Oh, I'm sorry. I should have clarified. Yes, I know the bobbin case is round. When I said solid tube I mean in the center of the round bobbin case there needs to be a solid post to fit into the hollow tube that is on the machine. There are a few different versions of the round bobbin cases that have the solid posts on them. Some are smooth and have no latch on them, some have a latch to grab onto like a modern bobbin case does. I have recently found both types and do not know which one would fit this machine.