As Rrog and Elizabeth inferred, it would depend upon the plate voltage (voltage difference between anode and cathode) and DC current being run through the tube. The product of those two parameters is "plate dissipation" in Watts. Look up a tube data sheet for a 12AX7. It will give you the max numbers for each of those 3 parameters. If the actual plate voltage, plate current, and plate dissipation (information which you would have to obtain from the manufacturer, if you don't know how to measure) are in the midrange of the recommended max values, then the life of the tube will more depend upon sample to sample variations within the tube itself.
I am surprised at the story about the SP14, because Audio Research was notorious for running their tubes very "hard". I guess that applied to the output tubes in their amplifier designs. The old Dual 50 and its variants used to blow output tubes all the time.
I am surprised at the story about the SP14, because Audio Research was notorious for running their tubes very "hard". I guess that applied to the output tubes in their amplifier designs. The old Dual 50 and its variants used to blow output tubes all the time.