Mullard 12ax7 dating
6GK6 code: AX1 (NOKEY)9J1 - NOKEY is Heerlen, Netherlands 4 seams on top, 9 means 1969 in code.
Note the pointed pins which are characteristic of a Russian made tube.
In a separate cartouche there often is a two character alpha code which denotes manufacture date eg AA.
The first letter denotes the year and uses the entire alphabet except for I and O and started in 1945. In a similar fashion the second letter denotes the month of manufacture with A denoting January and M denoting December.
A microphonic tube will ring, howl or produce general feedback problems.
It will be more noticeable at louder volumes or when used in close proximity to a speaker, typically in combo amps.
date code 1960 - date in white writing is (1 14) 1961.
ECC88/6DJ8 code: GA4 (NOKEY)0L2 NOKEY manufactured in Heerlen, Netherlands. Note that in 1961 the globe logo is used on the box.
Some tubes were clearly better suited in one application or usable in both.For each tube used in the test, two were taken since there was no pre-screening involved.The aim was to get a sample that would be practical to work with but allow for variations in the tubes or prevent picking the only dud in a lot. One was a 100-watt Trace Elliot Speed King with 4 x 12 cabinet, and the other was a Fender Blues Junior combo amp.All tubes were used at the input amplifier stage of the amp since this seems to be where most people develop their perceptions of how good a pre-amp tube is.
Back to top The musical detail or ability to reproduce the sound of the instrument is a key factor in assessing a tube for guitar amplifiers. Each one has strengths, weaknesses and certain factors that contribute to its overall ratings.An additional system of dating is also used which records the date of valve labelling, this dating system utilises a four digit numeric code in which the first two integers denote year and the second two integers the week of the year. These are all European made tubes (except the Russian fake). Bugle Boy - 4 seams on top - note the 50s logo on the box. Evidently this tube sat for 4 years before being boxed. Note that this box has North American Philips written on it.