It is quite common to find tubes that have a “brand name” printed on them, but were actually manufactured by a different company. Yes, you can easily find “RCA” tubes made in a Sylvania factory, “GE” or “RCA” tubes manufactured in a Tung-Sol plant, etc.
So how do you know which company actually manufactured your tube? Most tubes have a Factory Code on them, an EIA code (Electronic Industries Association), and that code tells you the answer.
Here are the most common factory code numbers found on tubes:
- 111 = This is an Amperex code, but many of the tubes marked 111 are obviously GE manufactured
- 158, 171 = DuMont
- 188 = GE / KEN-RAD (188-4, 188-5, 188-20, etc.) [info]
- 210 = CBS Hytron [info]
- 247 = National Union [info]
- 260 = Philco
- 274 = RCA [info]
- 280 = Raytheon [info]
- 312 = Sylvania [info]
- 322 = Tung-Sol (USA)
- 323 = United Electronics
- 336 = Western Electric
- 337 = Westinghouse
- 722 = Sylvania-manufactured for OEM equipment makers (Allen Organ…)
- 1022 = Fisher branded Mullard/Telefunken/Amperex
- 1109 = Raytheon (Japan)
- U.S.A.3 (found on the glass envelope) = Tung-Sol
- Xf1,Xf2,Xf3,Xf4 = Mullard
- 7C,8I = Toshiba (Tokyo Shibaura Electric)
(1) you have an “Admiral” tube with “188-5” codes. GE made your tube.
(2) You have an RCA 5881 with “U.S.A.3” code on the glass. Tung-Sol made your tube.
There are many other “clues” also. For example, many Sylvania 12AX7A tubes have the tube designation printed in white-gray letters, vertically stacking all three designations of “12AX7A ECC83 7025” Only the Sylvania factory used this exact marking.
It just looks like an RCA…
distinctive markings of the Sylvania 12AX7A
Example: Stacked printed designation “12AX7A ECC83 7025”.
Here is an example of unique Sylvania-manufactured designation for 12AX7A tubes, where the tube is printed as “12AX7A ECC83 7025” vertically stacked. In this example, this tube is branded as an RCA, but was in fact manufactured by Sylvania. Compare to the Sylvania-labeled tube.
When I have more time, I will continue to add more clues here to help you determine who manufactured a tube when it does not have a factory code number.