-=> Quoting John Allen <=-
1. How long should you expect to get out of a set of tubes?
Depends upon the tubes; 2 hours to 400 hours, depending upon the
tubes, matching, and how they are handled is a decent estimate
of the range.
2. How long should a tube unit be allowed to "warm up" 
before playing?
Give them 15-20 minutes for the best stability; it varies.
 
3. Do the "tube covers" (look like little tube sweaters)
actually increase tube life?
No, not in themselves; some vibration damping is a good thing, but
most tube covers tend to insulate the tubes, causing higher 
temperatures, and output tubes are hot enough, even with good air
flow.
4. What things are tube amps/pre-amps sensitive to/about
   that might cause them NOT to play at their optimum?
Lack of ventilation, poor matching, physical vibration, impedance  
mismatching (higher speaker impedances are worse than lower), and
transformer degradation (often due to heat and vibration).
5. Are tube units more or less sensitive to vibration and
   should extraordinary effort be put into their isolation?
More sensitive than solid state, yes. Isolation/damping is important, 
but do not impede the airflow, since this will kill the tubes.
6. Can a tube amp like I described be placed in an enclosed
   (no back) entertainment center without overheating etc
   or - - must they be in the open air due to heat? (I have
   often seen them called "space heaters").
You can easily ventilate the amp with a small fan, and this will
solve any problems.
7. Am I nuts for even considering getting into tubes?
NO! Tubes are wonderful, with their even-order harmonics 
charactaristics! They need to be replaced more often, and rebiased,
but the sound of tubes is superb, especially if you are listening to
(often harsh sounding) 16 bit digital material. Take care, and 
keep the tubes! 
... You're a Redneck if you think people who have electricity are uppity.
--- Blue Wave/DOS v2.30
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