Ammeter gauge shows slight discharge

Hello, I have a '70 Dart, and recently, I noticed the ammeter is showing a slight discharge whenever I have any electrical stuff on...such as brake lights when stopped, the ammeter shows a discharge. I put the A/C on, and a much bigger discharge. Once I accelerate, the ammeter stays right in the middle.

It used to be not like this, it would always slow a slight charge.

I just changed the voltage regulator to a new one from YearOne...could that be the cause? Does that need to be grounded correctly to the firewall? maybe I didn't tighten the two bolts firmly enough to the firewall?

thanks