I replaced my 6AK5's last Saturday. Although I checked them on a Hickok tV7 last December and they tested fine, I recently aqured a Mighty Mite IV tester and the life test/emssions showed they were poor. They had been installed for several years without issue, but there was a substantial improvement after the replacement (i.e. after 45 hours burn in). I have found the V2 6C33C to fail most frequently, followed by the 5651, V1 6C33C and finally 6N6p. 6AK5 is at the end of list as I have rolled the 12AX7 position several times to have any meaningful life expectancy judgement. The good thing about a failed V2 6C33C is that it can be used in the V1 position and vice versa, as the amps only use half the tube. So when I lose a V2 6C33C I just put it in a box for when a V1 goes sometime down the road.
