> The other reason is that 300B's characteristic curves are not so correct.
> In your plot, the curve for Eb=350 is more straight than that for Eb=300 and
> the curve for Eb=450 is more straight than that for Eb=400.
> Don't you think that this is strange?
There was only one curve Eb=300V (the red line) in the attached file, I will try at lower voltage and see how they look. Thanks again for your kind advise.