The valve coding used by Philips and associated manufacturers seems mysterical, but the markings are basically rather simple to decode. Let's take an example of the typical, I believe later, 3+4-character code. This should work with the 3+3 and 2+3 codes, too.
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This beautiful rectifier decodes as follows.
IS |
Manufacturing code. This is the Philips-group internal code for GZ37. There can be several internal codes for the same commercial code. These are different versions of the valve type. These codes are available at R1 or R2. |
| 1 | Revision number. This is the original version of this internal code type. 2, 3, etc. would be revised versions. (Ignore if not present.) |
| B | Factory code. B = Mullard, Blackburn Works (later
possibly Simonstone Works), UK. These are available at R3.
N.B. If you think that your valve shows the digit 4, look again. It's
more than likely the very similar-looking triangle |
| 2 | Date code. Last digit of the production year. In this case most likely 1962. |
| H | Date code. Month. August. (A = Jan, B = Feb, ...) |
| 3 | Date code. Week. 3rd week of August. (Ignore if not present.) |
This is my limited knowledge about the Philips/Valvo/Mullard coding. I learned this on the net. This is to say that possible errors are not necessarily only mine. :-)
P.S. Factory and manufacturing codes in one file: R4, R5.
P.P.S. More information about this fascinating matter. I lately found R6.
= Philips, Heerlen, the Netherlands.