820 



BELL SYSTEM TECHNICAL JOURNAL 



in keeping power requirements down. Heretofore the use of oxide cathodes 

 in high voltage power tubes had been found very difficult, particularly 

 where filamentary cathodes were employed. Any spark or momentary 

 discharge in operation usually resulted in the burning out of the relatively 

 fragile filaments. This result was caused mainly by the fact that a con- 

 siderable amount of energy was of necessity available from the power supply 

 equipment. However, in pulse service it is possible to limit the amount of 

 energy available so that a momentary tube breakdown will not result in 

 damage to a reasonably rugged equipotential cathode. Also, in the interest 



OSCILLATOR 



MODULATOR 



(a) SERIES MODULATOR CIRCUIT 



CURRENT-LIMITING 



IMPEDANCE STORAGE 



^±jO ^^b MODULATOR CAPACITOR 03^,,,^,^, 



(b) SHUNT- MODULATOR CIRCUIT 

 Fig. 2 — Typical pulse modulator circuits. 



of conserving control power it is desirable to build high perveance tubes 

 which require very close control-grid to cathode spacings. This is much 

 more easily accomplished with rigid cathode structures rather than fila- 

 mentary cathodes, especially for service conditions under which extreme 

 shock and vibration may be encountered. 



Conservation of drive power requires that the modulator tube have high 

 power-gain. This is most easily provided in the tetrode which provides a 

 high over-all amplification factor with reasonable drive characteristics. 

 Lining up the control-grid and screen-grid wires is of course advantageous 

 in the interest of minimizing screen dissipation and getting the largest 

 possible jiortion of the cathode current to the plate. The control-grid to 



