VACi'i'M TLHES R)R RADAR I.I-. A M I' LI II ERS 401 



would be exi)cctcd due to thermal noise from /, because of the effect of j^rid 

 noise. North and Ferris^ showed that the grid noise can be taken into 

 account by assuming that the input resistance of the tube is a resistance 

 noise source whose absolute temperature is about 4.8 times ambient, if the 

 input loading is due to transit time effects alone. One consequence of this 

 input loading is that at high frequencies the best signal-to-noise ratio is 

 usually obtained with an input circuit of lower impedance than that which 

 would be used at low frequencies. 



Actually, as was brought out in an earlier section, the loading in tubes 

 like the 6AK5 is probably due largely to lead inductances between the active 

 tube elements and the external circuit components up to a few hundred 

 megacycles. According to Pierce^ the effect of the cathode lead inductance 

 feedback is to reduce the signal component in the output current while 

 leaving the noise current due to screen interception noise unaffected. Input 

 loading may be a limiting factor in tubes like the 71 7A and 6AK5 when the 

 frequency is of the order of 100 mc or higher, both because of its effect on 

 gain in some cases and because of its adverse effect on the signal-to-noise 

 ratio for early stage use. There is good evidence that the 6AK5 structure 

 would be useful at much higher frequencies than is the case at present if the 

 circuit connections to the tube elements were improved by more advan- 

 tageous mounting of the structure and the use of more suitable sockets or 

 external connectors. 



Noise measurements made by a number of workers at Bell Telephone 

 Laboratories'" indicate that an average noise figure of about 2.8 can be 

 obtained with the 6AK5 at a midband frequency of 60 mc, with a well- 

 designed input circuit, and bandwidths up to 10 mc. At a mid-band fre- 

 quency of 30 mc the noise figure is about 2.4. At 100 mc it is about 3.6. 



5. Description of the Design of the Western Electric 6AK5 Tube 



In order that the reader may have a full appreciation of the dimensions 

 and other requirements of design to meet the characteristics discussed 

 above, a detailed description of the 6AK5 tube follows. 

 5.1 Mechanical Description 



The 6AK5 tube is an indirectly heated cathode type pentode employing 

 the 7-pin button stem and the T-5-1/2 size miniature bulb. The outline 

 dimensions are shown in Fig. 11. A photograph of a mount ready to be 

 sealed into a bulb is shown in Fig. 12. Figure 13 is a photograph of a 

 transverse section through the tube at the middle of the structure in a plane 



* "Fluctuations Induced in Vacuum Tube Grids at High Frequencies," D. O. North 

 and W. R. Ferris, /. R. E. Proceedings, Vol. 29, No. 2, February 1941. 

 ' Unpublished Technical Memorandum, J. R. Pierce. 

 »« S. E. Miller, V. C. Rideout, R. S. Julian. 



