I see lots of "who made this tube?" or "I don't know who made these..." posts. While manufacturers made tubes for each other all the time, it's often easy to figure out who made a particular tube. GE's markings are very distinctive, but several manufacturers (most notably Tung-Sol) used an octagon very much like RCAs. In these cases, there's usually a three-digit number printed with the markings. (In some more rare instances, it's a four-digit code) Amperex (US production) = 111 GE = 188 CBS/Hytron = 210 RCA = 274 Raytheon = 280 Sylvania = 312 Tung-Sol = 322 Western Electric = 336 Westinghouse = 337 If you find something odd, usually a quick Google search will turn up who that EIA code belongs to. Here's one page that's a good reference: http://www.triodeel.com/eiacode.htm The codes aren't always present, but when they are, there's no need to wonder who made your tube!