Why aren't new vacuum tubes as good as old ones?

I confess I use Mullard CV4024s, but mostly because the JAN Phillips 12AT7s they replaced self destructed after about a month, and I like the "start-up flash" the Mullards display...we need more tubes with that start-up flashing...it's exciting. Note that the EH 12AT7s were fine, I was simply overcome with curiosity. I do like the new Mullard 12AX7s in my Burriss guitar amp, but they, sadly, don't flash on start-up.

All these new tubes are basically 845,300B, etc. If your looking at signal tubes like 12ax7,12au7,6922,6SN7, etc. the new old stock from the 40-60's sound way better than any tube from China or Russia in my opinion.

Jwm, I sort of agree although there are great sonic differences among the NOS also. I think either the skill of those who were making the NOS or the QC then with many tubes discarded.

Yes of course the RCA's sound different than the Sylvania's. I don't know why that should be the case since factories know the specs on how the tubes should be made. It may be slight differences in materials or tolerance in the parts.

I'm encouraged by the sonics of many current production tubes, especially some coming out of China. Many rank right up there with the legendary tubes, though more often than not, the old American and European variants still hold the edge in absolute terms. From my experience, I think it's in ruggedness that the old tubes hold a big advantage, particularly over many current production European tubes.

Shuguang tubes possess the best reliability in the industry reliability over the past decade. While the premium Chinese products get most of the attention, the run of the mill Shuguang 12AX7B, EL34B, KT88, and 5AR4 offer surprising sound on top of that reliability. Speaking of surprising, the dirt cheap NOS military Shuguang metal base 6SN7 (labeled 6N8P) will as well.

Personally, I find buying old used tubes that most pass over in favor of spending a lot more for NOS finds on sites like ebay to offer tremendous sonics, longevity, and overall value.