CHOOSING SUITABLE VALVES.

Radio Record, Volume I, Issue 32, 24 February 1928, Page 11

CHOOSING SUITABLE VALVES.

(Continued)

seem a matter of indifference whether with a valve of low resistance or a transformer of low ratio with a valve ef high resistance. In order to cona transformer of high ratio is used sider this question, two facts must be borne in mind; first, that, in the case of any given construction of valve in which only the density of the grid mesh is varied, the ratio of the mapnification factor to the internal resistance is approximately constant; secondly, that, in the case of transformers of a given type, but having varying numbers of turns on the primary winding, and consequently different stepup ratios, the appropriate valve internal resistance varies inversely as the square of the ratio. Thus, if we have a 3:1 transformer, which is recommended for use in conjunction with a valve of internal resistance of 24,000 ohins, a 6:1 transformer of the same type will operate in conjunction with a valve having an internal resistance of a quarter of this value, namely, 6000 ohms. But the former valve, if constructed in the same manner as the latter, would have a magnification factor four times as great; hence, since the total magnification is equal to "magnification factor’? multiplied by "step-up ratio," we shall obtain twice as much magnification from the valve of high internal resistance with the low ratio transformer as we obtain from the other combination. There is, further, the advantage of a lower H.’. consumption, which is by no means a negligible factor. Valves for use in the earlier stages of. a set should be built to have as high a resistance as is consistent with an adequate range of working characteristic, and with as higl a magnification factor as the particular form of construction will allow. RESISTANCE AND CHOKE L.F. COUPLINGS. For the purposes of low-frequency amplification, where either a choke or a resistance is employed, there is an opportunity for using valves of exceptionally high magnification, and this class of valve has recently come into prominence to a considerable extent. If we can succeed in building an impedance, either in the form of a choke or a resistance, which is large in comparison with the internal resistance of such a valve, we get, practically speaking, the whole of this amplification at eacls stage, though there are complications, of a character by no means negligible, introduced as a result of the inter-elec-trode capacities of the amplifier, However, it may be said that the results

obtained on these lines have led to a very great advance in the design of resistance-capacity amplifiers, and have established these valves of high-mag-nification factor firmly in the popular favour. The advantages offered by such valves, apart from their actual efficiency in operation, is their ex-’ tremely low H.T. consumption. |