I would assume (never a safe thing to do) that the type numbers for CRTs were assigned in an ascending order. In other words 15AP22 should have been the first 15" color CRT. If that assumption is correct, were there ever 15AP22 through 15FP22 CRTs? Prototypes, maybe?

Also, what organization was responsible for assigning tube type numbers?

Number were assigned to tubes ignoring the phosphor. 15AP4, 15CP4, 15DP4 and 15EP4 were manufactured. 15BP4 and FP4 were probably registered by someone but never made.

Thanks, Steve. I knew there had to be a rationale in assigning tube types. Makes sense to me.

On the RCA CRT data sheet I managed to come across, they list the EIA number, and the developmental number in the next column ...ie C19676. Not every tube managed to get an EIA number. Perhaps they floated some designs, and if a manufacturer ordered, they registered the tube?

Maybe someone on this thread can answer an unrelated question in the same verve: How did the 1DN5 (a diode-pentode receiving tube similar to the 1U5) wildly jump the queue of the 1**5 sequence?

I remember reading someplace, that a second letter in the sequence (in CRTs following bottle diameter) meant it was a better design tube. (i.e. 21YP4 vs. 21FEP4, etc)

So the string for a 21AXP22A would be:

21 ____________ AX________________________P22___________A
Bottle size_______Tube model Type_____________Phosphor type__"A" = aluminized

Charles

I remember reading someplace, that a second letter in the sequence (in CRTs following bottle diameter) meant it was a better design tube. (i.e. 21YP4 vs. 21FEP4, etc)

So the string for a 21AXP22A would be:

21 ____________ AX________________________P22___________A
Bottle size_______Tube model Type_____________Phosphor type__"A" = aluminized

Charles

I knew that the "A" often referred to the aluminized bell inside the B&W CRTs, but didn't think that's what it meant for color CRTs, so I just looked it up.

The data sheets indicate the difference in the 21AXP22 and 21AXP22A is the addition of a high resistance coating inside the neck of the tube to eliminate the need for the external resistor between the CRT and the anode power supply. A high-resistance coating certainly wouldn't be made with aluminum.

http://pdf.datasheetarchive.com/indexerfiles/Scans-008/Scans-00173622.pdf

I'm also not sure the first part of what you're trying to say in your post is either clear or correct.

.

And somewhat OT, the "A" in a vacuum tube could mean controlled warmup filament, or less microphonic, or higher voltages. I guess it would be too much for us to expect some kind of order ;) !