Audio Research REF3 tube question clarification

Tube life depends mainly on cathode current - the same tube used in different circuits can last for very different times.
Most tube preamplifiers work in class A, meaning that tube current is constant. Unless you have all data from your preamplifier you should rely on manufacturer advice.
The referred values for the 6550 (1500 hours) are typical for a power tube in an Audio Research amplifier operated at 65mA bias and do not apply to the Ref3 - my Ref3is now over 4300 hours and the 6550 still measures over 8500 micromhos.

I agree with microstrip. In an amp,the 6550 life expectancy is around 2000 hours plus or minus but, in the power supply of the Ref-3 it's coasting and will have a much longer lifespan. My ref 3 is at 6300 hours and sounds wonderful. A swap with a new 6550 made that point very clear. If you talk to the tube guys, the knowledgable ones, they'll agree.

Yep, I'm over 5000 hours on my Ref-3 ... and still in amazement.