• 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.

6H30 data

you will soon learn that increasing the B+ supply increased the cathode currents and so increased the gains...

what then is your limitation? the plate dissipation ratings, make sure you do not approach it, small signals use a fraction of the plate dissipation ratings when considering the tube operating point.....