When's that "sound" an bad tube?

I'm real new to the tube game, but I've now got a tube pre-amp and a tubed source, so I guess it is about time I asked the inevitable question -- how do you know a bad tube when you've got one? I've been through a whole array of uninvited sounds from the preamp (a VTL 2.5) but today I got a new one. Sounds like static or crackeling, more than anything. Intermittant, irregular, varrying volumes, often silent, and only from the right speaker. It's not source dependant, so we can rule that out, and the amp is quiet as a churchmouse--so all fingers point to the preamp (which is a noisy little beast, it hisses, sometimes even hums, and now it appears to have caught a case of sporadic crackeling--what next?). So, as I haven't owned tubes long enough to have had one go on me yet (unless this is it) the tubes are immediately my first suspect. (And besides, this sounds kinda like what I would imagine a failing tube to sound like). My question? How do I (short of going out and getting a new set of tubes, switching them out, turning it on, and seeing if it still crackles at me) know whether I've got an ailing tube? Oh, if it helps, the tubes have no more than 4-500 hours of use on them (I think life expectancy is somewhere around 3,000?). Also, if anyone's got a good URL or reference on the care and feeding of tubes, it would be a great help. Many thanks.
Switch tubes from cracckling side {left to right} if problem moves it is tubes. I 've bought sets of tubes on E-bay and theres always one tube that crackles and pops. Get good matched sets of tubes. Try audioasylum.com ...tube {archives} section for great advise... Bill
Sure enough, the bum tube was the first one I switched. Better yet, cause the thing is only two months old, I get a new one for free. Question is, does the fact that a tube went sour after only two months mean that I should be worried about something else...? Thanks.
Since your preamp is new, you should be able to insist on a complete new set of valves from your dealer - if not, remember this next time the upgrade fever strikes. Low signal valves usually have a long life, depending on the circuit they're in (eg. how much they're stressed) and the amount of times they're switched on and off. I leave my preamp and phono stage on all the time, only switching my power amp off at night. Unless you're going through a complete set of valves every year or so, the cost of replacement is not great (assuming you're not into exotic 300Bs or 845s) for the sonic benefits. Pearl coolers are a good addition - they not only keep the valves cooler and therefore lasting longer, they also damp microphony. Proper mounting of your valve preamp is also very important - I use the Cetech carbon fibre composite shelves which make a very big improvement to the sound.