In article , Arthur Shapiro wrote:
The kitchen has a dropped ceiling with two pairs of standard 4-foot bulbs - I
guess that's what is called T12. This fixture has never been very reliable in
the 20 years I've owned the house. Bulbs don't last as long as they should,
and on colder winter mornings, while the heater is still warming up the house,
they often take several minutes to come on.

The last straw came two days ago when a two-week-old pair of Phillips bulbs no
longer would light, period. One of those tubes already has blackened ends.
The other pair in this fixture is working OK.

What is likely to have failed to cause such an early failure on these bulbs?
I assume the ballast, but would like an expert to confirm. I'm interested in
purchasing electronic replacements, while realizing they are more expensive
than traditional magnetic ballasts, but an intimidated by unfamiliar concepts
such as "ballast factor".

Can anyone provide practical troubleshooting and replacement/selection advice
so I can purchase the correct pair of ballasts?

Many residential dual-T12 4-foot fixtures have some call the
"residential grade" ballast. It is maybe 1.5 inches shorter than a "full
size" dual-4-footer fluorescent lamp ballast. I call those ballasts
"stool specimens" to put it politely.

Ballast factor is ratio of light output using the ballast in question to
light output on a "reference ballast", which is some standard used to
determine the catalog figure for rated light output.

Most ballasts for which ballast factor is made known are for T8 (1 inch
diameter) bulbs. T8 ballasts tend to be electronic and better. I would
recommend replacing the ballast with an electronic one for T8, and use T8
bulbs.

As for a usual dual-4-footer rapid start magnetic ballast becoming prone
to blacken the ends of the bulbs quickly: The output series capacitor is
probably shorted. The ballast can easily overheat from this. The ballast
needs to be replaced.

- Don Klipstein )