1964 Ampeg Jet Questions

[Tom Phillips]

...The label on the inside of the cabinet says Jet 12-A but it can't be very different from the 12-B, right?...

Mainly different power tubes. Attached is a J-12A schematic.

...I don't have much experience with single coils as I'm currently borrowing a strat from a friend, but the single coil buzz when the amp is at 6-10 is really loud compared to the humbuckers. Like not even in the same ballpark. Does that sound right?...

Yep. That's the way there are. The relative sound levels in your video didn't surprise me.