I often run the tree lights with U.S.-made early vintage series-wired C-6 (plus several 1970s-present C-7 120V) bubble lights all day long, up to about 12-16 hours with no obvious permanent harm done. My C-6 string (just one) is modified with 10 sockets instead of the usual 8 or 9, and I usually put 5 bubble lights and 5 regular "cone" bulbs in it. Several of the modern bulbs and 1 or 2 of the vintage ones have an odd problem when run for several hours: The dye in the liquid "latches" onto the boiling chips at the bottom of the liquid vial, gradually turning the liquid clear. After the lights have been off overnight, all return to normal.
