AFAIK, x-rays are the result of electrons slamming into metal at high speed from a high voltage accelerating potential (>20kV). The glow inside Mercury rectifi

AFAIK, x-rays are the result of electrons slamming into metal at high speed from a high voltage accelerating potential (>20kV). The glow inside Mercury rectifiers comes from ionization of Mercury vapor. Various other voltage regulator tubes glow from the ionization of low pressure gases inside the tube. Common beam power tubes and power pentodes sometimes glow blue from residual gas in the tube even with plate potentials ~500V.