Using the correct bobbins is essential. Machines are designed with a specific type of bobbin size, this means the thread smoothly unwinds whilst sewing.  Using the wrong bobbin can cause tension and bunching up problems. 
 
These Bernette clear bobbins are suitable for models b70 / b77 / b79 / Chicago 7 / Chicago 55.  The machines are designed to use these plastic bobbins.
 
Metal bobbins are not suitable for these machines.
 
Pack of 10