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differences btw time needed to fire up MHs?

I recently installed 2 CoralLife 5500k metal halides over my 90g
tank. I'm using CoralLifes single-box ballast; in other words, one box
containing the ballast(s) with two leads going to the lamps.

Now I've used MH bulbs before, but always only one at a time. The
interesting thing about my current setup is that the bulbs do not
"fire" at the same time. By firing I'm talking about the moment they
first emit any light whatsoever, *not* the moment they final come up
to full intensity.

When power is supplied to the ballasts, one usually fires within 30
seconds or so. But then there is almost always a delay that can
stretch up to several minutes before the other one fires! Both bulbs
are new, bought at the same time from the same supplier. And it gets
more interesting than that - the bulb that lags behind is not always
the same bulb!

I know very little about metal halides, so could someone enlighten me
to what is happening here? Thanks.
Dan Resler                            email: dresler at vcu_edu
Dept. of Mathematical Sciences          
Virginia Commonwealth University           
Richmond, VA 23284-2014 USA