• 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.

Heater Wiring - the Good the Bad and the Ugly

If you have a uTracer you could trace a set of curves with the two different heater voltages, keeping to the two extremes of the voltage @ 300mA and 6.3V. Keep the anode voltage as stated in operating characteristics, sweep the grid from cutoff to zero, and capture both sides by measuring the other side with the screen (g2) measurement, and tie the grids together.

It will be easier to visualise the effect than the just the single point on the curve as seen on the oscilloscope.

Ra looks to be high and current low in the quickest view, pointing to a tired tube?