*
HONG KONG DAILY PRESS, THURSDAY, DECEMBER 25, 1930.
RADIO
FEATURES
LOW-FREQUENCY AMPLIFICATION.
WHAT IS ANODE IMPEDANCE?
DIFFERENT METHODS OF LOW-FREQUENCY COUPLING.
Between the detector and the output valve an intermediate valve is nearly always required. This is because the low-frequency voltage, separated by the defector from the high frequency of transmission, is not big enough to load the power valve, which cannot, therefore, operate the loud-speaker.
The
Yet a study of the average threo- valve set of to-day will not, at first, show the presence of a low-fro- queriry valve. The first valve may he the high-frequency amplifier, the second the detector, and the third the power valve. Where, then is the low-frequency valve?
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B.C. 3049. SCREEN GRID FOUR PORTABLE RECEIVER.
CABINET MODEL, IN SOLID MAHOGANY. An entirely new model incorporating the new Loud Speaker movement and "STORK" “HIFLUX" Transformer.
PRICE COMPLETE: £26.0.0
B.C. 3036. SCREEN GRID SHORT WAVE RECEIVER, THREE VALVE MODEL,
For wavelengths of approximately 14 to 100 metres. Metal case, finished dark brown crystalline enamel. Incorporates two tuning condensors with double spaced vanes. A potentiometer is fitted for varying the bins on the detector valve. A screen grid valve proceded the detector and this enables the set to oscillate easily on all wave bands irrespective of the length of the aerial; there are "blind spots. By using appropiato coils, this set may be used on ordinary broadcast wavelengths.
no
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The Burndept Screen Grid Receiving Set.
ACKNOWLEDGED TO BE THE FINEST WIRELESS RECEIVING SET IN THE WORLD.
All British,
WORKS DIRECT FROÙ THE MAIN, SO THERE 13.NO WORRY OF BATTERIES,
HAS WAVE LENGTHS FROM 188a, 220-300, 800-2100 AND HAS A RADIUS OF 12,000 MILES IN LOUD SPEAKER STRENGTH
T
COILS NEED BE INTERCHANGED IN QEDER
GET THE VARIOUS" WAYE LENGTHE
ON THE SWITCH.
ASK FOR A DEMONSTRATION AT
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|
The answer is that the detector and the power valve, in addition to their main functions, also work as low-frequency amplifiers. So much so that, with a suitable coupling between the detector and power valves, a further valve for the express purpose of low-frequency amplification is often not wanted.
HAVE YOU HEARD THESE?
Here are lists of long-wave and short-wave stations which should be picked up by anyone. in Hong Kong who has a mode- rately good not suitable for re- ceiving such signals. Success in plcking up these (stations also depands very largely upon fav- ourable atmospheric conditions. Readers are invited to add to this list should they succeed in picking up any station not in- cluded in either of these lists.
Before talking about the different | methods of coupling together low- frequency valves there are two guid. ing rules. Firstly, we must ensure an even response curvo., That is a technical way of saying that we must make sure that the output from the low-frequency amplifier is of the same form as the input, differing only in amplitude. At the input the high notes, middle regis-7 ter, and low notes will bear a cer tain relative prominence to one another. This same relative make up of the musical scalo should appear at the output. The job of low-frequency amplification is to amplify, not to modify. Sometimes the input is not pure, in that certain notes aro, unduly prominent. Only then enn low-frequency amplifica tion be allowed to distort the input, in order to overcome some initial and undesirable emphasis.
Firstly, then, an even response, curve; Secondly, to obtain even amplication, an observance of the impedance rule. The impedance the anode circuit of the low-fre- quency valve must be not less than twice the impedance of the valve *itself. Now the grent difficulty about this rule is hardly ever re ferred to, but I intend to put be ginners on their guard. It is all very well to say the impedance of the anode circuit must be twice that of the valve, but just whht in, the impedanco of the valve under work. ing conditions?
No amateur has any way of an- swering that question. The imped ance of the valve is usually mea- sured at zero grid volta and 100 anade volts. Assuming that the ancdo voliage actually applied to the valve is about 100 volts, one mast think of the altered condi-i tions brought about by the bins en' the grid, often very different in: value from the zero volts determin. ing the nominal impedence of the valve.
In practice, the impedance of the valve is usually something greater than the value given in the maker's list of characteristics. By making the anode impedance three times the impedance of the valve, one can usually be sure that the impedance requirement for good quality is being observed. Just what is this anode impedance {
It may be the primary of a trans- former, a choke winding, or a non- inductive high resistance, depend ing on whether transformer, choke- capacity, or resistance capacity coupling is employed. The trans former is by far the most used low- frequency amplifying coupling.
The primary winding forms the anade impedance of the valye prej oeding it. If the valve has an im pedance of 90.000 ohms, the primary of the transformer should have, an impedance of not less than 50,000 ohms, at a stated frequency. Here the beginner will again find a song in relating theory with practice, for the transformers on the market Aro seldom accompanied by ; any useful data, Only a general rule enn be observed to conform with the impedance ratio requirement.
This rule is based on a limitation common to all transformers, name. ly, that only a certain amount of wire can be uacil for the secondary, beyond which very bad effects are produced. So when a high ratio. of secondary to primary turns is wanted, the primary is reduced, in- stead of increasing the accondary. The fewer the primary turus, ather things being qual, the lower tha impedance of the primary. So if a high-ratio transformer is used, in order to gain a big step-up in volt- ago, it is wise to choose a medium- impedance valve, any 20 000 ohms, A townsatios transformer in zaste likely la
than che with a high ratio. But with a good low-ratio transformer one can use a much higher impedance valve. And since a high-impedance valve usually has a high amplifica- tion factor, it greatly offsets the disadvantage of the low ratio.
(Continued on" Page 3.).
LONG-WAVE STATIONS.
Wave length
Call Kilo- (Metres) Station Sign cycle
200 Manila
K.Z.I.B. 1,183 277. Shnughai K.S.M.8. 1,085 280 Tientsin C.R.C. 1,070
K.R.C. alo Shanghai
907 320 Peping
C.O.P.K. 943 J.O.A.K. 345 Tokyo
870 333 Hiroshima J.O.F.K. 850 846 355 Hong Kong Z.B.W. 337 Bombay V.U.B. 840 301 "Sapporo
J.0.1.K, 890 J.O.D.K. 300 Koijo
820 370 Nagoya J.O.C.K. 810 370.4 Calculia V.U.C 809.0 380 Kumamoto J.O.G.K. 790 390 Sendai J.O.H.K. 770 323 Dairen J.D.A.K. 700 V.U.R. 754 308 Rangoon 400 Onaka J.O.B.K. 750 C.M.B. 739 410 Canton
K.Z.R.M. 413 Manila
758
SHORT-WAVE STATIONS.
STATION
CALL SIGN
HSARJ. 2.B.L.
K110
CYCLES
TIME (HOND KONG)
or Wonking
4,484 Mon., Wod. & Fri., 0 pm, & 2 a.m. 4,000 6-10 p.m.
7,142 Daily 080 p.m. & 13 p.m.
7,810 Not regular
7,730 Daily 1 p.m.
4.108 Tues., & Fri., p.m.-1 a..
| 71108.
1,230 Not
0,503 Not
0,620 Daily 75.m
9,620 Fri. 8., Sat. 8 am, & 10 am.
0,500 Not regular
67.68 0.12 Khabarovsk (Russia)
Navon 50. Moscow #43.6 Homo
Dobinti (Germany)
A.F.K.
B.A.9%.
A.G.J.
6,201 Not regular
R.F.N.
1. M.A
6.800 Sunday midnight”.
0,003 Tue, Thurs., Sat., 8 p.m.
Perth
0.4.0.
41.3
Singapora
V.S.LA.B.
39.8
Kootwijk (Holland) '
P.C.L
87.
Bangkok (diam)
82,
Bydney
$1.65
Melbourne
3,1.0.
81.48
Schenectady
W.2.X.A.F.
81.20
Eindhoven (Holland)
P.C.J.
31.28
Sydaay
2.P.C.
81.
Nairobi (Kenya)
7.1.0.
28.5
Sydney
2,3.B.
10,626 Not regalar
27,8
Bandoong
PLE
26.53
Chelmsford (England) | 5.8.W.
24.5
Manila:
K.1.X.B.
28.35 18,89
Bandoong
Schagectady
W.2.X.O.
12,850 m. Wod, Fri, Bat
PLE.
184
Kootwijk (Holland)
F.C.L.
17,4
Bandoong
18.0
Bangkok
10.3
Kootwijk (ilolland)
15.74 15.0
Bandoong
P.C.K P.LE.
Nancy (France)
10,351 Daily B..
13.08
Pittsburg
W.8.X.K.
21,540 Not regular
P.L.F.
IL.6.1.2.3.
[Allowance must be made for
9,677 Midnight dkily.
11,020 Midnight-3 m. daily
11,761 7.80 p.m. & 8 am, daily, except
Saturday and Sunday
12,240 Nightly--
16,109 Daily 6.30 p.m. to midnight!
16,304 Daily 7 p.m.
17.280 Daily 8 p.m. to midnight
17,761 Bundays 7 p.m. & midnight
18,404 Each afternoon
10.220 Daily 5.30–7 p..
summer-time" in most Euro.
pean countries, which is one hour ahead of true time.]
MUSIC FROM MANY COUNTRIES
by wireless with
TELEFUNKEN 40 Long-range- All-Electric receiver with station selector for use with batteries or lighting
mains.
+
with TELEFUNKEN LOUDSPEAKER ARCOPHON
For Demonstration and further particulars apply 10':—
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TELE
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KEN
Office-Pedder Building, 4th Floor. Showroom-27, Nathan Road,
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