• 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.

Measurement of OT inductance by series resonance method

The graphs makes sense.

Peak inductance point for 2.5k primary is located at 40mA DC bias. for 5k primary impedance it is 28mA. DC flux density is proportional to the number of turns.

28/40 = 0.7
(2.5k / 5k) ^2 = 0.707

Both ratios match enough.