Quote:
Originally posted by jshorva65
. . . The fact that the 6L6-driven flyback HV supply of the Farnsworth (listed in the Sams index as Capehart) 610P, 651P & 661P was obviously a part of the original prewar schematic makes perfect sense given this information. The set's postwar modifications which were incorporated prior to its reaching the market in 1948 included a 13-channel tuner . . . .
The almost-exclusive use of prewar octal tubes with the exceptions of the few miniature tubes in the tuner, video det, video amp, audio IF and horiz. AFC strongly suggest that set was a prewar prototype with some postwar modifications.
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The weak point in the pre-war designs was the available tubes for VHF use. These included the 1852 and 1853 octal metal RF types used for RF amp, mixer, and IF amp. These types will not work above 100 MHz, and are the main reason why pre-war tuners were low band only.
There is an interesting article in RCA Review (1946 I think) about wartime tube development. New VHF types were needed immediately for radar, and the first design was the 6C4 in 1942, followed by the 6J6 in '43, quickly followed by the 6AG5, 6AU6, and 6AL5.
When the war ended, RCA and others had millions of these new types in surplus, and they were just what was needed to upgrade the pre-war TV designs. In 1946 the RCA 630 used (3) 6J6s in a new tuner design that added the high band channels, and 6AG5s and 6AU6s in the IFs. The 807 was given an octal base and became the 6BG6G, but the 5V4 damper used in 1939 remained. Even though all TV manufacturing halted during the war, TV design did not. The upgrades were added to the designs during the war as the new miniature tube types became available, in preparation for post-war manufacturing.