WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Sounds like the P/N junction of a diode drop will clear this up just fine getting you 6.1V..........get a healthy current rating & PIV rating..lots of designers fine tune voltages this way.
PS, At 6,8V.....just about any PIV rating is gonna do.
That's my preference also. You can more easily fine-tune the voltage if you need, and as Chris says, the resistor will limit the inrush current when the filament is cold. (Since a cold filament has much lower resistance, the resistor will drop a much greater share of the voltage until the valve has warmed up.)
6.8 volts loaded is fine because you are probably losing .1v on each pin connection bringing it down to 6.6 across the filament which is within 10% Good enough for a power amp, for a pre convert to dc. You could drop a resistor in the circuit but i see no need.
I have frequently used both diodes and resistors; even for out put tubes, 811A's 4 amp heaters. Don't find they get very hot certainly not with a driver tube. You can easily work out the size of resistor to use with Ohms law. In Perth west australia the ac voltage is all over the place; supposed to be 240 volts but often rises to 255 volts or more.