A lot of questions have been asked about when a tube is made, so here you go, a quick guide on how to decipher the date codes.

For the most European tubes made after 1955 date codes are located on the bottom of a sidewall of a tube (not always the case)

In this case, it’s B0D (this is for Mullard, Blackburn – 1960 – April)

The code is broken down into 3 or 4 characters. 1) Factory code, 2) Year of Manufacturing, 3) Month, 4) Week (this is usually on tubes manufactured after 1960)

The first 2 characters are the most important ones since the factory code would show where a tube was made and year, well, this one is super important since sound changes quite a bit for the same tube from the same place depending on a year. For 3 character codes 8 means 1958, for 4 character codes, 8 means either 1968 or 1978 (that depends on change code – more about that later).

Finding the code could be very easy, but occasionally it’s quite challenging. Start with the naked eye and if you can’t find it use a magnifying glass. Date codes are usually done with acid, so they are burned into glass. Some tubes, like Telefunken, don’t have acid date code, it’s usually printed on the glass as part of the silkscreen and very easily removed.

Most popular factory codes:

Code	Manufacturer	Location
A	Wiener Radio Werke “WIRAG”	Wien, Austria
B	Mullard	Blackburn, Great Britain
C	Hendon Works	Alberton, Australia
D	Valvo	Hamburg, Germany
d	Ned. Instituut voor Fysisch	Onderzoek
E	Hyperlec Brive	Correza, Spain
F	La Radiotechnique	Suresnes, France
	Marconi Osram Valve Co.	England
G	Mullard	Fleetwood, Great Britain
J	Mullard	Tottenham, Great Britain
L	M.B.L.E	Bruxelles, Belgium (Mazda)
N	Matsushita	Takatsuki, Japan
n	Matsushita	Kyoto, Japan
R	Mullard	Mitcham, Great Britain
r	Philips	Ontario, Canada
V	Bharat Electronics	Bangalore, India
X	Philips	Sittard, Holland
x	Philips	Chile
Y	Philips	Sittard, Holland
+	Philips	Sittard, Holland
Y	Philips	Sittard, Holland
=	La Radiotechnique	Cinchy, France
*	Amperex	New York, USA
±	La Radiotechnique	Chartres, France
¡	Siemens	Munchen, Germany
	Philips	Heerlen, Holland
	Ei	Nis, Yugoslavia

Month codes are quite straight forward:

• A – January
• B – February
• C – March
• D – April
• E– May
• F – June
• G – July
• H – August
• I – September
• J – October
• K – November
• L – December

The next step would be to consider which Change Code the tube has been developed under. That’s the 3 character code above Date code. Usually looks like I61, etc. This is much more complicated since there is no standardization on that, each factory used their own codes.

Some most popular change codes for ECC83 (dates are very approximate):

• Mullard
  • mC1 (Long plates) – 1955-1956
  • F91 (Long plates) – 1957-1958
  • F92 (Long plates) – 1959
  • I61 (Short plates) – 1959-1964
  • I62 (Short plates) – 1964 (transitional tube)
  • I63 (Short plates) – 1964+
• Siemens
  • mCP – before 1956
  • mC1 – 1956
  • mC5 – 1957-1958
  • mC6 – 1958-1959
  • I61 – 1960-1961
  • I62 – 1962-1966
  • I63 – 1967-1969
  • I64 – 1970 (end of production)
• Philips/Amperex
  • mC2 – 1955-1956
  • mCN – 1954 – 1955/6
  • mCA/mCB – 1956-1958
  • mC5 – 1957
  • mC6 – 1958-1959
  • I61 – 1959-1960
  • I62 – 1961-1962
  • I65 – 1963-1969
  • I66 – 1970-1972

Some most popular change codes for ECC82 (dates are very approximate):

• Mullard
  • K61 -1956-1960
  • Gf1 – 1962-1964
  • Gf2 – 1965-1970
• Siemens
  • k62 – 1955-1958
  • k63 – 1958
• Philips/Amperex
  • k61 – 1956-1957
  • k63 – 1957-1959
  • Gf3 – 1959-1960
  • Gf5 – 1961-1961
  • Gf6 – 1963-1969
  • Gf8 – 1965
  • Gf9 – 1970+

The list goes on and on and on…

So there you go, hopefully, some of the mystery is gone from the tube world.

Please leave your comments if you have any questions or corrections.