A HIGH POWER VACUUM TUBE S 



Virginia, in which speech was first transmitted across the Atlantic 

 to Paris and across the Pacific to Honolulu, required the use of nearly 

 300 of the most powerful tubes then available, each capable of han- 

 dling about 25 watts, and the difficulties encountered in operating so 

 many tubes in parallel gave added impetus to the development of 

 high power units. 



It is the object of the present paper to deal with the various steps 

 in the development of high power tubes as carried out in the Bell 

 System research laboratories at the Western Electric Company. 



The usual type of vacuum tube consists of an evacuated glass vessel 

 in which are enclosed three elements, the filament, the plate, and the 

 grid. When the tube is in operation an electron current flows between 

 the filament which is heated by an auxiliary source of power and the 

 plate, the magnitude of this current being controlled by the grid. 



The passage of the current through a thermionic tube is accompanied 

 by the dissipation in the plate of an amount of power which is compa- 

 rable to the power delivered to the output circuit and wrhich manifests 

 itself in the form of heat. This causes the temperature of the plate 

 in the usual type of tube to rise until the rate of loss of heat by radia- 

 tion is equal to the power dissipated. Some of the heat liberated by 

 the plate is absorbed by the walls of the containing vessel which con- 

 sequently rise in temperature. These factors, together with a con- 

 sideration of the size of plate that can be conveniently suspended inside 

 a glass bulb and the size of glass bulb that can be conveniently worked, 

 set a limit of about 1 to 2 k. w^ for the power that can be dissipated 

 in the plate of a commercial vacuum tube of this type. The plates 

 are generally constructed of molybdenum or some other refractory 

 metal and the containing vessel made of hard glass. 



The use of quartz as the containing vessel offers certain advantages 

 which tend to raise the power limit somewhat and this material has 

 been used for power tube purposes in England. 



It is apparent then that in the development of vacuum tubes capa- 

 ble of handling large amounts of power means other than radiation 

 must be used for removing the heat dissipated at the plate, and 

 development of tubes along these lines was undertaken by Dr. E. R. 

 Stoekle and Dr. O. E. Buckley. 



Dr. Stoekle had already worked for some years on the problem of 

 removing the heat dissipated at the anode of a thermionic tube by 

 making the anode a part of the outside wall of the vessel and thus 

 making it possible to convey the heat directly away from it by means 

 of circulating water. This was clearly the right principle but as is 

 obvious to those who are familiar with these devices, great difficulties 



