I won a pair of NOS RCA 2A3 tubes (grey S-plate variants) on eBay and have them in my Stereomour right now. They sound great, but I have a question about the filaments: each tube essentially has two plates, and therefore, two filaments. Each filament is "hung" from two points, like an inverted v. When lit, this means that four lit "points" can be seen when looking down into the triode assemblies. On one of the tubes, one of these points is lit when turned on, but then fades after a minute or so, leaving just three of the corners lit. Then, it will sometimes get bright again. The other tube does not show this behavior. Is this a problem, or simply the result of uneven heating, impurities in the filament wire, etc?
