I should have been more clear - for sure I was talking about the amount of thread left on the bobbin. It appears to me that the disk spring that is in the bobbin case creates a drag force on the metal bobbin to ensure it does not continue spinning when sewing stops. A quick google search shows two types of "anti backlash springs". One is an almond shape that seems to touch on the Outside diameter of the bobbin and the other is a C shaped spring with a bump that seems to touch on the center of the bobbin. My bobbin has the almond shape one. I will be sewing with a normal bobbin tonight since I need another color and I will report back. I might purchase some of the C shaped springs and try those.
Hello - does anyone who uses prewound bobbins on their 206 see tension differences based on the thread diameter? I haven't used prewound bobbins to this point - but just purchased some to try and I notice a tension difference when they are full vs when they are nearly empty. After playing with the bobbin case it seems that maybe as the diameter changes the disk spring force changes. This wouldn't be an issue with a normal bobbin as the metal would always have the same contact with the disk spring. I was contemplating removing the disk spring and seeing what the result was but I wanted to check with others. It seems that when the prewound bobbin is new the bottom tension is tight and when it is nearly empty the bottom tension is light.