Alternator Gauge Reading High With 300 Watt Stereo System Installed---1969 Electrical System

First, you may or may not have a charging problem regardless of the reading. The thing to do is get the battery "normalized" IE run it after starting with accessories......including the stereo.....OFF

Next with the engine warm and battery charged, read battery voltage with engine running to simulate "low to medium cruise" RPM. You should read 13.8....14.2, and not below 13.5 and not above about 14.8 FIX THAT FIRST. If it is high it most likely is a harness voltage drop issue. I've posted "a bunch" on this

After that, the high reading is likely because your junction point is on the "wrong side" of the ammeter connection, IE say, the starter relay. That will cause the wrong reading.. If you are going to run this high power stuff you have very little choice........if you tap off on the other side of your ammeter, NOW you have invoked the deadly "Mad electrical" disease, read this:

Catalog

So now you have a choice.........leave it as it is, and hope you don't do or have the kind of damage in the article, move the connection so the ammeter is happy, you will almost guarantee damage to the system.

You might want to think about going ahead and doing this mod or similar. Crackedback (member here) sells a jumper wire for this purpose.