While looking at some preheat fluorescent tube ballasts to add to my collection, I noticed that some ballasts of this type say “straight tube only” on the etch for some reason like these Advance ballasts shown below:
https://www.ebay.com/itm/273514613604To me, it seems like they might not mind running circline lamps or pin based CFL lamps without any issues. Why are these specific ballasts marked “straight tube only”?