WIRELESS

Nelson Evening Mail, Volume LXIV, 21 February 1931, Page 3

WIRELESS

THE CHOICE OF VALVES QUALITIES WHICH DETERMINE SUITABILITY The qualities which determine the suitability of various valves for different positions in a receiver are denoted by numbers which indicate the anode impedance, amplification and mutual conductance of the valve. A general indication of the purpose for which a valve is most suited is given by a rough examination of its impedance and amplification factor: comparison between one valve and another of the same class is made by a more careful comparison of their impedances and amplification factors and of tlieir mutual conductances.

IMPEDANCE The term “impedance” or “differential resistance” as it is sometimes and more correctly termed, denotes the effective internal resistance of the fila-ment-anode path of the valve, and is expressed in ohms. Because its value depends upon the value of the anode voltage' and grid voltage (also, i(n screened grid valves and pentodes, upon the auxiliary grid voltage), the impedances of valves, as given in makers’ catalogues, are measured under some standard conditions, usually at anode volts 100, grid volts zero. The importance of knowing the impedance of a valve lies in the fact that for best results the internal impedance of the valve should bear a definite relationship to the impedance of the external circuit, or, in the case of an output valve, to the impedance of the speaker or of the output circuit (choke or transformer) if used. If the impedance in the external anode circuit is known, it is not difficult to calculate mathematically the most suitable value for the valve impedance, but practical considerations necessitate the use of valves which differ somewhat from the ideal.

In high frequency amplifiers employing tuned coupling's, a very high internal impedance is required in order to obtain the maximum gain per stage. The valve possesing the highest impedance are those of the screened grid type.