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Yeah, the other end of that "x" will be found at the power transformer on one of its secondary windings. This cuts confusion, as actually drawing in all these filament supply lines would make for a very busy and confusing schematic.
Some sets of that era have fusible links for filament feeds. The bottom line is that you need to actually measure the AC voltage between pins 3&4 of the seven-pin tubes to verify filament supply. On the 12AU7 measure the filament voltage between pin 9 and the tied-together pins 4&5 as is usually found.
Both the 6AU6 and 12AU7 can be deceiving, as their construction and cathode characteristics are such that sometimes it's not easy to see the glow. This also may be true of some 6AL5s. And the duties each of those tubes has are all relatively low power. A 6AL5 seldom gives off any significant warmth, and an AGC keyer is not likely to get all that warm either.
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