Do I need a new alternator

[davidpriebe at cablespeed.com]

I need some help diagnosing what is wrong with my charging 
system. On Friday, driving home from work, and sitting in 
traffic, I casually scan the gauges, and notice that the 
voltmeter is reading just barely below 12. I haven't seen 
it ever read this low, so I turned off all of the 
accessories, AC and radio, and wait for the radiator fan 
to cycle off. Typically I see a little bump up when 
everything is off. But this time there was no bump at all. 
Normally it reads well north of 12 volts. So for the rest 
of my trip home I keep a close watch on the gauge, and see 
it slowly decreasing to about 9 to 10 volts. When I got 
home I checked the belt tension and saw that I could turn 
it over 180 degrees, a bit loose. So today I went ahead I 
tightened it back up. But no changes in the gauge reading, 
still below. So now I'm thinking that there must be 
something wrong with my alternator. The current alternator 
was put in about 18 months ago, a rebuilt. The battery was 
replaced about 6 months ago. So do I need a new 
alternator, or is there someother problem? If I do need a 
new one, any recommendations for a supplier?
Thanks, 
David Priebe
Carnation, WA
91 200tqa 182k (his)
95.5 S6 114k (hers)
66 MGB 41k