Aftermarket ammeters (amp gauge) are often real ammeters and measure actual
current.  Factory amp gauges measure a voltage drop across a known
resistance and do not have actual current through them.  So you could have
blown something with a real amp gauge.

The alternator for an '82 is an internal regulator (1969-up) and is easily
damaged, often by transient spikes such as during jumping another car, loose
battery connections (arcing a cable to a terminal), and so on.

What frequently happens is that you lose one phase on the alternator.  This
will test find on the crude analog testers in the typical parts store.  It
will usually only show slightly under a heavy load.  But what happen is if
you imagine three sine waves 60 degrees apart so that you have evenly spaced
peaks.  On full wave rectified, you have three peaks, then the lower half is
flipped on top by the rectification and you have peaks 4,5, and 6.  This
brings you back to the beginning of the cycle.

Now if you lose a phase or if one phase rectification goes out, you will
either have peak, peak, zero, peak, peak, zero on voltage or you will have
peak, peak, peak, peak peak, negative peak.  Either of these will allow a
cheap tester to show a full 12 - 14 volts but the actual voltage and current
is severely diminished.

Return your gauges and wiring back to original, replace the alternator.