on Full power 20 watt ballast can over drive them. but it will also be tad bit harder on 20 watt ballast due lower lamp OCV. lamp ocv around 40 volts. if u have 30 watt ballast. put two in series. these barely ran in series here on 120 volt OCV using incandescent ballast on early floor lamps. here USA have some what universial short fluorescent lamp chokes. 14-20 watts 360 mA. how this ballast works is drive 14 watt tube bit overdrive 15 watt tube ran spot on, underdrives. 20 watt tube runing them about 16. watts. i like running 17 watt HF tube on it. running them nice and bright with choke running cooler. the HF tube.

I was thinking of slapping a couple in series on a 30W as i'm sure these things'd run fine on 120v each. A while ago I got some dual HF dimmable 18w ballasts that actually run the tubes 16w. I ran them on that and they seem fine. I may build a fitting with a pair on the "2x18w" fittings but I would rather run them in series on a 30w. I'm going to the bootsale this week and there is always cars with 15w blacklight fittings. If I see any i'll grab them to make some proper fittings for them.

Also worth mentioning the tube runs a lot better on the HF ballast. It ran brighter than the 20w magnetic and the internal arc seemed more dense on the 20w. At times it would shift to one side of the tube making one side brighter and one side darker.