82 BELL SYSTEM TECHNICAL JOURNAL 



The chief difficulty to be overcome in the design of a separate cavity type 

 of TR box has to do with the need for a low-loss contact between the internal 

 portions of the tube and the external cavity. A copper-disc sealing tech- 

 nique, developed at the Bell Laboratories in connection with the construc- 

 tion of water cooled tubes* and later superseded by the now conventional 

 Housekeeper seal, had previously been applied at ultra-high frequencies 

 in the design of oscillators and amplifiers. This technique makes possible 

 very satisfactory high-frequency connections by simply clamping the ex- 

 ternal portion of the disc between machined surfaces. The flexibiHty of 

 the copper discs is sufficient to compensate for minor machining errors and 

 for differential thermal expansions while the relative softness of the copper 

 insures a continuous contact around the entire periphery. The goodness 

 of contact provided by these contacts is evidenced by the fact that Q's 

 of 4000 and greater are obtained at 3000 megacycles with discs of the 721 A 

 type. This technique w^as therefore adopted for the 721 A tube and the 

 724B tube and for one electrode of the 1B23 tube. The second high-fre- 

 quency electrode of the 1B23 was made in the form of a rod terminating 

 in a ball for convenience in replacing tubes since the accompanying loss of 

 Q can be tolerated in the frequency range where this tube is used. 



In an external cavity type of TR box the over-all goodness of the design 

 is largely determined by the design of the gas-discharge tube. It is the 

 tube designer's responsibility to determine the optimum shape and size for 

 the copper discs and for the glass tube envelope and to determine the opti- 

 mum gas composition and pressure, with due consideration being given 

 to such matters as mechanical ruggedness, manufactureability and freedom 

 from undesirable ambient temperature, pressure and humidity effects. 



With the copper-disc type of tube the system designer has at his disposal 

 the ability to vary the design of the external cavity, and to arrive at any 

 specific compromise between the various conflicting performance criteria 

 which he feels to be the best for his particular apphcation. For example, 

 in systems employing vacuum tube converters it is customary to adjust the 

 TR box for a low^-level loss of 1 db or somewhat less since receiver protection 

 is of minor interest while in systems employing crj^stal converters it is cus- 

 tomary to fix the low-level loss at 1.5 db or sometimes as high as 2.0 db. 

 Certain cavities, notably the one shown in Fig. 5, have to be designed to 

 have an extended tuning range, in this case achieved by a piston tuner with, 

 however, some loss in Q, while other cavities, the one show^n in Fig. 11 being 

 typical, do not require this same tuning range and a different tuning mecha- 

 nism (in this case, tuning plugs) can be employed. 



An extreme example, illustrating the advantages to the system designer 



* W. Wilson, "A New Type of High-Power Tube," B. S. T. J., vol. 1, p. 4, July 1922. 



