http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

http://www.lh-electric.4t.com/tutorials.htm

http://groups.yahoo.com/group/singleendedtriodes/

http://www.symphonysound.com/articles/tubefriendly.html

https://www.tubeworld.com/index_low.htm

http://hktubeaudio.homestead.com/files/tubecode.html

http://www.duncanamps.com/

Tubes last as long as tubes last. Different tubes in different circuits have a longer or shorter lifespan. Unless the component has a soft start facility the full, sudden surge of voltage at turn on is always the hardest on a tube and will shorten its lifespan.

http://ken-gilbert.com/vacuum-tube-faq

http://www.ultrajosh.com/tubeamp/

http://www.vacuumtube.com/tubes.htm

http://www.vintagetubeservices.com/

http://www.tubedepot.com/

http://www.tubedepot.com/whisbipo.html

and most importantly ...

http://www.worldtubeaudio.com/

I had a simple EL34 based integrated amp I bought in 1996...I listened to it about 10 hours a week until the first tube began to fail in 2008...

My new amp uses KT88's...I had one fail 6 months after I bought that amp...