Hi Kay, IIUC, apart from them being power tubes, there is also a connection to the glass, and the amount of cobalt it contains. Cobalt is in the glass so that the metal in the glass-metal bonds is wetted, and will seal hermetically. The cobalt ions released from the glass cause the fluorescence. Different glass sources have different compositions of the glass, so some companies have a greater reputation for fluorescence (e.g. Svetlana).To your 5B/255M pic: These STC tubes are nothing else than modernized 807's without top caps, without pinch and in straight glass, loctal based envelopes. So their blue halo also resemble those of 807's shown some postings above.