I just did this job on my '80 300SD. It's good that you stopped for
you're right, the hex nut is the tensioner and works very well once the
alternator's security bolt is loosened. Specifically, the 17 mm bolt at
the rear of the alternator that's AWAY from the engine's block (there
are two bolts about 2" apart). The 17 mm bolt is accessed from the
bottom. Then, from the top, turn the hex nut clockwise to tighten the
belts. Then tighten the security bolt.
Replacement belts ought to be a matched set for their slightly different
lengths will make them work against each other and so develop a glaze.