Dating ge vacuum tubes
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.
It has also come to my attention that there are some perfect "fake" copies produced in China which are difficult to tell from the genuine ones.
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.
I know that I would sell more tubes and make more money if I did, but I'm not in the business of pedaling B. This usually isn't a problem as series circuits are only normally found in TV sets.
The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonic response. Late versions produced by Philips/ECG don't have the extra support and don't perform as well as the older 5751's.
This is a special non-microphonic and low hum version of the 12AX7 that was produced only by Sylvania.
Then, I will follow up with short descriptions of some that I currently have for sale. There is no way I or anyone else can tell you how any given tube will sound in your equipment.
There are too many subjective variables for me or anyone to tell you unless they have your equipment and your ears.
All of the tubes will have odd voltages other than 6 and 12.