Mullard 12ax7 dating
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.
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8 in code means 1958 - (6009) 9th week of 1960 is the date in white marking. In transitional years they may have used up the old boxes while simultaneously using boxes with the new globe logo.
PQ6689/E83F PQ (Premium Quality) logo is on other side of tube.
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.
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.Input pre-amps are the most sensitive areas of the amplifier.When used in this application most tubes will generate some noise if you tap on them with a pencil during operation.This allows the high power, high gain crowd and the more conservative players to get the fairest evaluation possible.
Some tubes were clearly better suited in one application or usable in both. 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. In the last years of Amperex the tubes were mostly General Electric.