Explanation of the Telefunken method of naming valves
This historical series of RE-.... tubes are probably the finest tubes ever made. No American manufacturer ever came close to the quality of those. Just imagine that some of the smaller tubes had a filament current of just a few miliamperes. It is a miracle how they could ever get so much emission out of this. That would make a historical battery radio work on a set of vintage batteries for many days.
One of the secrets of those extreme efficient filaments is, they had a method to vapor the filament coating on the filament after the glass was closed. Something similar like a getter. There are filament coating pills in the middle of the plates. This was their secret to those extreme thin filaments, that could work so well on so little current.
Some of the special dual grid triodes (yes!) could work on voltages of 12 Volt just normally. These were variations of the normal four pin triodes, and a "help" grid (5th connection, with a screw) was electrically connected. The "help" grid would delete the space charge between the filament and the normal "triode" grid. The coming of the normal mains voltage in households ended this wonderful period of tube making, and then tubes with more filament current were introduced.
NOS RE-tubes are very rare and highly sought today.
There was second source of many types, but they all were under completely different type numbers, without much of a system. So it ended up in a big mess, later. The TFK part numbering is well choosen, and very clear.
On some tubes, you find extra words, like "NEUTRO" or "SERIES". These are extra quality types, by selection.
The tube types are composed according to the following scheme:
Note 1) We learn from this that you may only use DC heating with RE tubes, unless you have an "N" tube, like RE134N. I do not know the exact reason for this. One thing is for sure, the filaments of RE tubes are miracles of technology. It is absolutely amazing how these tubes can work so well, and use less filament current than an LED. It means these filaments are 1/3 the diameter of a human hair, and their heat capacitance is extremely small. They can heat up and cool down in fraction of second. So.... it is well possible that if you use "DC-only" filaments on AC, that the filament temperature might follow the 50Hz a little bit, giving hum on the signal. Special AC-filaments would not do so.
Note 2) Warm filaments are like a constant current user. An increase in voltage will almost not change the current. This means, if the series current is different, you can not serialize them. Just as well as you can't parallel a 15Volt Battery with a 20Volt Battery, you can't serialize an 80mA user with a 60mA user. You can not parallel different voltages, and you can not serialize different currents. Period. If you try, something will go wrong. Suppose you have these tubes: 4Volt-80mA and 4Volt-60mA. If you serialize these at 8 Volts, the 60mA will act as a fuse for the 80mA. To serialize them, they need to be factory selected for drawing all the same current. Later modern TV and radio tubes has special "series connection only" part numbers. Telefunken used to factory select the voltage driven tubes for using all the same series current. It proves how well their process control was....!