I would think a 300b operating at those points would last a lifetime.
And that's what a JJ 2a3-40 is, a 300b with folded filaments.
It does have the mu of a 2a3 but the same limiting values as their 300b.
450v, 40 watt, 70ma fixed bias and 100ma cathode biased
It has the same inter electrode capacitances and that's interesting because if the mu is different then the spacing between elements should be different but still the inter electrode capacitances are list as being the same.
I really think the two, the JJ 2a3-40 and the JJ 300b, are the same tube with the filament center tapped and the center tap is connected to one filament pin and the two ends are both connected to the other filament pin.
Tre'
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