I am like Nomade and CK, I don't understand where this "Steel" tube nomenclature ever came from.
They are composite Glass/Ceramic tubes. Even some of the Russian tubes are Hybrids,
I guess it stems from the Anode cooler on top, "appears" to be
steel but they are not.
Nomade is correct (as others have stated) the "C" in the tubes like the 4CX250/4CX1000-1500-3000 etc. is a Glass/Ceramic Hybrid.
The (4) in front means Tetrode (4 elements) ... Filament/Grid/Screen/Anode
The (3) in front means Triode (3 Elements) Filament-Cathode/Grid/Anode
Ex: 3-500Z ... 3-1000 ...3CX800 ...3CX3000
However some Tetrode tubes can be operated as Triodes 4-400 ...4-1000 and others.
However, in doing so, they can require more drive to get approximately the same power as their Triode cousins.
I have an amplifier here that is designed to use a 3-1000z Glass, I can also sub in a 4-1000A or 4T-1100 tube in the same amplifier. The 4-tubes require more drive in Grounded Grid configuration than the 3-tube. The way to compensate for that is to have an adjustable Plate voltage supply. Thus being able to increase the Plate voltage gives the 4 - tube the same "Gain" as the 3 - tube. (or in some cases more)
However again may require the additional drive to do so.
OK CK/Shockwave you or others/ Engineers etc., if I have misstated anything,
please correct me.
I am just a Layman wire twister and this is how I have learned to understand this.
The Russian tubes have a different number or identifier system. However many of their configurations are direct subs for the American (USA) versions. They also have many of their own designs which work well in amplifiers. Ex. Some of their Ceramic/Glass examples are the GI (6)7 series ...GS31 ...GS35 and many more. It just depends on the Frequency and supply voltages/ drive requirements etc. parameters they were designed for.
A good source for many Transmit Datasheets here:
http://www.ok1rr.com/index.php/rustr/2P302A.pdf
https://mafiadoc.com/eimac-power-gr...ce-catalog-1969_5a13405a1723dd3e08081208.html
Russian tube source:
http://qro-parts.com/index.php?cPath=21
http://rutubes.com/category/power-vacuum-tubes/
https://archive.org/details/vacuumtubemanuals
OK...Got to run...
All the Best
Gary