RADIO RECORD

Greymouth Evening Star, 10 September 1931, Page 8

RADIO RECORD

NOTES FOR LISJENERS-®

(By

“Reception”)

MODERN VALVES.

EFFECT. OF THE PENTODE

In contrast to the almost violent modifications that, receiving sets have undergone in the last two years, aerials in the main are just the same as they were at the beginning of broadcasting when crystals were the rule, and valves a rarity, says an English writer. Following on a. series of experiments with a screened-grid-pentode receiver, the writer is of opinion that yesterday’s aerials are obsolete, and in many cases a hindrance to good reception.

The outside aerial dates back to a time when it was necessary to get the last ounce of power into a weak set. Until the S.G. valve appeared, an amplification factor of 60 for the first valve of the receiver was very high. The mag. of the modern S.G. may be 1500! The consequence of such a figure is best, told in the writer’s own experiences with a new set employing such a valve. Volume on London—the set lives in the S.W. suburbs—was so overpowering. and the spread on the condenser scale so large, that remedy was sought in a rejector circuit. The result was so disappointing that a small experimental aerial in the loft was tested: bat the improvement was not enough. Finally the set was tested with no more aerial than the short lead from the set to the rejector. The set then came into its own!

The slight loss of volume produced by earthing the disconnected outside aerial proved that some benefit was due to it from induction. (All sorts of freak results can be obtained from unorthodox connections to outside).

SUITABLE AERIALS Of real interest about the “rejector aerial” were the stations logged by introducing only a little reaction—and in some cases none at all. Careful tuning brings in all the giants of Europe, while by retaining the two larger aerials for reception of weak distant foreigners only, a total of nearly one hundred stations has been logged on the loudspeaker.

By this selection of an aerial to meet the needs of whatever transmission is desired, another difficulty can lie overcome. Efforts to cope with damping led the writer to weaken the coupling in the set till London was controllable. This meant that the receiver had to work on a fraction of its power for DX* owing to the nuisance of frequent alteration of the coupling and the difficulty of refinding the correct setting. The coupling is now set at optimum for really weak stuff, and by going all-out a station such as Juan-les-Pins comes over on the speaker. But by using an aerial apparently of ludicrous inadequacy for local reception, proper volume and control is still obtainable.

It is only in this way that the full benefit of the remarkable S.G. can be obtained.

Any efficient detector-pentode circuit working on an outside aerial, will bring in the local at full loudspeaker strength, and the addition of reaction ropes in most of Europe as well. This is particularly true when an acutely sensitive speaker such, for instance, as the “inductor” is used. In a very large number of cases the above arrangements are used, plus a scroened-grid valve with an amplification factor running into three figures or more. What is the result? The detector valve is presented wjth a terrific extra magnification when the set is used on the local—amplification which is neither needed nor desirable.

Orthodox grid-leak rectification is unab 1 '- to cope with the situation. The pentode, which should essentially operate on weak grid input, receives too much energy from a signal already distorted by detector overload. Contrary to a wide belief, the pentode is very easily overloaded, which is why there is a general recommendation that a pentode shall be the only L.F. valve in a receiving set. If it receives more than a small signal, it distorts horribly—as only a wrongly worked pentode can! In the case of such distortion, therefore, the bias should be carefully checked against I-I.T. voltage and meter reading. At its maximum of 150 volts, the normal small pentode requires no more than about, 9 to 10J volts at the outside.

This being done, the input must be reduced. .For a really fine quality of output such as is essential for coil reproduction, it is of little use to employ a volume control across the secondary of ( the L.F. transformer. The distortion is still there, for the detector is still being overloaded; the aerial input, must be reduced, or the S.G. stage weakened. There are several ways of doing this, apart from the use of a smaller aerial, or no aerial at all. In the case of a battery-operated set, a panel-controlled rheostat in the negative filament lead of the S.G. is a wellknown and recognised volume control, though some people profess to regard it as unscientific and crude. Of course, it is possible to adapt the set to handle more power, if the big volume is wanted. In this case a change to power-grid rectification will be necessary, involving little alteration beyond a i-megohom leak and .0001-mfd. grid condenser. The detector H.T. should be increased to about 100 volts or more, and it will be desirable to shunt the primary of the transformer via resistance and condenser in order to avoid the risk of the bigger anode current being more than it can carry. Lastly a power pentode, with not less than 250 volts, and the auxiliary grid decoupled; and sufficient volume will be obtainable. free from distortion, to operate an outsize in moving coils for ' concert work,

LACK OF SENSITIVITY

I Often a receiver will perform fairly satisfactorily, but will lack the seusitivity or the selectivity that might be • expected. Generally the fault is due • to the bad design or careless construe- > tion of the tuned circuits or the. threw- ■ ing of excessive loads upon such cir- . cuits. In the grid circuit of an r.f. i stage there are three possible souices of loss, viz., the coil, the tuning con- • denser, and the valve. The reduction of any losses occurring in the coil i must be beneficial, although it must I not be forgotten that this may lead to instability and, in extreme cases, i to loss of the high notes of the transmission. The high frequency resist ance of a coil depends upon many fac- , tors, amongst which are its physical ■ dimensions, type of wire, the nature of the material in the former upon which it is wound, the proximity of ■ other components or shielding material, and the spacing of leads or terminals. As a general rule, the larger a coil is the better it is, but improvements may be effected in other ways, and if it is not. possible to include a larger coil the question of winding an improved unit, of the same size should be considered. Preferably the ratio of length to diameter should be less than unity, a winding of liin on a 2in former being a suitable proportion. An increase of size, of course, means an increase in the external field, and hence it may be necessary to extend the screening, if any. The tuning condenser is in parallel with the coil, and thus any loss of energy which takes place in it must be added to that which is wasted in the coil. If the condenser has been made by a manufacturer with a good reputation it is not likely that its losses will amount to much, but sometimes a definite improvement may be noted by substituting another. The valve and its cap are in a similar position to the tuning condenser, but here little can be done beyond replacing it by one of better quality or characteristics. The socket, too. must not be forgotten, and its replacement by one designed to avoid losses as far as possible will always assist in thegener-

al improvement. These points must be considered in every grid circuit, and since the losses there are always much greater than in the primary circuits, the plate circuits may be practically neglected. If the detector operates on the leaky grid principle, it is nrobable that reaction is provided, in which case there is not the need to investigate fully all possible sources of loss, because regeneration will bring about a reduction in the resistance of this circuit, but the detector plate circuit cannot be entirely neglected. Here there may be reverse reaction through the capacity of the valve itself, which has the effect of throwing a load on the grid circuit. Rectification efficiency may be improved by connecting a larger capacity between the plate and filament circuits, but care must be exercised, because too large a value will bring about a reduction in the high note response. The importance of the points mentioned above may be gauged from the following example:—ln a 25in coil of about 60 turns of No. 20 s.w.g. wire, which is of comparatively heavy gauge for this purpose,, the actual resistance of the wire is about a third of an ohm. At a wavelength of 300 metres the r.f. resistance of such a coil would be about 4 ohms, due to the loss of energy caused by the creation of eddy currents in the turns of the coil itself. In addition to this, there is the loss of energy in the material on which the coil is wound, the tuning condenser, the loading of the valve, and its associated parjs, so that ultimate’ v the r.f. resistance may be 10 ohms, or 30 times the d.c. resistance. The grid leak should be placed across the grid condenser, and nor. from the grid to fijament plus or cathode, where it would be in parallel with the coil, especially in a power grid detector, where the leak is probably about 250,000 ohms, and is, therefore, comparable in value with the dynamic resistance of the coil. SCREENGRID AND GRID BIAS

The matter of grid current in amplifying valves is an important one. So many screen-grid high-frequency stages are used without grid bias that it is necessary when possible to avoid valves which pass a fair grid current when the grid bias is zero. Some valves pass much more than others at this point, and you would therefore expect the performance to suffer with a valve passing current as compared with another passing a lesser current. There are considerable differences between the grid current characteristics of valves, and these are reflected to an extent in the other characteristics. If you just plug in one valve, note the results, and then try another valve and obtain different results; grid current may be playing a part. It lias been noted that a valve having a heavy grid current at zero bias tends to broaden the tuning and to “deaden” the circuit. When grid bias is used, such as negative .9 volt, the results are not likely to be so different. In fact, provided there is no grid current at this bias, the valve having most at zero bias will probably provide the strongest signals.

CONDENSER SPECIFICATIONS Condensers used for radio work have to have a certain safety margin, that is. they have to be able to stand up to an actual test voltage considerably higher than the voltage at which they are meant to operate in ordinary working conditions. The margin of safety differs a good deal according to the type of condenser and the kind of voltage with which it is to be used, and a good.deal upon the quality of the condenser and the repute of the manufacturer. Clearly, a reliable firm will take every precaution to avoid the possibility of any of their condensers breaking down in use whilst a cheapjack condenser such as some of the foreign ones which find their way tipon the market, may be made only “to sell,” and with little or no regard for its reliability in service. In regard to condensers, almost more than any other component in the set. you should be careful to go in for reliability, because in many positions the very purpose of the condenser is to safeguard some other component, such as a valve or

loudspeaker, or, indeed, even to safeguard the user of the set, which is yourself! SCREENING Constructors do not always realise that it is just as necessary to screen condensers as well as all coils on the high-frequency side of a set. The potential variations on the vanes of a tuning-condenser set up spreading fields of static force, which are just as liable to eive rise to back-coupling as the magnetic flux from a highfrequency coil. This liability to elec-tro-static interaction between different circuits is, of course, greatly increased when several condensers are “ganged” together in close proximity to one another, so that care should always be taken to see that they are adequately screened from one another.

UNINSULATED WIRE

No useful purpose is served by using insulated wire for the aerial or for the down lead. The only reason why enamelled wire is used on aerials is to avoid weathering wearing out the wire. Unless one is situated very close to the sea a hare aerial should last several years. The coating of oxide that forms on a bare aerial does not lower its efficiency. Copper wire has a lower resistance to highfrequency currents than any other metal except silver. The use of tinned aerial wire is not so good. High-frequency currents flow for the most, part on the outer surface of wires. It flows along the tinned portion of a tinned aerial. Tin has a resistance many times that of copper. The size of wire used in an aerial makes very little difference for reception purposes provided it is larger than 22 gauge. The 7/22 copper wire sold for aerials is vei*y suitable for the purpose. If anything, it errs on the heavy side. Very few people have either the space oi* the money to erect 50-foot masts 300 feet apart. An aerial of this type, however, should lie able to pick up 4YA, Dunedin on a crystal set in the Dunedin district. On exceptionally good night it should be possible to hear some of the stronger Australian stations on a crystal. If any reader is able to indulge himself in such an aerial the results he obtains would be interesting to hear about.

TO-DAY’S PROGRAMME.

IYA, AUCKLAND (875 Kilocycles). 5.0: Children’s session. 6.0: Dinner music session. 7.0: News and market reports. 7.40: Talk, under the auspices of the N.Z. Manufacturers’ Association, “The Making of Nails.” S.O: Studio concert by the Auckland Society of Musicians. 2YA, WELLINGTON (720 Kilocycles).

5.0: Children’s session. 6.0: Dinner music session. 7.0: News, market reports and sports results. 7.40: Lecturette —Mr. Johannes Andersen, Librarian, Turnbull Library, “Maori Folk Lore.” 8.0: Relay from the Blue Triangle Hull of Concert by the Orpheus Musical Society. “Tubalkain”, “The Coffee Cantata.” Soloists—Miss Jeanette Briggs, L.A.8.; baritone, Mr. Ray Kemp; tenor, Mr. Roy Hill. Instrumental solos by Mr. Claude Tanner, ’Cellist—“Sonata in G. Minor” (period 1670 to 1742 in four, movements); “Largo.” “Allegro Con Spirito,” “Adagio.” ‘‘Vivace.” 3YA, CHRISTCHURCH (980 Kilocycles.)

5.0: Children’s hour. 6.0: Dinner music session. 7.0: News session. 7.30: Talk —Mr. H. J. Geddes, Lincoln College, “Some Cultivation Problems.” 8.0: Specially Recorded International Programme. Dr. Eugene Ormandy and His Salon Orchestra. International Singers. Abroad with the Lockharts. Weather forecast and station notices. International talk—Mr. L. R R. Denny, M.A., F.R.H.S., “Java.” Ambassadors of Melodyland. Hywide and Handsome. Nat Brusiloff and His Lido Venice Orchestra. 4YA, DUNEDIN (650 Kilocycles). SILENT DAY.