Tube Help--I'm trying to make sense of the codes on this tube.

The etched code is the way to determine exactly what you have.

Gf is the tube type, in this case a 12AU7

2 is the change code which tells you what revision of the tube you have, like a model number.

The second row of codes tell you the factory it was made at and the date it was made.

B means the Mullard factory at Blackburn, England.

5 is the year of manufacture. Since these are orange label, which were made in the late 60s and the 70s, the year is 1975. If the labels had been white, I would say 1965.

J is the month, 10th letter of the alphabet is the 10th month, October.

2 is the week of the month.

Phillips was a Dutch conglomerate that owned many tube brands such as Mullard, Amperex, Siemens, Valvo and many others. Tubes made in one factory were often labelled with another factory’s brand if an order needed to be filled or whatever. Phillips also sold tubes to independent distributors like Los Gatos and National who put their own labels on them. Other manufacturers like Telefunken and RCA had their own ways of coding tubes.

Phillips tube codes can be found here:

https://frank.pocnet.net/other/Philips/PhilipsCodeListAB-v8.pdf

Another good resource for identifying NOS tubes is the Tubemonger picture library, which contains pictures of many, many NOS tubes:

http://www.tubemongerlib.com/gallery2/main.php

Enjoy!