Singer 301 tension
I usually fill several bobbins before starting to sew. I then test the tension. Once I have it correct, I start sewing.
However, EVERY TIME I change bobbins on my Singer 301 I have to play with the tension - even if the top thread and the bobbin both contain exactly the same thread as the one that just ran out AND the fabric is exactly the same.
It is really frustrating.
Recently I spent over an hour playing with the upper and lower tensions. Many times the tension 'looks' good but if you tug on one of the thread tails the thread pulls right out of the fabric.