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Amperex tubes were originally produced in Holland (The Netherlands) and are very high quality tubes. Some of them say "Made in Holland" and in fact are Japanese.
Many prefer these to the Telefunken and are probably the second most sought after brand of 12AX7. The earliest versions from the 1950's have a "treble clef" logo or just the name Amperex (looks like the lettering above).. 1970's versions have the "world" logo in orange printing. The easisest way to tell the difference is to look at the side of the plate in the center.
THIS TUBE HAS A DIFFERENT PINOUT and CANNOT BE USED IN 6BQ5/EL84 SOCKETS UNLESS YOU HAVE VERIFIED IT WILL WORK!
These have gray plates and usually have a black graphite type coating on the inside of the glass which is to minimize electrical radiation.
The 12AX7 is one of the most widely used double triodes in audio circuits. In most cases, they can all substitute for each other.
Original Telefunken printing is very poor and will wipe off easily with your fingers or just a little moisture.
Prices are the same as 12AX7 Telefunken produced some of the best tubes that have ever been made anywhere at anytime.
They are remarkably devoid of any microphonic problems and tend to last a very long time.
This should make an excellent replacement for any audio application that uses the 12AX7 in the circuit.
It isn't very well known because they were only made for a few short years as it was never designed into circuits by manufacturers. These are usually standard 12AX7's which have been specially tested to assure they will perform well in circuits with /- 20% variation in filament voltage.
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So if the tubes you see have really indelible printing, suspect that they are fakes.