1973 220D Battery not Charging

[73220downer]

Today I drove a few miles to the Post Office, did my business there, came back to my car, put the key in, pulled the starter knob and then "clickclickclickclick".
I got a jump and the car started just fine. Lights and accessories worked but seemed as though they weren't getting enough voltage. Revving the engine helped temporarily.
Got home and now the same problem.
What might be the cause of this? Alternator? Voltage regulator?
The belt for alternator is kinda loose but it still turns it just fine, might that be it?