ULTRA-HIGH-FREQUENCY POWER AMPLIFIER 23 



amplifier at frequencies up to at least 300 megacycles. It is the pur- 

 pose of the present section to describe the methods and apparatus used 

 in testing this tube and to set forth the results of some of the tests. 



An attempt to study the operating characteristics of an amplifier 

 tube at ultra-high frequencies brings up many new problems. Such 

 fundamental properties of the tube as amplification factor, transcon- 

 ductance, and plate impedance do not convey as much information 

 about the behavior of the tube at these frequencies as they do at 

 lower frequencies. The presence of unavoidable stray inductances and 

 capacities makes it much more difficult to separate tube problems from 

 circuit problems. Consequently, at ultra-high frequencies we are vir- 

 tually forced to consider the tube and its associated circuits as com- 

 prising a single piece of apparatus. If the circuit design is carefully 

 made the stray inductances and capacitances can be greatly reduced in 

 magnitude and so localized that their effects upon the over-all perform- 

 ance of such a piece of apparatus can, to a certain extent, be computed. 



Circuit Design 



Some idea of the extreme attention to detail required in designing 

 amplifier circuits for use at ultra-high frequencies may be gained from 

 the following considerations. Computations indicate that even with 

 the tuned plate and grid circuits placed as close as physically possible 

 to one of these push-pull pentodes, at 300 megacycles, the radio-fre- 

 quency voltage actually applied to the grids of the tube may be as 

 much as twenty-five per cent greater than the voltage developed across 

 the tuned grid circuit. At the same time the load presented to the tube 

 plates may be as much as twice the load actually present across the 

 tuned plate circuit. These discrepancies are a direct result of the in- 

 ductance of grid and plate leads which, in the case of this new tube, 

 have already been reduced well nigh to the minimum possible. 



In studying the performance of these tubes we wished to be able to 

 check experimental results against theory at every possible point. 

 Consequently the simplest auxiliary circuits were chosen, namely, 

 shunt-tuned antiresonant circuits from grid to grid and from plate to 

 plate, with screens and filaments by-passed as directly as possible to 

 ground. In their mechanical design these circuits embody a number 

 of features intended to reduce and localize stray inductances and 

 capacities, into the details of which it is not possible to go at present. 

 A simple arrangement was evolved to provide a maximum of conven- 

 ience and flexibility for experimental work. The single stage amplifier 

 unit consists of three sections, an input circuit section, a tube housing 

 section, and an output circuit section. This arrangement permits 



