THE POWER VALVE

Evening Post, Volume CVI, Issue 51, 7 September 1928, Page 15

THE POWER VALVE

WHEN AND HOW TO USE IT

More and more use is being made of "power-valves" in the last stages of loud-speaker sets, always with good results when the proper methods are adopted. There are cases, ho*vever, when the adoption of a power-valve not only makes no improvement, but may actually cause trouble. To begin with, the writer is of the opinion that if a comparatively small and light loudspeaker of the horn type is used, tho advantago of a powervalve is so small as to be scarcely worth while. Any standard horn-typo speaker can be , adequately driven by a "small" poijer-valve such as tho R.C.A. 120, the PM4, or PM6, or others passing a plate current of somewhere about 0 or 8 rnilliamperos, and a plate voltage not over 135. Most good loud-speak-ers will, safely stand that amount of windings. Bi every case care must be taken to connect the Jour-speaker correctly so that tho terminal marked as positive is joined to the battery, otherwise there is a serious risk of demagnetising the unit, and ruining it. Some speakers have what is called a balanced armature; in these the connection may be made either way. High-grade modern speakers may give quite gpod results in a moderatesized room with a small powor-valvo, but in nearly all cases they will respond well when a "super-power valve is used, and the heavy feed given such a value is really necessary if a largo volume of sound is required along with undistorted output. A good cone-type loud-speaker will_ handle an amount of power which is simply overwhelming in a living-room, and there is little risk of overloading the loud-speaker itself. But it is seldom sufficient merely to take out the last valve and replace it with a high-power valve.and increase the battery supply. A circuit change is practically imperative. In the first place, a high-power valve will pass a plate current of anything from 10 to 30 inilliamperes, depending on the type used and the plate voltage,, This comparatively heavy current may injure tlie windings of the magnetic system, and will almost certainly have the effect of producing distortion by "magnetic saturation" or mechanical displacement of the armature. Hence some device is needed to side-track the plate-current into a course where it will do no harm, and yet pass the "speech frequency" currents through tho loudspeaker. Two devices are used, each with its advantages. One method is to use an "output transformer.'' The plate current passes through the primary coil, which is designed to carry it safely, ana the soundfrequency currents are transferred to the secondary and the loud-speaker.-The advantage of the transformer is that it enables the speaker to be "matched to the power valve. \ > The other method is to uso a choke and condenser combination, often called a "filter." An iron-cored inductance is connected between the plate of the valve and the plato battery. A largo fixed condenser, of at least the microfarad and preferably twice that capacity is connected to the plate, and the other side to the loud-speaker. Tho free terminal of the-speaker is connected to the negative end of the plato battery. It is often reeommendod that the speaker sliould be connected to the positive sido of the battery, but if this is done, the sound-frequency currents have to pass through the plate battery, and one of the chief advantages of the filter circuit is lost. The effect of these currents in the battery may not be noticeable when the batteries arc new ana of low resistance, but if the resistance increases, the vigorous audio-fre-quency currents set up a reaction eftect which is often quite destructive of good reception, and may cause continuous howling or that extraordinary performance known as "motor-boating f or which no more appropriate name can be found. If an eliminator is used, "motor-boating".is very liable to be experienced unless.the set is carefully designed to avoia low-frequency reaction. There is a simple remedy for. otherwise stubborn cases of motor-boat-ing though it; may seem a retrograde step. That is to use the eliminator for the power valve alone and use dry batteries for the plate supply of the other valves. In the case of a set using four or five valves, properly biased, largesize batteries are not required, and tho battery does'not add.much to tho running cost.