 For a source of 25 kΩ, the noise generated by tube and source are the same, so the total noise power at the output of the amplifier is twice the noise power at the output of the perfect amplifier.The noise figure is then two, or 3 dB.For higher impedances, such as 250 kΩ, the EF86's voltage noise is formula_4 lower than the sources's own noise.