Yeah, the "leakage" is by design in AA5s—imperfect cathode-heater insulation in the 35W4 allows DC coupling, centering heater potential at ~half B+ to kill hum. See Riders' AA5 schematics or "Understanding Tube Amplifiers" by Morgan Jones, p. 45-50. The tap is grounded, isolating full B+ from the string itself. Conflicting info often ignores the intentional capacitive coupling.