CAUTION

Pin 1 of a 6BD4 is the cathode connection.

Pin 8 of a 6BD4 is not used.

Unfortunately, in 1954 RCA  used pin 8 of the 6BD4 octal socket in a CT-100 as a tie point.*

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Pin 1 of a 6BK4 is the cathode.

Pin 8 of a 6BK4 is internally connected to pin 1.

So...

Replacing a 6BD4 (or 6BD4A) with a 6BK4

will shunt the cathode and grid with a one-megohm resistance

(applies to the early boost B+ configuration;

150K if it's the revised boost B+),

which alters the high-voltage regulator circuit.

*The CT-100 uses pin 8 of the 6BD4 high-voltage regulator as a tie point for the junction of 3C213 and 3R258 (RCA) or C141 and R183 (Sams) .