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.
For a filament transformer designed for 115V at 400 Hz but used at 60 Hz, the input voltage must not exceed 115×60/400 = 17.25 (VRMS), so obviously the secondary voltage will be practically useless.