Bloody hell.. I just replied to this at length.. hit "post" and its not here!
Ok.. here goes.. revised edition.
The plasma tubes work the same way as a neon tube.. voltage determines just how much can be run.. current its brightness. If your voltage is OK but a higher resistance is present then the current drops I=V/R.
Over a long period of time the Edison effect takes place... electrons are normally released with each AC wave back and forth.. on release the electron is gathered on the opposing charge side.. sometimes this doesn't happen and the electron gets deposited onto the inside of the glass.. it will take on a blackened look and the plasma glow start to appear slightly greenish.
If the burn in doesn't work ( this revapourises the mercury inside that has accumulated on the cathodes sending it back into the argon gas and creating UV- blue light).. then clean the contacts.. if this doesn't work then Edison has proved himself again and the tube is failing... but.... if all tubes are appearing the same then I doubt Edison is very present.. it only really happens due to manufacturing faults.. ie dirty glass, a vacuum under 10-4 or impurities in gas or mercury.
