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Here is an example of a Siemens tube with the metal tag factory and date code. Often quoted by tube experts, and just as often misread and/or misinterpreted by the same experts.
The list reproduced here is about the most comprehensive I have seen after researching original Philips documents (poor quality scans of these are available on various websites on the Internet) and several other partial sources.
But it is nice to have these codes available if you are not sure what type of tube you have.
I have decided to post the scans of the original Philips Holland lists that were created at their headquaters in Eindhoven, The Netherlands.
This is for reasons of finding tubes that could be put together in matched pairs.
It is more likely that tubes from the same date code and factory will match up better than tubes from different batches and factories.
The theory that they pertain to individual batches of tubes or "production runs" seems to be the most plausible, and is the purpose I have accepted as to their meaning.
For the most part, tubes made after 1956 are what is commonly found on the vintage market today.Not only can it be done at home, but it is free, easy to prep, relaxing and super fun!I am constantly amazed at the amount of information you can find on the internet – including, and especially on You Tube! You name it and you can probably find it on You Tube.This will reduce the number of code choices you have, and the tube type codes of the tubes you use the most will become committed to memory, as they never change.
Here is an example of a Mullard Blackburn, UK factory date code: The first line is r X2. 2 is the change code, probably a 2nd production run with updates from run #1. The B is Blackburn, UK Mullard factory, the 2 is 1962, the J is October, and the 5 is the 5th week of the month. Mullard tubes are the only coded tubes I have seen that SOMETIMES do this, another clue to a genuine Mullard tube.
Therefore, batch 0 was 0, batch 1 was 1, batch 10 was A, and so forth until the end of the list.