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.
You may try also in right-handed triode mode. It is similar to pentode mode in terms of curves and high anode voltage allowed, but needs positive drive of the grid (all grids connected together)