COLD CATHODE TUBES FOR AUDIO FREQUENCY SIGNALS I.3K3 



400 



350 



(0 



I 300 

 O 



250 



O 200 

 O 



150 



100 



50 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 AO 4.5 5.0 



-^XI03 



Fig. 9 — Resistive component of impedance as a function of density (pressure 



at 25°C) of gas filling. 



the plane cathode through changes in tube current, cathode area, density 

 and type of gas filling is limited. A reduction by a factor of five or ten 

 times over the values present in commercial tubes is certainly possible. 

 But the possibility of obtaining values of less than +10 ohms or negative 

 values seemed remote. Consequently, development effort was concen- 

 trated on the hollow cathode tube described below. 



HOLLOW CATHODE TUBES 



The static voltage-current characteristic of a hollow cathode was 

 shown in Fig. 2 to be below that of a plane cathode of the same area when 

 operating at currents in the abnormal glow region. It has been found that 

 by proper choice of the cathode dimensions and the kind and density of 

 the filling gas, desirable transmission characteristics can be achieved. 

 The follomng discussion illustrates the manner in which some of the 

 variables are interrelated. 



We ^vill consider a "U" shaped cathode which has been formed by 

 folding a piece of molybdenum sheet in the form illustrated in Fig. 10. 

 The choice of dimensions of the hollow portion will be discussed lat<»r. 

 The anode is a cylindrical rod placed in front of the cathode. The anode- 

 to-cathode spacing is selected so that the anode is always within the 

 Faraday dark space and hence does not influence the impedance ap- 

 preciably. It is assumed that the structure of Fig. 10 is sealed in a bulb 



