• 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.

Drive requirements.....

As mentioned, u need to swing the grid above the cathode potential, so u need more than 50volts each way as mentioned. But if the OPT is for AB driving, the impedance is likely too high for any benefits of AB2. The plates will saturate (too low voltage) before the positive going grid can do any good to the plate current.
U could try to hook a lower impedance load to a higher impedance output tap, such as a 4ohm speaker to 16ohm tap, which probably works, but the OPT may not be dimensioned for the increased current at max signal.
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