How the Socket Power has Helped Radio

Radio Record, Volume III, Issue 10, 20 September 1929, Page 40

How the Socket Power has Helped

Radio

A.C. v. D.C. Operation

(By

W. M.

DAWSON

MIW, A.MIRE)

ig

HEN comparing. present-day radio receiving sets with those of 15 years ago it is interesting to note that nowadays a set requires more: sources. of current. Though at first this may seem a retrogressive step in reality these conditions are

due to the character of radio reception. ‘The energy received by an aerial is extraordinarily small even in. the neighbourhood of a powerful transmitter. It is therefore remarkable that not only can a transmitter be successfully received over thousands of miles, but that such reception can be amplified to loudspeaker strength. -It is most difficult to measure the amount of energy which is induced in a receiving aerial, and what small energy there is can only be indicated by the. most sensitive ‘instruménts. , The volume produced by the loudspeaker, which is sometimes so powerful that there is vibration, is not the direct result of the power in the receiv-’ ing aerial, but is due to power derived from other sources, e.g., "B" battery or power unit. _ For about 15 years, amplification was not possible, as the radio valve, as

it is now known, had not.been invented, In these times the received energy | was the only source of power from which audible signals could be obtained, and therefore reception by. telephones only was possible. — ‘The reason why reception can. now be amplified to any desired strength must not be attributed solely. to the fact that transmission efficiency has improved, but to the development of receiving apparatus which enables a considerable amount of energy, ‘to be freed by. the use of local sources of current supply. These auxiliary sources supply the current for heating the valve filaments and also the voltage which is applied to the plate or anode of the valve. First dry cells or accumulators were used

for hoth current supplies, ie. one or more accumulators for the filament and dry cells for the plate. These two sources supply energy ‘for loudspeaker reproduction, and it goes without saying that- the greater the amount -of current used,. and provided the valves are of the correct type, the greater jay be. the. volume, . An accumulator; ‘ especially when only of one or two célls, proyides an excellent and constant source ‘of ¢urrent if it is kept fully. charged. . Recharging, however, has one drawback. ' Some accumulators are very heavy, and are therefore difficult to transport to a charging station, and in addition there is the danger of leakage of acid. One way out of this is to charge the batteries where théy are used, and-to do thig energy will be required. Provided electric mains are available the necessary current supply can be derived from them. The first problem to be _ considered is that an accumulator must be charged with direct current. In most cases the mains supply is alternating current, which cannot be used for accumulator charging. without its first being converted or rectified to direct current. Direct current is also necessary for other purposes in radio work. Rectifiers, Mechanical and Otherwise. HERE are various methods whereby alternating current can be converted into direct current. While we -do not intend to discuss them all fully it is necessary to give a few details. By mechanical rectifiers is meant apparatus which converts alternating current into direct current by méans of moving parts. In the first place there are rotary convertors, small machines with a rotating armature which are fed with alternating current. The direction of the current is kept constant by means of brushes which make contact with the commutator. For charging accumulators as would be used in the usual type of recéiving installation these machines are not used as they are too complicated, too expensive, and too heavy. Besides convertors, there are alternating current motors which are proyided. with interruptors by which’ the current can be rectified. These motors also have a _ Totating movement. Finally, the current can also be rectified by a vibrating metal spring. If the vibration of the spring synchronises with the perioditity of the alternating current such current ean be interrupted sq that only one phase passes into circuit. As a matter of fact, there are rectifiers which, work on this principle. A transformer (a safety device to prevent short-circuit-ing should the contact fail), a rheostat, and an ammeter alre absolutely necessary.

It is of great importance that the ammeter be of the moying coil type, as otherwise the charging current cannot be tested correctly. Vibrating rectifiers have various drawbacks which have resulted in this type almost disappearing from the market. Such a rectifier is not by any méans silent in ‘operation, and in addition adjustment is rather difficult, ofteri requiring readjustment. through alterations of frequency, heating, etc. Sudden alterations in alternating current frequency often oceur when cables short circuit give rise to fusing or sparking of the contacts. Chemical Rectifiers. | "te (QHEMICAL rectifiers’ are: based on the principle that certain elements only: allow the current to pass in one direction when the electrodes are correctly selected. As an example we may mention a pointed aluminium electrode inserted into a solution of ammoninmphosphate. In this case a transformer is nécessary to reduce the supply voltage.and an ammeter and rheostat (or tap switch) cannot be dispensed with. A rectifier of the electrolytic type is the so-called colloid rectifier, consisting ‘of. a colloidal silver: deposit in. concentrated sulphuric acid with a silver and nickel iron electrode. This rectifier has one advantage, viz., that in this electrolyte the losses are small. Less heat.is developed, and a much smaller, quantity is sufficient, while. a highe! output cdn still be obtained. A drawback is, however, the uncertain life of the ‘cells and unreliable working. -The simplest types of rectifiers are those of the third class, such ‘as Mercury vapour, and rectifiers with oxide -eoated filament. Both types are based on the principle that a glowing surface emits negative electrons which are attracted by a positive potential and cause a current of electrons (that is, an electric current) from the glowing surface to the point of positive potential. No current can possibly. pass in the opposite direction, he dry copper oxide rectifier ‘has recently received some Its chemical action is not fully understood, but it. may be taken-as being elecrofytic in character; the two rectifying surfaces being respectively a metal and an oxide of copper held in close contact by-means of a bolt. The resistance to the passage of ¢urrent in one direction being considerably greater than in the other rectification can be fnade to take place. é The Mercury Are. : With mercury are rectifiers the glowing surface consists of a quantity of mercury on which a s0called cathode spot appears when @ lighting are is introduced between a higher electrode. Conditions of working are high vacuum and an excitation of the are by a contact between electrode and mercury surface. The difficulty is, however, that the drop in voltage in the are between elec Coy" trode and mereury is very large, viz’, about 30 volts, and that a minimuin current intensity. of a few amperes is necessary te. maintain the are. Therefore, the mercury are rectifier is not suitable for the charging of small aceumulators.

e! Thermionic Rectifiers. A YERY favourable solution is found in rectifiers with oxide-coated filament where the negative stream of electrons is obtained by a filament. Even small current intensities can be obtained, and the drop in voltage betwee the filament and auxiliary electrode is small. If such a filament and electrode are placed in a bulb we have a rectify ing valve. The bulb must be evacuated of air and then filled with gas (argon) or vapour (mercury) under low pres sure. In the’first case the Voltage-drop inthe valve is larger, but the valve can be used for the rectifying of -highe: voltagés. In the second case a rectify ing valve is created. which is very ‘suitable for the charging of small ac. cumulators. dn order to use such a valve in com bination with A.C, mains a transformei is necessary, which not only steps down the voltage necessary for feeding — the rectifying valve filament, but which also enables adjustment of the current voltage for the second electrode (the plate) or anode to obtain the charging current. | . A special quality of rectifying valves ig that once the filament is first set glowing they can work without furthe: filament feeding. The collision of the ‘positive gasions against the filament releases such an amount of heat that the filament remains glowing. This experience has, however, no practical use, as the current consumption of -the * filament is but small, and only a.very small saving can. be obtained, and. on the other hand the risk of overchargg the filament is very great (concentration of emission). \ EE

Rectifiers with oxide-coated filament have proved to be most suitable for the charging of accumulators. Philips manufacture many types for 1.3, 3 and

6-amp charging currents, while for anode or H.T. accumulators and for the slow charging of filament current

, accumulators, 100 milliampere rectifiers have been placed on the market by many manufacturers. Practice has shown that this continuous .charging is in no way detrimental to the life or officiency of the battery. \ ‘Vilament-rectifiers have the advancage that they can be easily..handled, and are absolutely reliable and proof igainst short circuit. -Therefore, they ean be charged overnight without any supervision. The fact that ‘filament rectifiers can work a long time without jupérvision or cost or upkeep makes them ideal for use with electric cloqk ‘nstallation, signal devices, switching purposes, ete. Of ,course, pure ‘direct current is not obtained, but a pulsating direct current which, with the rectifier type,. 450 contains 100 impulses per sec., and with the small rectifier type 1017, 50 impulses per sec. This is vhe reason why an accumulator -as used ‘or a radio receiving set cannot be substituted by such a rectifier. In. this sase filament feeding could not. be vffected with direct current, but instead Shere would be a pulsating current yvhich would cause hum by the periodic thange in emission -of the, electrons. However ‘well a filament rectifier aight work in this connection there is 10 advantage in this.method by feeding he filament with alternating current (the number of pulsations being equalty high. This is a problem which entails further consideration. _ *B" Supply. S a second source of current for the. radio receiving set we mentioned plate feeding or anode current. In the early days of the valve a battery of dry cells was always used for anode

voltage, The first, types of receiving valves only required an anode voltage of about 80 volts, and a plate curvent of 1 or 2 milliamps only. With modern radio reception strong loudspeaker reproduction is required for which 3 or more valves are necessary. Therefore, there is need for a last stage valve, requiring an. anode voltage of 150 volts, and a plate curr ent, of about 10 mA, the latter Alepending _ upon the correct negative grid voltag@, applied. When one considers that’: the: other valves of a receiving set, together take a plate current of 6 to 10 mA, it. will be evident that a modern 5-valve receiving apparatus takes an anode cutrent of some 20 mA. ‘With such a current supply the life of the "B" battery is considerably reduced. Moreover, the voltage of the battery becomes exhausted, volume decreases very much, and in addition parasitic noises are heard as cells become defective. oo From this it may be deduced that a modern radio receiving set intended, for loudspeaker reproduction cannot be satisfactorily fed with an anode bat- — tery except for short laboratory | tests or demonstrations. ; "An accumulator can be used for. plate feeding, "but as such a_ battery consists, however, of about 80 cells, it is rather a bulky object, which cannot be handled very easily. A short circuit in the set can damage this battery and various other parts of the upparatus, When the difficultieof "B" batteries» were realised, attempts were made to obtain plate current from A.C. supply, with the sesult that ‘high tension supbly units came into being. ; In these high tension supply units & -_-- -_- ---_- --

rectifying valve is nearly always used. This valve generally is a two-electrode ‘valve, with heated filament. With a valve which has a filament and an anode it is evident that when the filament is heated it will emit negative electrons. ‘ Characteristics. of Power Units. VOLTAGE depends on ‘the current 2 derived, and the internal resistance of the rectifying valve. This must be low with respect tq the external resistance (Philips 378, valve about 300 ohms at 4 volts). Tests for "ripple" (hum) have been. made when high tension supply units are used and compared with ‘‘B" battery supply, and also. tests for’ induction by lighting supply (reception test of Daventry). Everything depends on the smoothing condensers and choking coils for the current derivation and voltage. (With a certain percentage ripple. ) Alternating Current Valves. THE simplicity of working connected with high-tension supply units brought -about the idea that filament current could also be. derivéd from the alternating current supply. There were however, many drawbacks. ‘The filaments of radio valves .at present are very thin, and this being so their heating properties are very small. It therefore follows that when a filament is fed with alternating current the filament oscillates with the same frequency as the alternating current supply, that is to say, a maximum and a minimum ‘are obtained twice during each eycle. The emission, therefore, shows the same fluctuation and a hum is clearly audible in the

telephones when filament current is so applied. This drawback can be overcome by applying sufficient heat to the filaments. This will cause a much higher filament current consumption, but as they are not fed by an accumulator it matters very little. The feeding transformer must supply more current, it is true, but indicated in watts the ineréase -is practically negligible. This, however, is not "the only diffieulty. The alternating. current voltages passed to the grid of a radio valve must circulate in the grid circuit. This. grid %irenit: consists of resistances, capacities, self-induction or .combinations. . These generally are -connected on one side to the grid of the valve and on the other side (generally negative) to the filament. If this is done with. a valve where the filament is fed by alternating. currents, one will realise that the potential of the filament. is not constant with respect to the grid, but instead, a variable one, corresponding with the frequency of the alterrrating current. For this reason this side of. the filament alternatively has a positive and a negative potential with respect to the grid, and. this causes alterations in the anode current which is audible as hum. In order to avoid this the filament must be shunted by a resistance of which the centre is connected to the grid. circuit. This can be accomplished by means of the sliding contact on a potentiometer, or the. transformer usedmust have the centre of the secondary winding tapped. If sufficient precauttons are taken "that those parts carrying alternating current: (transformers, filament current’ mains,- ete.) cannot cause induction, good results can be obtained in this: way with amplifying valves. Even ordinary power valves can be used on a modest scale: in low-frequency: amplifiers. At present better types have been developed in the D105 and D148 with a filament current of .65 amps., and the E series as well as other specialised valves. The greatest difficulties, however, are caused by feeding the detector valve with alternating current. The working of the grid condenser with leak resistance to obtain..rectification renders the valve still more sensitive to the influence of alternating currents and the capacitive charges of the grid with respect to the end of the filament, more especially when reaction coupling is applied which causes a hum as the result of anode current oscillations produced as stated above. The methods for dealing" with this trouble,.are not at all sufficient, and only when a new principle is. used (separate ‘heating ) has "it become possible to. construct a detector valve which works absolutely free of hum with alternating current. -The emission of electrons is not .obtained by a filament but by a separate tubular. surface which. is indirectly heated by a filament fed with. alternating current. This filament is connected to an.alternating current voltage of 2.5 volts, and consumes 1.5 amps. Valves of this class are the 227, H102T, and F215. This latter has the. same characteristic as H415, while the former and second are 201H type. Connection to the grid circuit is, of course, not by means of the filament, but. by the electron emitting cathode which has no electric contact with the sfilament.. For connection to the. grid

‘circuit. a connection is provided with the cathode (on the cap). These alternating current valves provide a solution of the problem of filament current supply, and when replac-' ing the negative grid voltage battery by -a resistance, shunt a condenser in the anode circuit, or, still better, use a small high-tension supply unit. ‘The modern trend is towards the use of indirectly-heated valves in all sockets of the receiver but the last, it beitig found satisfactory to feed the ordinary D.C, valve from an A.C. filament source when the valve is not followed by a further amplifying stage, i.e., on the ‘power’ or output socket of the receiver. Due to uniformity of electron emission all along the cathode -and other, reasons which are rather technical to be of general interest in these columns, it is yossible to design and ‘manufacture independently-heated A.C. valves having characteristics far surpassing the best possible from a directfilament valve, ie., batteryoperated type. * Valve Characteristics.

HERE are three main factors or characteristics which serve to indicate the "goodness" of a’ valve. The first of these is the "amplification factor" of the valve, giving. some indication of the voltage gain of the valve, ie., how.many signal volts may be expected in the: plate circuit from one volt applied to the grid. The next factor is plate impedance, which indicates how much of the valve opposes the setting-up of the desired signal currents variation on its plate circuit, The impedance normally should be low. Generally speaking, a high amplification factor: is: desirable, but this is usually accompanied, by a rise in impedance, the latter disadvantage offsetting the former advantage, so that these two factors must be jointly considered to arrive at a true estimate of the merit of the valve. This characteristic is known as "slope," or ‘mutual conductance"’ It is usually expressed in ‘milliamperes per volt" and gives valuable information as to the number. of milliamperes of signal current released in-the plate circuit of the valve for each volt of applied signal grid potential. . _ Thas the steeper the slope of the valve, ie, the greater the M,A.’s per volt, the more effective it is. To indicate: the enormous advances. -made in A,C.. valve manufacture we may compare a.modern example with the D.C. general purpose quarter ampere valve. .

DC. AC. Amplification factoy . 8 24 Impedance ...... 10,000 8,000 Slopes... se eee 0.8 3.0 From these figures it is readily seen that it is possible to design an A.C. set not merely a "convenient" set in that it eliminates battery troubles, but a definitely superior set to one .battery operated. The initial troubles experienced in the manufacture of A.C. valves have been overcome by at least the léading manufacturers, so that there is now no reason why the modern A.C. valve should not last practically as long as its D.C. brother. ‘The first general purpose A.O, valves (directly heated) gave trouble due to an "opening" of the filament circuit because of a manufacturing trouble ex-

perienced in getting the "pinch" of the; filament support to bite deeply enough: through the oxide coating of the filae: ment to maintain :good permanent contact .with the metal beneath. The earlier indirectly heated valves, as used in the detector sockets of the, first American A.C, receivers, had a distressing habit of burning out after a very short life. This was wont to take place just inside the insulating thimble of the cathode. Both of these troubles have now been overcome, and the reader need not hesitate to "go A.C." on. account of shortlived valves, "provided, of course, that he. buys a high-grade receiver in which the necessary protection demanded for A.C, valves has been taken‘into serious account by the designer. The use of receivers deriving all their power requirements from -the A.C. mains has greatly helped alotig the development~-of "real" power ‘valves. When operating from battery supply the use of a "real" powér valve imposed such a drain on the financial resources of the average user-due to incessant 2nd expensive battery re-newals-that the use of.valves suitable for giving really big volume without distortion became prohibitive. All this is changed with A.C., operation, because even the largest "combination" sets draw quite a small amount of "power" from the house supply mains, and we need no lohger starve the speaker on account of prohibitive running costs. "We can thus enjoy realistic volume, and our thanks are largely due to A.C. operation. In the early stages of A.C. development it was only feasible to make large sets for A.C. operation, but there are; bow on the New Zealand market com-' pletely A.C. operated receivers as small; as two valves, of moderate price, and completely satisfactory operation. The.use of A.C. has then conferred the following benefits on thé’radio user. It has provided excellent ‘battery .substitutes and charging | aids by which the user may, with existing D.C. set, enjoy better reception. with less maintenance worry. It has provided means whereby the D.C. set may be modernised for complete battery elimination, and at the same time improved in results. It has enabled better sets to be built, and these to be completely operated from an A.C. power source. It has made possible realistic volume and. tonal quality hitherto impracticable, and last, but decidedly not least, it has reduced radio operation costs. Hente it is hardly necessary to remark that A.C. operation of radio receivers has definitely come to stay, and represents one of the greiitest silvances that have been made in rad io of recent years. \