12au7 should be a 6 volt filament tube
Because of the center tapped heater the 12AU7 can operate on 6 or 12V.
The two heater sections are wired in parallel for 6V and in series for 12V.
The BHK preamp has a switch to change the wiring for either 6V or 12V.
The BHK power amps do not have a switch for different filament voltages.
Noted with thanks