by reading between the lines.
What does that mean???
It means that there is nothing written there that slam-dunk guarantees max emissive life when the tube is operated at the upper limit of its filament V. range .
I don't have access to the latest and greatest Care and Feeding publication so I can't comment on that. But I was able to find this on CPI's website: http://www.cpii.com/docs/related/23/Tube%20Maintenance.pdf
I took note of three things:
1)"The operator, by adjusting the filament voltage, can control the operating temperature. Each tube is unique; while one tube may make full operating power at a filament voltage of 7.3 V, a replacement device may require 7.4 V to attain the same power."
This tells me that you can't make wholesale generalizations about reducing filament voltage.
One can quite safely say that operating the filament potential 2% higher than the PEP decrease point is a practice that will not damage the Th or unnecessarily decrease the life of the ditungsten-carbide emissive layer.
2) It is very clear that as you lower filament voltage you must monitor the tube to ensure it is meeting all specifications. You can't just say "reduce your filament voltage by 0.25V for greater tube life".
But we can say: operate a Th-W filament 2% above the PEP decrease point and you will not be flushing your electron-tube $$ down the crapper. However, if the tube is slightly gassy and one inadvertently operates the filament below the minimum rated V, the resulting Th degradation by O2 and N2 can be reversed by operating the tube at the specified filament-V. -- so it's not like spilled milk.
3) I consider this to be the most damning to your argument: "A power tube operated in this manner will generally yield life 50 percent greater than a tube run continuously at rated filament voltage." So if you lower filament voltage in the way CPI dictates and sanctions you can only expect tube life to be 50% greater. This doesn't fit your formula either...
My Ham-friend in San Diego, who gets 22k-hours out of 4CX15.000As in the FM broadcast transmitter he maintains, pretty much confirms that Eimac's supposedly only theoretical statement that each excess 3% filament-V = 50% decrease in emissive life is not just "nonsense" Scott.
••• Rich, ag6k