COLD CATHODE TUBES FOR AUDIO FKEgUEXCV SU.NAUS 1379 



0.02 



0.04 0.06 0.08 0.10 



ANODE CATHODE SPACING IN INCHES 



Fig. 5 — Resistive and inductive components of impedance 

 anode-cathode spacing. 



as a function of 



IMPEDANCE OF PLANE CATHODE TUBES 



Let US consider first the impedance of a tube with a plane cathode or of 

 a cathode with a radius of curvature large compared to the combined 

 thicknesses of the cathode dark space and the negative glow. An example 

 is the Western Electric 313-C which has a gas filling of 99 per cent neon, 

 1 per cent argon and a cathode surface of barium and strontium oxides. 

 At a fixed steady current of 25 ma flowing through the starter gap, it is 

 found that the resistive component of the impedance varies with fre- 

 quency as sho^Ti in Fig. 6. In the middle of the voice band it has a value 

 of about 200 ohms. There is also an inductive reactance of about 65 

 ohms at this frequency. At a fixed frequency, it is found that the resistive 

 component of this type of tube decreases with current approximately 

 inversely as the square root of the current. This is shown in Fig. 7. The 



