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RE: How about the new 45 by EML?

Well, transconductance depends largely on cathode-grid spacing, or at least the length of the electron path within that spacing. Since this is the smallest critical dimension in the tube, it is not unreasonable to ascribe a majority of the variability of performance with operating point to this variation. And in fact the curves of the modern designs are especially linear. Whether curve linearity is the major influence on subjective sound quality is I believe unknown and unknowable.

I personally am more suspicious of the cathode substrate and coating. It is reasonably well known that excess noise increases with cathode current, while Johnson noise decreases (because of increased transconductance) - hence the cathode size affects the current density and you might reasonably expect a quieter tube to have a larger cathode, all else being the same which it never is. Alternatively, (Jeff will love this) you might reasonably expect a tube to be quieter when run at a lower than maximum current.

The other known effect is that of the cathode substrate (the filament wire itself for DHTs). It's traditionally nickel, and certain impurities (silicon for example) are known to result in eventual formation of cathode interface, increasing noise. Silicon (again, just an example) also makes it easier to "form" the cathode, making it cheaper to manufacture tubes - no free lunch.

I have not even touched on the cathode oxide coating, about which I know practically nothing. But speaking as an engineer, I am certain that the things I don't know are even more important than the ones I do!

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