Amperex, Mullard, Philips, and Valvo Tube Codes
Most Amperex, Mullard, Philips and Valvo tubes are marked with a group of symbols which contain coded information relating to the type, place of manufacture or country of origin, and the date of manufacture. Apart from the interest of knowing when and where a tube was made, the code group can still provide useful type identification when the commercial markings have become illegible.
There were two types of code formats that used the same tube type codes, which were in use from 1948 until these companies stopped the manufacture of standard receiving tubes. The code format changed about 1955 or 1956. There are some deviations to this rule, and in certain manufacturing plants not all of the code protocol was followed exactly, with every batch of tubes. Prior to 1955, Mullard used a purely numerical code for encoding the tube type, but then adopted the code change as listed here after that. Therefore, for tubes made after 1955 by all three of these large European manufacturers, the code is fairly uniform and straightforward to decipher using these code lists. Of course, as mentioned above, there are deviations in the code that vary from factory to factory and from year to year. This list is presented as found on original Philips documentation. Brent Jessee Recording makes no claims as to accuracy, and presents this information for your use "as is" and "as found".
THE ELEMENTS OF THE TUBE CODE:
The digits are often found near the bottom of the glass on the side of the tube. They consist of two or three lines of numbers, letters, and sometimes symbols. They will nearly all use the following convention, from left to right, top to bottom of the code group:
This may be one, two, or three alphanumerics or symbols. For example, the code GA means the tube is a type ECC88, which is also a 6DJ8. These codes will be listed later in this webpage. Pretty straightforward, huh? Take notes, it gets stranger from this point on......