HPS bulb question
 

A question.
How do high pressure sodium bulbs tend to burn out?
I've used a 400 W MH fixture for several years. I've replaced a few MH
bulbs and they just burn out like any other bulb. Three or 4 years ago I
got a sun systems fixture with a 400 W MH AND a 400 W HPS together in
the one fixture. I remember thinking how yellow everything looked under
the new light.
This week, all the plants came in. About the 2nd day, I realized the
plants didn't look yellow now. Things looked about the same as the MH
only fixture. I peered under the hood and found the HPS bulb still
working but glowing a deep orange that wasn't very bright.
Is this how they die of old age? Don't even tell me the ballast is going
bad!

Steve