Tube smell?

Some 5687 tubes I just installed have a mild but definite
burning-rubber odor. It's not extreme, but it is there
if you sniff around them (and I can also detect it around
the room). Has anyone ever had this? Could it be something
oxidizing because they are newly installed?

It's a bit hard to tell, but it seems to be all 4 of them,
not just one.

The tubes work fine, but look brighter than
my old cheaper 5687s. They sound absolutely great,
nice and smooth and mellow.

It's not the amp, as the old tubes don't smell.

Thanks for any help.
I bought a tube preamp from someone that smoked cigars and ended up getting rid of it because it stunk up the house so bad when I turned it on.
On the PrimaLuna amps, as I think Art Dudley noted at one point, the stock tubes have kind of a smoky smell when they warm up. If you have to put your nose right up to the tubes to smell it, I wouldn't worry too much. If you can smell it from your listening chair, then maybe it's time to replace the tubes -- or have the entire unit checked out.
Part of the charm of tubes. Mind you it is subjective only and cannot, indeed need not, be explained scientifically, but the sense of smell is involved in the emotional response to music.