Bottom tension is factory set on machines that use drop-in bobbins. The only thing you can adjust is top tension. That's why it's so hard to get it just right.
Although having to pop your bobbin into a case, then stick both into the machine is a bit less convenient, there's a little screw on the case you can use to adjust bobbin (bottom) tension. It's especially helpful if you want to use specialty threads to do the quilting or to embellish. I've held onto one of my oldest machines especially for those purposes.