CHOOSING SUITABLE VALVES

Radio Record, Volume I, Issue 32, 24 February 1928, Page 10

CHOOSING SUITABLE VALVES

ee ene Assuming that the experimenter has made himself familiar with the published data regarding the various classes of valve, there remains the question as to which valve should be selected for sny specific purpose. We may deal first with the case of so-called power valves. These are intended for connection to a loudspeaker, or for ‘any other purpose where an output of considerable power is required (as opposed merely to the production of amplified voltage). In the consideration of a valve of this type a distinction must be’ drawn between its efficiency when reproducing a weak signal and its capacity for handling power without distortion. As au example we may compare thie per-

fat ach eh areola in At tanh nlatatahiaht dale tee tna talreetahye thant" re ~ é formance of two yalves in a receiver which is sensitive enough-to produce ample voltage from a strong local sta-~ tidén for application to the grid of the last valve. If we insert our valyes in such a receiver and adjust the volume until it as loud as possible, while being free from distortion, we shall obtain a reliable estimate of the second characteristic of the valve; but if we tune the receiver jn to a very weak station and, in mak‘ing the comparison, refrain from altering the tuning controls, so that precisely the same small signal vyoltage is applied to the grid circuit of each of the valves under test in turn, we shall get a comparison under the first heading which may give us an entirely difterent result. In fact, we may say, as a general rule, that if s valve is constructed so as to be capable of large undistorted output, it will be less efficient as a reproducer of weak stations than a valve of similar type in which the undistorted output is less. DYNAMIC CHARACTERISTIC, Power valyes are usually designed to have an internal resistance of less than 7000 ohms, and not infrequently as low ag 2000 or 3000 olims. In comparing two valves which have equal internal resistances, the valve with the higher magnification factor is the hetter, thongh this does not quite coustitute a complete survey of the problem. It is highly desirable that the valve used in the last position should have a straight characteristic of considerable extent, the limits being, on the one hand, the bottom bend, and, on the other hand, the point at whicl the grid circuit begins to flow. Information on this point can be obtained from published characteristics, and it. will be found that in general the lower the resistance of the valve the greater the ‘‘straight range." A word may be said regarding the correct adjustment of grid bias in the case of a loudspeaker valve. Manufacturers’ published characteristics are taken without any load in the plate circuit, and it is customary, in the case of a loudspeaker, to make the average impedance of the windings equal to the resistance of the valve, so as to get the optimum efficiency of reproduction, Needless to say, this cannot be achieved at all frequencies, and it is not intended to do more than emphasise that under practical conditions there is impedance in the plate circuit, and in consequence the valve will haye a more gentle slope than appears in the pubiished characteristic. We can assume that the theoretical conditions are complied with and that the loudspeaker is in fact a resistance equal to the internal resistance of the valve. We next come to valves which have been classified in a somewhat indeterminate manner as H.F, and L.F. am. lifiers. These valves do not proess to have any very great range of straight characteristic, although the class of L.I. amplifiers mav be used for a moderate volume of loudspeaker reproduction. Setting aside this use we may consider for what remaining purposes in a set various types of valves under this heading are suitalle. _ In a low-frequency amplifier employing transformers it may at first sight (Continued on Page 11.)

Construction -¢ ontinued