistance between the points marked -B and -40 volts. The method employed to determine the value of the resistance required to produce; any. given voltage. drop .(and consequent bias) is a simple applica-

tion of Ohm’s Law. Yn Its common 4 form: "ot u I= — . i R oy it is not very convenient for our. purpose, so we multiply through to turn it to the form * . oo B \ . z Ir : We wish °to.. obtain in the first instance 6 volts negative bias for a group of 226-type valves, and we, will assume that a measurement: (or’a ‘calculation) of the total "B" current (ineluding the ‘few milliamps that me "wasted" by passing right through the tapped resistance) shows its value to be 48.9 milliamps or..0489 amps. Knowing that the: resistance required is found by dividing the . m.f. in volts by the current. in: amperes, we put 6: . R = -- = 123 ohms. ’ .0489 . So that to. secure the six volts bias re- ° quired we must put.a resistance of 123 ohms between the, point marked -B — (and to which the filaments are connected) and that marked -6 volts. To obtain the additional 34 volts negative bias to make the..required. total of 40 volts negative we require » | 7 84 -~="i‘ , .0489 . oe or 695 ohms resistance between the? points marked -6 volts and -40 volts. ' But.it is not essential that’ the whole of the plate current should pass through one resistance in this manner . and, in fact, there will be less chance of undesirable interaction if this is not the case, (incidentally, an A.C. cir- © cuit embodying the 226 type. of valve in R.F. and first audio). Fig. 3 shows ; in. a practical manner the method of biasing a receiver employing % 171 or similar type valve in the last stage, an indirectly heated type 227 or similar detector, and a number of type 226 valves, of which only two are shown. It will be seen that here R4, | connected between the filament centretap of the 226 valves: (obtained by the ‘eentre-tapped resistance) and B-, _ is virtually in parallel with R5, which similarly bridges the centre tap of the .- | 171 filament and B-. Thus, in cal-/’ culating :the value. of R4, the total plate current of fhe 226 type valyes only would be taken into consideration, while the basis- of: calculation for. Rd would be the plate current of a single 171. type valve. . "as ; The Cathode Valve. . "THE introduction of the indirectly | heated cathode valve renders it possible: to go still further in the

direction of having a separate biasing resistor for each valve, an aim which the writer, heartily approves as reducing any possibility of interaction. As there is.no connection between ‘the cathode and the: transformer, there is no obstacle -to providing a separate. path between cathode and "B-" for each yalye, and the insertion of a suitable resistance in each such path will. automatically provide bias. Regarding: the respective merits of methods 1 and

3, the wtiter is not prepared to make. any statement except that both are successfully used by. different, .manufacturers, and either is capable, is a proper ‘design is adopted, of giving every satisfaction: in the hands of the amateur. constructor.

Hum Contr ol and Bias.

Problems of A.C. 1 By the early days of A.C, valves the question of: the reduction of hum ‘and grid bias was a difficult one, If alternating current is applied. to the filament of a D.C. valve, the* fluctua. a ee a Pr Orn ~

tions in temperature cause fluctuations in the plate current. This results in A.C. hum. The development of the 226 valve with a broad strong filament'to a very great extent overcame: this, particularly so when the centre tap of thy A.O, winding was connected with earth, This. means that ‘the difference in potential between the céntre tap and the two extremes of the winding was direc ly proportional to the numbers of turns on either side of this tap, If this tap is not directly in the centre, it can be seen that there is an element unbalane-

a> oe ed in the valve, and as the grid is also at.earth potential, A.O, hum is the result. . _* To overcome this, the centre tap must be at the.exact centre of the winding, and when the transformer jis amateur wound there is a possiblity that the halves of the windings ‘will not be perfectly balanced. Where this.is the case i. centre tan of resistence of almost any value providing it is reasonably low (40-4000hms) may be shunted across the winding and the céntre tap connected with the ground, "Were a vari« able resistance used, it may be varied so that all bum is ‘eliminated, Com-mercially-madeé transformers rarely require this potential divider (potentiog ameter) across them, as the centre tap hag been made with the greatest of precision. When grid bias is required, thé procedure is much the samé, except that a resistance is placed in series with the céntre tap. The resistance is of -a predetermined value, and will pass a definite amount of current, so that according to Ohm’s law there will-be a definite difference in potiential between: the énd that is connected with the ground, and that connected with the centre tap, Thus the grid which is connected directly to the ground is. still an earth potential, but tha filament, although the B.D, existing between either leg is directly proportional to the windings, yet the difference between this and the ground can be varied by the resistance value, In this manner the filament igs made positive .to the same extent, that in D.C. operation. the grid is made negative, ‘The p#Hinciple in the indirectly heated vathode type of valve has been explainPe ee eS ra —

— ed in another article. The grid bias proposition remains the same. A resistance is placed in series which makes the: cathode positive in relation to the grid. A centre tap with this latter type of valve jis rarely necessary, though where hum canhot be reduced otherwise, the centre tap conneted with the earth may improve matters. .- Summing up theh, the usual cnuse-of hum is an unbalanced filament brought about by the centre tap not being cons nected to the electrical centre of a filae ment winding, .

Short-Wave Expansion

HE commercial démand in the Upite _ -¢q¢ States for permission to construct and operate ‘short-wave radi¢ stations is widespread. Radio cons panies, telegrapli companies, Press Associations, oil prospecting’ con¢erns, Government bureaus, aeroplane operate: ing enterprises, public utility co operations and even ‘apple-growers in. the Far West are among the diyersified industries that have applied to the Radio Commission for short-wave-licenses. a Nor is this agitation for the construction of short-wave stations confined to the United States. Practically . every ‘country in the world is hastete ing to erect commercial stations using: the high-frequency channels. Canada, France, Germany and Brazil and many other countries are increasingly make ing use of short waves fora variety of communicating purposes, — 7 senatndneammmeaienelll ----w