ever-changing
Yes for the better nearly every time, it’s called evolving. Tubes if anything have gone backward, that’s why the real tube die-hards prefer NOS, than to todays tubes. Like NOS RCA black plates, Seimans, Brimar, GE, Telefunken, Mullard ect ect. than todays rubbish from China and Russia
Back in the old days I was told by a tube guru they had things like nickel plates in them, today that’s been outlawed. Also thicker glass in yester-years tubes, bakelite bases ( polyoxybenzylmethylenglycolanhydride) and it had asbestos mixed in as well, also outlawed. Todays are plastic bases and microphonic thin glass. Flick and NOS GE green label mil spec 6550 with you finger nail, it goes thunk, todays Chinese or Russian 6550 goes tinkle!
Cheers George