THE AMPLIFIER

Evening Post, Volume CVIII, Issue 77, 27 September 1924, Page 22

THE AMPLIFIER

OVERLOADING OF VALVES.

Tho importance of keeping the work imposed on amplifying valves within their capacity is not, one fears, realised as fully as it should be, even by designers of costly receiving sets. The function of an amplifying valve is to magnify either the voltage impressed upon its grid or, if it is the valve supplying the final output to translate its grid voltage changes into a power output. " The voltage amplification factor of the overage amplifier valve is about 7. That is to say, a change of grid potential of the volt will make such a change in the volt plate current as would be effected by a change in the plate battery voltage of 7 volts; and with an efficient apparatus for passing on this change to the next valve, the good potential becomes changed to that extent. If the coupling arrangement is. as it is generally, in an adio-ampuiying circuit, a _ transferomer, the voltage stepup between one grid and the next 13 further increased. Assume, for the sake of argument, that the average step-up per stage is 10, and that the voltage change delivered to the first amplifying valve after the detector is only one-tenth of a volt. If there are three amplifying valves, the second will receive grid varistions of 1 volt, and the third of 10 volts. It is not often that a detector ■will deliver changes of such magnitudes as one-tenth of a volt except with very strong signals, .but it can easily do so if a _ stage or two of radio. frequency amplification precedes the detector. It will be seen, from the examination of the^ characteristic curve of an ordinary receiving valve, such as those recently published in this column, that none of fhe smaller capacity valves has room within its range for a 10-volt sweep of grid voltage changes without trespassing on the bend at the bottom of the,curve or on the positive grid potentials.' This supply means that to use such a valva in such conditions will ensure its being overloaded. It .cannot possibly give true , amplification; and it may even fail alto- j gether in bad cases to add volume beyond that produced in the second stage. The effect is then altogether disappointing—no increase in volume, but a serious amount of distortion. The plain fact is that, given signals of good strength in the antenna, two stages of audio-amplification with valves of tho crdinary receiving types should fill the !;ist valvo " full," and that full output of the valve, not overloading, should wrrk a loud speaker satisfactorily, unless great volume of sound is required.

If the second valve is thus working up to its capacity, it will obviously be useless to add another of the same cappcity in the third stage. A similar valvo will be useful only when ..weak signals are being received.and this second stage output is not sufficiently loud. 'I he only way to secure a satisfactory lurther increase in volume is to use a following valve of larger capacity, and to choose one which will accept a wide sweep of grid voltage changes. This in general means what is commonly called a " power amplifier." Probably the smallest valves which will give good service in this capacity ar& those having ihe^ characteristics of tho well-known UV2OI-A. These valves havo quite a ir.rge current passing capacity—that is, they have a large power output, and if they are operated with a high plate voltage, 100 volts or more, will accept a powerful input. But even these can be overloaded by an operator who is xeally ambitious in the matter of volume of sound, and it becomes necessary then lo adopt a " power valve"—a small transmitting tube of, say, five watts rating. This at all events will not overload, and will prove distortionless amplification of whatever isl put into its giid circuit. At the same time, anybody who wants more noise for houso-!-old purposes than a 201 A will give is a glutton. It may be of interest to note that Captain Eckersley, one of ,tho most prominent British broadcasting experts, stated in a lecture recently that for perTectly satisfactory loud-speaker work the lust valve should have a grid voltage sweep'on the straight part of the curve of 20 volts, without the grid becoming positive. Consider how far from this ideal is the remarkably efficient' little 199, which is often used in second and even third stages, with Jt3 negative sweep of approximately 8 volts. In the earlier part of this article' a ■voltage amplification factor of 10 was assumed to cover both valve and coupling transformer, in tho audio-frequency amplifier. It may be thought that, with a valve'having an amplification factor of 7 and a step-up transformer ratio of 3 or 5 to 1, this figure is too low. It may be; but if so, it merely strengthens the argument for looking after the capacity of following valves. However, the efficiency of the transformer coupling system is a much disputed matter. Resistance coupling and impedance coupling givo of themselves no step-up, and necessarily take some fraction off the amplification factor of the valve; but they are not nearly so inferior in the result to a transformer coupled apparatus as the loss of the nominal step-up would indicate. It is evident that a coupling transformer does not in general deliver the stepped-up voltage implied by its ratio number.