I’ve this portable small T5 4W UV-A 300-400nm black light tube lamp with me. I’m trying to see if it’s possible to use a 4W UV-C lamp in the same fixture. But I noticed that the lamp has the violet glow only to one side which makes me think the supply is pulsed DC likely. But I noticed that the tube has started to darken on both filament ends as shown in picture. 

Anyone has any idea what might be causing this or is this tube Int meant to be run on an AC type ballast. 
[![Blackening at filaments](https://i.stack.imgur.com/Hc712.jpg)](https://i.stack.imgur.com/Hc712.jpg)

[![enter image description here](https://i.stack.imgur.com/ubzWa.jpg)](https://i.stack.imgur.com/ubzWa.jpg)