A Diagnosis of Radio

Radio Record, Volume V, Issue 12, 2 October 1931, Page 17

A Diagnosis of Radio

By the Technical Editor

A CORRESPONDENT requests that, before this series be closed, a few notes should be given on push-pull amplification. Others have written on the same subject, one remarking that he does not know the difference between push-pull, pull-push, push-in, pull-out, and so forth, and asking for an explanation. The explanation of push-pull is a little complicated, but we shall try to keep out technicalities as far as possible. Ij must be borne in mind that all rg#¥o amplification can be carried out only because of the pulsating nature of the high frequency currents. They surge backwards and forwards similar to the waves breaking on the beach. We have the pulsating movement taking place in the primary of all transformers. This is picked up by the secondaries. If one end of the secondary is connected to the grid of a valve, and the other end connected to earth, as we explained last week, the pulsations on the grid will alternately be high and low. A push-pull amplifier is, on the other hand, one in which the grid voltage changes are applied simultaneously to two yalves, not one and the earth, and connected so that increasing grid voltage on one valve is accompanied by decreasing grid voltage on

the other valve. The resulting plate current changes, which are opposite in direction for the both valves, are passed through an output coupling device in such a way that their effects are added together. One valve tends to push plate current through the coupler while the other tends to pull it in the same direction, this action accounting for the name of the amplifier. Let us now look at the component parts of a push-pull amplifier. There are two valves instead of one, and two transformers. We notice that these transformers are slightly different from the ordinary. The first transformer, known as the input, has a centre tap which is connected to grid bias, or what we might, for the time being, regard as earth, a: far as high frequency

current is concerned. The second transformer (output) likewise employs a. centre tapped coil, but this time it is primary, and this centre connection is made to "B+-" potential. Let us follow the movement of the electrons. In the first case we get a surging backward and forward in the primary of the input transformer. This is picked up in the secondary, but, instead of surging taking place from the one extremity to another, it takes place from the extremities to the centre tap. Thus while the electrons are piling up on the grid of the valve, there is a deficiency of them on the grid of the other valve. When the cycle is reversed the high pressure of electrons on the first valve eases off by their passing to earth, but there is a pile of electrons on the valve

which previously had a deficiency. Thus the grid voltage rise and fall in the second valve is always opposite to the rise and fall in the first valve. . We know from our study of the valve itself that each alteration in the potential on the grid of a valve is magnified in the plate circuit. Thus, while the electrons are piling up on the grid of one valve, the flow of the electrons from the plate is being restricted, Blectrons are negative and tend to inhibit the flow of current from the plate to the filament, When the grid becomes less negative more current flows in the plate circuit. Now as the valves are "out of phase" we will find that in one plate circuit the high frequency current is flowing one way, while in the other it is the reverse. If we can connect these two plate currents we get a continuous pulsation, and the effect of this can be seen from the diagram. ‘The outputs from the plates of the two valves combine in the split primary of the output transformer, and their combined energy passes to the secondary of the output transformer. In a general way we have now explained how push-pull amplification takes place. Let us see some of the practical factors involved: Push-pull amplification implies1. That the amplifier is able to handle (Continued on page 29.)

at i more volume than previously, for the simple reason that the volume can be split up between two valves instead of one. 2. The amount of bias that can be applied between G.B. and earth is double, or more, than for the single valve. The reason is not apparent without the use of valve charts, but it is such that, not only can a greater bias be applied, but also, by applying greater bias, greater signal variation can be applied to each valve, so that really we put on far more than double the voltage (a.c.) to the grid than we could have to one valve. 8. Less plate current. Because a higher grid bias can be used, the plate current is automatically diminished. This makes push-pull amplification admirably suited to receivers which de- |

pend entirely on batteries for their supply of current. 4, Because of the compensating ac: tion of the two valves, certain forms of distortion will balance out and leave true reproduction of the input voltages. 5. When the amplifier is operated with a.c. on the filament, hum will be, 7 &° a large extent, balanced out. 1 6. The valves must be carefully matched. 7. A push-pull amplifier requires twice the input grid voltage required by a single valve of the same type, if the push-pull arrangement is to show an advantage over a single valve in volume of output. 8. Ordinary trausformers may be used instead of the split primary and split secondary type by connecting them as shown in the diagram. Re sistances across the secondary of the input transformer should be about 500,000 ohms. The output transformers are two ordinary audio transformers of the same ratio. 9. Remedies for incorrect design may take the form of a .0001 to .00025 mfd. condenser between the grid of the pushpull valve and the centre tap or a 0 to 100,000 ohms resistance across the same position. A tone control, consistingapf a .05 condenser, and a 200,000 oliffs resistance can be employed in series between the grids of the valve. 10. A tapped output impedance can take the place of the output transformer. In this case, however, match-

ing is not possible, and consequently the system should not be employed unless the plate resistance of the valves being used is within the limits of 80 to 125 per cent. of the speaker output impedance in ohms. With low impedance valves and a high impedance speaker there will be opposite relation and an impedance matching output transformer should be used. Accompanying this article are several diagrams of push-pull amplifiers. PPE A.C. set follows the general lines Jaid down in the earlier articles in this series, only, instead of employing valves with a directly heated filament, a slightly different arrange-

ment is employed, whereby the electron emitting body is indirectly heated because of its proximity to a wire which is heated by a.c. voltage. Very few valves indeed can have their

heaters come in direct contact with the mains and, in order to reduce the mains voltage to a suitable value, a step down transformer must be employed. With the plate circuit, however, we cannot employ pulsating current (alternating current is pulsating), and we must take steps to rectify and smooth the A.C. current from the mains. For a moment we shall look at a typical power pack and see what happens. Between all A.C, radio apparatus and the mains themselves there must be employed a transformer. It is against the regulations to use anything else. This transformer will alter the A.C. mains voltage to whatever we require. If we are designing an ordinary set we shall want something like 180 to 250 volts smoothed, for the plate supply, therefore the secondary of the power transformer must be able to supply this. Grid bias must be added to the plate voltage and allowed for in winding or designing the transformer high voltage secondary coil. Further. more, certain losses will be occasioned by the chokes and the rectifier, although in modern rectifying valves the loss in voltage is very small. The secondary is then wound to supply, say, 60 to 100 volts extra, as the case may be. The voltage developed ucross this secondary will be in proportion to the number of turns wound on and the amount of current avail: able will depend upon several factors. not the least of which is the diameter of the wire used in the secondary wind: ing. This voltage is a.c., and as such is useless for plate supply. It must then be changed to direct current. This is brought about by rectification, a sys:

em by which the reverse pulsations re cut out, leaving the current to flow n one direction only. The eurrent s still rising and falling in voltage, jut is not reversing, as ordinary ‘pulsating current does. Im order to rectify the current a special valve is generally employed in what is known as a full-wave circuit. The connections for this are as shown. ‘The ends of the high-voltage secondary go to the plate of the fullwave rectifier and the centre tap of the filament winding of the rectifier supplies the high-tension positive. The centre tap of the secondary winding is the negative. Now between this -+ and — we have a d.c. voltage of a slightly lower voltage than that developed by the seconaary. If we were to use this in the set hum would be present to a marked degree. We must have some means of smoothing or ironing out these pulsations. This is done by a combination of chokes and condensers. A choke we have described, and usually two of these are employed with condensers on either side of them, the other side of the condensers connecting with earth and high-voltage secondary centre tap. At the end of this filler we have smooth current of a voltage equal to that delivered by the secondary, less that dropped in the rectifier and the chokes. This high voltage may be applicable to one valve, but we require a variation in the voltages supplied throughout the set, so resistances must be used. You will remember that when current passes through a resistance, the voltage is dropped aceording to the value of the resistance, and by using suitable resistances in the cireit we can drop the voltage to whatoyer is required. Thus we can turn

the a.c, voltage from the mains to any value of direct current. Recapitulation ND now, perhaps, we can see, even if vaguely, how a set as a whole operates, and how the various components play their part in converting the weak, rapidly oscillating current radiated by the station, into real music and speech. We have seen how the aerial collects as many of the vibrations as possible, and how, in spite of the fact that these impulses are only too ready to skip the short distance between the aerial and the earth, they are passeé on to the set. The aerial coil and the tuning condenser select the station desired by the operator, and they are passed to the grid of the first valve. They are magnified by this and the following valves until they are ready to be detected. Magnification before the detector is spoken of as high frequency or radio amplification. A coil and a valve suitably connected are known as a stage. It is a tuned stage if a condenser ig used across the coil. When the signals reach the detector their form is altered. The rapidly pulsating vibrations which have carried the slower currents from the transmitting station are no longer wanted, and they are stripped away. Further magnification takes place by means of transformers (usually) and

valves until the signals are strong enough to work a loudspeaker. The current in the audio stages should be the exact replica of the current in the microphone. It is the disturbances caused by the voice or instrument changed into electrical vibrations. If these electrical vibrations are reconverted into sound we can hear, and the miracle of radio has been performed. This last step is performed by the loudspeaker. Before a signal can be magnified power must be utilised, and this may be

drawn from batteries or the mains. Both systems are very similar. In the battery type of set, the filaments of the valves are heated by a direct current of electricity. In the mains valve the filament is heated because of its proximity to a wire, kept hot by virtue of its being connected with the mains through a suitable transformer. Tf we applied this current directly to the filament there would be a hum. In both battery and a.c. set the high voltage

supply is direct, and as the mains voltages are a.c., a system of rectification has to be employed. Usually the rectification process is carried out by a valve, but this valve in no way magnifies the current, so that when we speak of an eight-valve set "including a rectifier," @ seven-valve set is really meant. The eighth valve can be regarded as_ the modern high tension or B battery. We now come to the end, and it is hoped that some, at least, have a little more insight into the working of a radio receiver than they had previously.

It is hoped, too, that theoretical radic diagrams will present less of a puzzle than they did formerly.