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BELL SYSTEM TECHNICAL JOURNAL 



The 715B tube is essentially the same structure as the 715A except that 

 larger radiating fins are attached to the ends of the grid support wires. 

 Figure 7 shows the structure of the 715B tube. The characteristics were 

 identical but the ratings were changed, as indicated in Table I. The life 

 obtained in laboratory life tests under rated conditions averaged between 

 500 and 1000 hours. Failure was usually caused by grid emission or loss of 

 cathode emission. 



(OSCILLATOR 

 LOAD) 



SIGNAL 



it 



Fig. 9 — Non-linear coil modulator circuit with illustration of the current and voltage 

 relations in the 5D21 vacuum tul)e. 



TiiE 5D21 Tube 



In response to the demand for further improvement of this structure in its 

 ability to withstand higher voltage, the 5D21 tube was developed, Fig. 8. 

 It is of the same family as the 715A and 715B. It was found that higher 

 voltages could be used if the grid cooling radiators were removed from the 

 top end of the tube. The cooling of the grid was maintained by providing 

 copper wire connections from the bottom ends of the grid support wires to 

 the base seals. This and the use of a specially designed plate terminal cap 

 enabled the voltage rating to be raised to 20 KV. 



