404 BELL S YSTEM TECH NIC A L JOURNA L 



The plate has a rectangular transverse cross-section and is made of 0.005 

 inch thickness carbonized nickel. The carbonization increases the thermal 

 emissivity of the plate surface so that a reasonable amount of power dissipa- 

 tion in the plate can be tolerated. 



The end shields are made of nickel-plated iron. The reason for using this 

 material instead of nickel, as is more often the case, is to prevent overheating 

 of these shields during the exhaust process. At the temperatures used, the 

 iron shields pick up less energy in the induction field during the out-gassing 

 of the plate than do nickel shields. The function of the end shields is to 

 minimize the stray capacitance between the plate and the control grid. 



The insulators or spacers which hold the tube elements in proper disposi- 

 tion with respect to each other are of high grade mica. They are coated with 

 magnesium oxide to minimize surface leakage effects. 



The getter, which can be seen at the top of the structure in Fig. 12, con- 

 tains barium which is flashed onto the inside surface of the bulb at the end 

 of the exhaust process in order to take up residual gas evolving from the 

 parts of the tube during operation. 



Although the individual parts are extremely small and fragile, the com- 

 pleted tube is surprisingly rugged. The short supporting wires in the stem 

 and the support provided by the bulb-contacting top insulator result in 

 the stem, bulb, and mount structure being a relatively rigid unit. The 

 small parts assembled into the mount are very light in weight and therefore 

 exert relatively small forces on their supporting members under conditions 

 of mechanical shock. Shock tests performed at the Naval Research Labora- 

 tory and at the Bell Laboratories show that the 6AK5 tube stands up 

 satisfactorily under a steady vibration of rms acceleration 2.5 times gravity 

 and withstands 1 millisecond shocks of over 300 times gravity. 



The most important single geometrical factor in the tube is the spacing 

 between the cathode and the control grid. In the 6AK5 tube this clearance 

 is 0.0035 inch after processing. Before processing it is 0.0025 inch. The 

 manufacturing difficulties involved in assembling the structure and main- 

 taining such small clearances are obviously very great. However, as was 

 seen from the discussion above, this close spacing is essential in order to 

 obtain the desired high-frequency performance. 



It can be observed that the close mounting of the structure on the stem 

 provides ver>^ short lead lengths between the tube elements and the external 

 pins. This is of importance at frequencies where the inductances of the lead 

 wires become comparable in magnitude to the circuit reactances. 

 5.2 Low Frequency Electrical Charatceristics 



The usual static characteristics are shown by the curves in Fig. 14. The 

 ratings, nominal characteristics, and cold capacitances are given in Table II 



