I did not understand what you ask. But here it goes one more flower for you, my dear!

Hi Judy, I think you need to use size A prewound, I have a different machine but these are a bit higher/fatter than L or M so they dont jump around the bobbin case and basically they run a lot smoother.
They say if you use SA156 Part# of Brother its called CLASS A or Class 15 Bobbins,
Heres a place that sells them (size A prewound) and also has a good reference so you know which ones are the correct ones for your machine.
http://www.allthreads.com/Prewound_Bobbins.aspx

To figure out whether your machine takes L or A style bobbins, take a ruler and measure your regular bobbin because prewound A & L are very close in size

Size A the height is 0.4 inches or 10 millimeters
Size L the height is 0.3 inches 7.5 millimeters

From what I understand some brother machines come with a spare bobbin case that is for using size L prewound, if it did not come with your machine then its my guess that size A will fit better and not jump around in there.
I hope this helps

When I had a brother I used to use cardboard sided with no problem but in a singer I have now I must use plastic sided prewound or it sews out terribly.

Most websites will send you a few samples of cardboard and plastic ones for you to try out before spending that kind of money if you request it.
HeleneT

Metro has a list of which bobbin fits which machine. According to them your PE 750D needs a model A bobbin. See link.

You can use either A (taller) or L (shorter). If you purchase the L you must also purchase a bobbin pin from either a BabyLock or Brother dealer (or online) to use them - but they work great (I use that size from Metro in my 780D and Quattro). I use the plastic sided ones.