How to dim cold cathodes?

[lobsterpot]

:light: Greetings. I'm using standard 12" and 6" cold cathode fluorescent tubes outside my computer by connecting each dual set of tubes to a wall transformer outputting 12V 500mA DC. My question is how can I dim these tubes? googling on "high voltage potentiometer" (suggested by my Jameco tech) drew nothing but cyberflies.

Anyone?

regards,
bruno
cape cod