1122 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



space charge on the input capacitance, and include a total allowance of 

 3 mmf. for socket and wiring capacitance (input plus output). The L3 tubes 

 are somewhat better than the 404A and substantially better than the 6AK5. 

 The figures of merit tabulated will not check with the values shown in 

 Figs. 2-4 since the curves were calculated for cold tube capacitances and 

 zero socket and circuit capacitances. 



Table II 



Classification 



Heater voltage 

 Heater current 

 Plate current 

 Screen current 

 Transconductance 

 Input capacitance* 

 Output capacitance* 

 Plate-grid capaci- 

 tance* 

 Figure of merit** 



386A 

 Pentode 



6.3 



0.150 



7.5 



2.5 



4000 



3.6 



2.6 



0.025 



61 



6.3 

 0.175 



6AK5 408A 



Pentodes 



20.0 

 .05 



7.5 



2.5 



5000 



3.9 



2.85 



0.01 



72 



437A 

 Triode 



6.3 volts 

 0.45 amps 



40 ma 



— ma 

 45000 Mmhos 



11.5 mmf 

 0.9 mmf 

 3.5 mmf 



— mc 



* Cold capacitances. 



** This is the frequency at which unity voltage amplification would occur with a 

 simple parallel tuned circuit interstage. Allowances have been made for stray capacitances 

 and for the increase in input capacitance when a tube is energized. No figure is given for 

 the 437A tube because the relations derived for the earlier stages in the amplifier do not 

 apply to the output stage. 



3.3 Performance in Repeater 



The positions of the tubes in an auxiliary repeater are indicated in the 

 diagram of Fig. 13. The overall insertion gain is a little over 30 db at 4 mc. 

 While the noise contributions of the first 435A tube and the 436A tube 

 are important, they have been reduced to 48 db below one volt and 62 db 

 below one volt, respectively, for 1200 repeaters. The second 435 A tube and 

 the "lower" 437A tube appearing in Fig. 13 are the major contributors 

 to the modulation. The expected modulation levels from these tubes are 

 those associated with single tone ratios of about 34 db for the fundamental 

 to second harmonic ratio and 70 db for the fundamental to third harmonic 

 ratio, with a grid signal level of 0.1 volt r.m.s. 



Fig. 13 — Position of tubes in the input and output amplifiers for the L3 carrier system. 



