[Hallicrafters] Tube question for old-timers.

[WA1KBQ at aol.com]

The introduction of a good commercially produced superheterodyne  
communications receiver was possible in 1932 because of the then new type  57 and 58 
pentodes. These tubes had 6 pin bases with grid cap  connections on top. The next 
step in the evolutionary process was to  replace the 57's and 58's with 
improved type 6C6 and 6D6 which also had 6 pins  and grid caps; the 1934 receivers 
pretty much all used these tubes. The metal 8  pin octal line-up was introduced 
in 1935 which was a still further  improvement in tube performance. The 6K7 
is actually an improved 6D6 in a  smaller size metal envelope but still with a 
grid cap on top. The next  improvement over the 6K7 resulted in a single end 
design known as the 6SK7. The  grid cap disappeared with the control grid being 
connected to one of the 8 pins  underneath. The next improvement was the 
introduction of the post-war 7 pin  miniatures with the 6BA6 being an improved 
version of the 6SK7.
 
Regards, 
Greg Gore; WA1KBQ