Dating ge vacuum tubes
When the filament is first energized, it flashes very bright for a second and then dulls down to the normal soft orange glow. IEC Servicemaster bought the rights to market Mullard tubes during the 1960's and 70's, so many Mullard tubes are dual marked "IEC Mullard". Mullard also marketed a special version called their 10M series.
They are remarkably devoid of any microphonic problems and tend to last a very long time.The only way I could tell is by looking at the writing on the tube.Original Telefunken printing is very poor and will wipe off easily with your fingers or just a little moisture.It is fairly rare to see a weak or bad Telefunken tube.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.