DVDdoug:
If the string is working with 120V, pull-out one lamp. All of the bulbs in series will go out. If any stay-on they are in a separate series-string.If you have a series string of 50, that's 120V/50 = 2.4V per bulb. The energy/wattage from 120V RMS is the same as 120VDC. (Don't worry about the peak AC voltage.)
So with 5VDC, two in series would be slightly over-voltage but probably OK.
It is string of 100. 2 strings of 50 bulbs in series. The 2 srings in series are in parallel with the mains. A 2.5v drop per bulb sounds right.
The thing is at 120 vac the bulbs are dim and on the yellow side. At 5 volts they look bight like they should. THis makes me think these blubs were desiged for a 220v market. Instead of having 2 voltages of bulbs one set for 120vac and the other for 220 vac they just use the same bulbs in both markets.
The bulbs work fine at 5.5 vdc. They blow at around 8 volt. The black ring in the glass makes me think they are blowing from a voltage spike and not over voltage. I would also think they would all blow at about the same time if it were from over voltage. Since only one or two blow at a time I'm thinking spike.
As I side note I know these strings of lights are made by chineese prisioners. If they install 7,000 bulbs per day they receive a full ration of food. Less than 4,000 and they go hungry.