Even a poorly running stock system doesn't draw as much power as that. That sounds like a concern. They cycle heavily if not properly pressurized and noticably even when they are. Good luck on that one.

 Not only does it pull the idle way down, the ampmeter shows discharge when it's on and even more when I turn on the lights. Related problems or maybe I need an alternator with more amps??? Seems the more I do, the more problems I get.

The alternator problem is not a problem.  It's just turning too slowly to charge.

It's not unusual to drop 150 rpm with a A-6 compressor like my 75 and your 71.  The newer R-4 don't drop as much.

Ken... I have a new aftermarket system in my '71. The whole OEM system is gone, and I might add, along with my cockpit heat problem. I have a new Sanden seven-cylinder compressor, R-134. I don't drop 150 rpm's at idle when I turn it on. It drops between 400 and 450 rpm's. And, even at highway speeds, my ampmeter shows discharge when I turn on the a/c and fan. If I turn on my lights at the same time, it discharges even more. Maybe that explains my problem better? I dunno.  Any thoughts?

Are you sure that the compressor is functioning correctly?  That idle drop indicates a large load on the engine. Check out the compressor bearings and look at the belt to see if it is showing signs of wear or overheating. 60 rpm drop is ok 400+ is way off the mark. Are you sure it was charged right?  Sumpins up wit dat a/c youngin.

The system was just installed about a month ago. It's done that from the git-go.

Congrats on getting it going. Mines been disconnected since the rebuild couple years ago. Hope to have it back up and running sometime this summer.

On a newer aftermarket system, that seems like a lot of drop.  I wonder if the system is overcharged.  That would create a lot of extra drag.

That electrical load is also high if you see voltage drop when the engine rpm is up.  Again, I expect to see some at idle, but not at road speed.  That means the system pulls a LOT of amperage.   There are only two cures for that amp load.  A larger alternator, and a bigger capacity battery (more cold cranking amps).

Thanks Ken....How can I (or someone) tell if it's overcharged? I kinda figgered my alternator was weak even before the new a/c. Seldom saw the needle on the charging side except right after start-up.

The only way to check the charge is to connect a A/C gauge set, and take pressure readings.