Vacuum tubes fell out of the mainstream long ago so it's not surprising that I know virtually nothing about their care, in particular, how they should be handled. There's plenty of postings that talk about how cotton gloves are essential and that you don't want to get fingerprints on the tubes because they'll cause hotspots, and there's even someone selling a purpose built "tube glove" here:
http://www.tweakshop.com/TubeGlove.htmlI wrote Brent Jesse and asked him about this. Presumably, as a reputable dealer in tubes, he would know. As I'm sure he would not mind me sharing his response, here it is.
"Fingerprints do not harm the tube. Above all do not clean them, as cleaning will take off the fragile label! If you want to keep them as clean as possible, either handle them with cottom gloves, or carefully hold them only
by the top rim (or the black plastic base in the case of octal tubes) and do not touch the label. These NOS tubes have labels that are mostly powder with age, and rub off easily."
He further clarified regarding "hotspots":
"No, the power tubes do not get hot enough for the fingerprint to cause a hot spot. This happens with so-called quartz light bulbs (tungsten-halogen) since they burn at a bulb temperature that is nearly 1000 degrees on the
inside surface. The fingerprint actually blocks the light and heat and causes the glass to get super hot at that one point, which eventually will crack and fail. Bare halogen bulbs should never be handled without gloves.
"Electronic tubes, even those with very hot envelopes like an EL34 or 6BQ5 do not get nearly as hot on the glass as a quartz lamp. It is still a good idea to keep these tubes clean, as dirt and fingerprints can get "baked on"
and become very difficult to clean. Also, flammable material should be kept away, as the glass could become hot enough to melt plastic or cause a fire.
"Anyone who has experienced a 'hot spot' on a power tube is more likely seeing the result of incorrect bias on the tubes, and the result is thermal runaway. The plates become super hot as the incorrect bias causes them to
draw huge amounts of plate current. They begin to glow red hot on the sides, and if a fuse does not blow, the tube will actually start to soften the glass nearest the hot spot on the plate. I have seen some of these tubes that suffered this fate, and the glass actually had a huge bulge on the side near the plate that had gone red-hot."
So the fingerprint thing is, according to Brent, mostly a cosmetic issue and not a premature failure issue.
