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.
Audio Research REF3 tube question clarification
Audio Research REF3 kindly clarify.
Do the 5 Sovtek 6H30PI tubes have to be all matched to get the best sonic performance?
What do the 5 tubes actually control?
Hours on tubes, when should they be changed to get the BEST sonic performance as there seems to be different opinions.
Sovtek 5 tubes, some say change them up to 1600-1800 hours but other sugest 3000 hours, which is it?
Winged "C" (SED) 6550C tube, some say change up to 800 hours but others sugest 3000 hours, which is it?