1929-10-29 — Page 11

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RADIO TOPICS

YOUR UNIT

How To Derive Grid Bias From Mains

TO-DAY'S RADIO

BROADCAST BY Z.B.W.

ON 350 METRES

[By F. A. Turnbull] Having disposed of the H.T. bat- The following programme will tery by using an "H.T. eliminator," be broadcast to-day from the which can be a completely satisfac Hong Kong Broadcasting Station tory source of supply, there still re- | Z.B.W. on 350 metros. mulas the G.B. battery. And when

3:00-7.00 p.m.-Progranime of

| 20, 30 or even more volts are requir-European Music. (Vietor & H.M.V. ed to serve ឆ super-powerRecords supplied through the

COUPLING CIRCUITS

Enormous Difficulties at Beginning

(By Percy W. Harris M.L.R.E.) Accustomed as we are to using three or more valves in a wireless receiver without any special diffi- culty, it may come as

a surprise

to many readers to know that when the three-electrode valve was first Invented, the use of several valvea "in cascade" as it was then called, "Oberdon" (Weber), Overture,

enormous Albert Coates & the Symphony presented

difficulties. Orchestra.

One of the chief troubles was that' "Outward Bound" (Stanford),

no two valves were alike. "Drake's Drum" (Stanford),

| valve in regard to grid blas, the G.B. courtesy of Messrs. S. Moutrie &

battery is liable to assume the Co.). dimensions of a small H.T. battery. There are, of course, units avait- able which will give H.T. and G.B. from the mains and others from which L., also is available, but it Is quite a simple matter to take a little voltage away from an H.T. unit for grid-bias purposes,

All you have to do is to connect a resistance in verlus between the negative terminal of the mains unit and the IIT-terminal of the set. A lead taken from the units minus

Bass-Baritone....Poter Dawson. "Over the Garden Wal" Talking by

John Henry Assisted by Gladys Horridge. "Torna a Surrento"

(Come Back to Surrento), .

Florentine Quartet.

"Serenata Silvestri

(Serenade of Olden Times),

Neapolitan Trio "So Tired" (Little & Sizemore),

terminal then can be used as a grid- "Out in the New Mown Hay Penn.

bias connection.

The Variable Resistance Tappings at different points on the resistance can be taken to pro- vide other grid bias voltages of smaller orders. It is necessary to by-pass the resistance with a large fixed condenser.

The variable resistance will have to be one expable of carrying the total current flowing through the eliminator and must provide a smooth and reliable varia- tion.

A Clarostat can be employed for the purpose or a high resistance polentiometer. If you use a potentiometer only two of its ter- minals will be in service. A con- neetion is taken from the one which is joined to the alider and from: either of the other two.

Two Variable Tappings With a four point potentiometer (I believe there is one in the Cen- tralab range) you can take out two variable grid-bias tappings. in this case the other slider terminal is rus out direct to a grid-bias point. The Axed condenser used should be of at least 2-mfd. capacity or 4-mfd. if possible, and while it need not be of the very high voltage variety, it must be able to with- stand the pressure occurring across the resistance (this will be the grid- bins voltage arranged for).

It is important to remember that the voltage taken for grid bias in this way must be substracted from the H.T. voltage and it follows from this that as you adjust the G.B. so you vary the H.T.

Comedienne Ann "The Beautiful Calathen" (Suppe),

Overture....Berlin State Opern

Orchestra.

(a) Lowlands (b) Highland Laddie", (a) Blow the Man Down "(b) Tom's Gone to Kila",

Miss Billie Dauscha, the crooning radio contralto.

(a) Haul Away, Joe",

We have still a long way to ge yet before we get the same efficiency of energy in the high-frequency stages as we now get at the low- frequency end. In high-frequency coupling the two things aimed at are the greatest possible magnifica- i tion per stage with the highest practical selectivity.

Selectivity Not Always Important

In some cases where it is only desired to ten to one station selectivity is not Important, as it is unlikely that any other station will compare in strength with that the local to which we

desire to listen.

The reason why a circuit must

not be made too selective in the

high-frequency end is that in

broadcasting we

with one definite

are not dealing

frequency per.

fre

station, but with a band of quencies each side of that carrier- ነዚ ኖሮ For proper broadcasting all musical frequencies between about 50 cycles and 5.000 must be trans- mitted (although it is better, if eertain frequencies above this upper limit are also sent out), and if pro- per transmission of frequencies be tween these two limits is made, then we shall get exceedingly good re. production with a properly de- signed sot.

If our set is so sharply tuned the: there is an appreciable falling

in magnification of frequencies on either side of the carrier, then we shall get distortion.

The two leading forms of high- the tuned frequency coupling are anode and the tuned transformer. Modifications of these are the un- tuned anode impedance coupling where the high impedance of a good radio-frequency choke is used and the untuned transformer which transmits all frequencies fairly un-|

"b) What Shall We Do With The formly but with very little "step-

Drunken Sailor",

Fire Down Below,"

(b) Hullabaloo Balay", Baritone,

John Goss and the Cathedral Male Voice Quartet, Waltzing Doll" (Poldiai),

Unfortunately, the variation is not in such a way that the H.T. automatically follows the G.B. In "At Dawning",

"The

Victor Concert Orchestra.

fact, the more GB, you bring out "In Chineze Temple Garden",

the les H.T. is available.

With

"Sanctuary of the Heart",

resist- Organ Solo.....Ecginald Foort. 40-volts "I Got a "Code" in my "Doze""

"When Summer is Gatto",

Comedienne......Gracie Fields. "Lyric Sulte" (Grieg. Op. 54),

6,000-ohm Д ance I have" taken grid bias by this meana. For ordinary purposes. that la for obtaining grid 'blas up to about 25-volts, a resistance hav ing a maximum value of 1,000 ohms will be adequate.

It is impossible to give definte figures for the simple reason that the resistances of mains units and the circuits with which they aro used, vary considerably. If you know these factors, it is, of course, possible to work out a fixed re- Bistance to place in series, which will give you a definite, grid-blas voltage.

Calculating Voltage Drops The voltage drop across the added resistance will be equal to the total resistance in the whole of the circuit divided into the added resistance multiplied by the voltage available (in the case

Sir Landon Ronald and the Royal Albert Hall Orchestra. Shepherd Boy, 2.Norwegian Rustic March,” J. Nocturne,

1.

4. March of the Dwarfs. "Blue Bird, Sing me a Song", "Worryin'" (Fairman),

Baritone......Melville Gideon. "For You Alone" (Ccehl), "Maire My Girl", Tenor, "Marche Militaire", "Andanting" (Lomare),

up."

In the tuned-anode setrenie we can use ordinary plug-in calls, which I would like to emphasise are still a very highly efficient component when properly used, particularly as the leading makers now sell plug- In coils of far much improved efficiency,

A Popular Type of Coil

We can also use the very popular six-pin variety of interchangeable coil_in_which single-layer or slot windings

are used. As reaction -is-often- very desirable feature reaction windings, are fitted to the tuned-anode soils of the six-pin vuriety.

The efficiency of a single-layer six-pin tuned-anode coil is slightly higher than that obtainable with John Turner. most plug-in coils, but there is not the difference between them that some writers seem to suggest.

Organ Salo.......Arthur Heale, "La Boheme-Musetta's Waltz"

(Pucelni), "Bird Waltz" (Varney); Soprana, "Lilac Domino" (Cavilller),

Vocal Gems H.M.Y. Light Opera

Company.

Lucrezia Bari.

7.48 pim--Evening Weather Re-

port.

9.00 p.m.-Evening General Pro-

of a D.C. mains unit across the in-gramme of Chinese Music (from put terminals),

The extra components which figure in this grid-bias circuit, viz. -a fixed condenser and a resistance -can be mounted in the set, or in

Studio).

10.30 p.m.-Close Down.

the mains unit if this is home- LT. eliminator.

The efficiency of modern valves Is so high, and the mangification obtainable with well-designed cir- cuits so good, that it is becoming more and more essential to use some to prevent interaction between coil form of acreening between stages

fields, wiring etc.

that those readers who build their For this reason it is casential

sets from published designs should follow, the layout with great

cumcy.

AC-

The connections of evil is of considerable use in all The binocular or "feldless" type

forms of high-frequency coupling, as the held of the coll instead of

made, Additionally they can be are as follows. The G.B. resis- bulit up as a separate small unit tance should be placed directly in having on it the three necessary series with the negative input from terminals for the external con- the mains to the unit, the mainstaying all around is very closely nections.

confined owing to the particular end being the G. B. minus. "tap If you know the current that is ping." By-passing will have to alls of the binocular form are form of winding. Tuned-anode flowing through your eliminator | bo by electrolytic condensers in this and G.B. resistance circuit and the case, and choke smoothing arranged obtainable from the leading maken, resistance of the G.B. component, with the L.T. unit to cover the G.B. amount of screening required in a and cut down considerably the then Ohras Law quickly tells you Altogether rather more complicated the voltage drop created for G.B. than when only an H.T. unit is

'set.! used! purposes.

You take the ourrent In amperes However, should you be employ- and multiply it by the realstance ing separate uafts, both for H.T. and

in ohms, and the answer is the LT. then complete rebuilding le voltage drop in volts. Supposing advisables b the current were 10 milliamperes Combined Supply Unita (01 amperes) and the resistance Aseparate G.B device might 1,000 ohms, then ho potential drop prove quite practical, but where in 10 volts.

mains are concerned one is wise

If you happen to be using an to avoid any posibility of "cross

L.T. eliminator, the G.B. resistance connections.?

Useful in Portables The untuned anode coupling, ne exemplified by the radio-frequency choke, has galped considerable popularity in the last year of two owing to its suitability for portable sots where the selectivity is mainly obtained by the frame aerial, and simpHeity of control is a highly desirable feature.

Resistance coupling ordinary

and

will have to figure in the circuit of The⠀ntat step from having types of high-frequency valves this. Your H.T. unit will be elmfted H.T. and L.T. batteries has the merits of cheapness anding its minus connection via theis either a combined unit giving simplicity. When used in con- LT. minus, so it is obvious that we H.T., L.T. and G.Bba mains, set can ignore that,

t

With An LT. "Unit You will need to use a power resistance capable of handing all the "Juice" which passes through the

And of the two alternatives the Iatter is to be recommended, not so much on technical considerations as those of compactness and next

Junction with reaction excellent re sults can be obtained, but here, of course, we have to sacrifice "some amount of selectivity and sensitivity In order to obtain simplicity and cheapness.

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