On Monday, July 31, 2017 at 12:59:50 PM UTC-7, Leon wrote:

I was once told be a lighting salesman that the failing tubes can cause
a ballast to go bad. Cheaper to replace all tubes, maybe.

Replacing all tubes, if it's at all difficult to reach the fixture, is certainly
a good idea (won't save much money trying to get the last ten percent
of life out of a tube that's turning dark). The 'cause a ballast to go bad'
is probably obsolete, the old iron ballasts would sometimes overheat when
a tube went out.

The reason, is that the tube going out could result in accidental rectification of
the current (and magnetize/saturate/overheat the ballast).

New (electronic) ballasts don't do that.