Mullard 12ax7 dating
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.For that reason, allot of Telefunken tubes are devoid of most of their writing.These fakes have very nice writing and the printing will not rub off with your fingers or water.Any tube produced in the Telefunken factory should have a "diamond" molded into the glass in the base of the tube inside the circle of pins.If you don't see this diamond, then the tube wasn't produced by Telefunken. These are true Telefunken tubes as they have the characteristic diamond in the base.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.
The sound is characterized by many to be "mellow" or "dark" but really if you want to know how a tube sounds, you need to try it for yourself in your own equipment because it may sound totally different to your ears than someone else's.There are basically 2 different kinds of Telefunken 12AX7's.The "smooth" plate version has as large totally smooth plate surface. The later version has ribbed plates that look much like RCA plates.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.
There are many Telefunken tubes out there that don't have the Telefunken name on them, but instead an OEM brand like Fisher, H. This is true for other brand names of tubes like Amperex, etc.