New to tubes


I just got a Musical Fidelity X-pre preamp, which contains two tubes (KT-88's I think). How do tubes age? Will I hear a deteriation in sound? Or will this go so slow my ears will adapt to it, so I won't notice it? Or will they just quit working when there too old?
Basically: how do I know when it's time to change them?
Thanx in advance.
satch
I'll second the Tube pre and SS amp! (AR LS2B Mk2 and 3 Threshold S-300's!) As far as tubes are concerned, have some fun. Each tube will have a different sound and it is kind of fun fooling around with them. I agree with Marco, find someone with a tube tester or get one for yourself.

You will find the prices will range from low to outrageous. Decide on a budget and start buying some. You can usually recoup most of your cash in the resale market, if you purchase wisely. This will allow you to try many different tubes.

Hop over to AudioAsylum.com and talk with some of the tube guys over there, they can be a real help choosing. Welcome to tubes, it is quite and experience!
Ok guys, thanx for your help. There's nothing deeply philosophical about my set-up though. I changed speakers, but they couldn't get along with my ss-amp, so first I bought this huge class A power-amp, using my old integrated as pre. When funds allowed (or actually, they didn't....) I bought a secondhand pre-amp, which contained tubes, and combined great, so I decided to keep it.
I guess, since there is plenty of life left in the tubes, I first upgrade the interconnect between pre- and power amp, and then I might have some fun with tubes.
Sorry I couldn't chime in again last night Satch. I went to bed with a hangover...not because of your questions though. That may explain all the additional copies of my post! Yes, I'd concur; tube pre with class A SS can be a nice combo when done right. Hang onto your tubes, save your pleasure money for the prostitutes and enjoy the music!

Marco
I think of changing tubes like changing oil. I do it regularly (once a year or so) whether they need it or not and certainly before they get 3,000 miles on 'em and get all black and sticky :)

Besides, it's fun to get a little bit different sound every year or so.

TJ
one thing not mentioned so far: diagnosing pre-amp tube problems is a pretty simple exercise, and you can do it without a tube tester. If you get bad noises (of any kind) in one channel only, turn off the pre, unplug it, switch the tubes from L to R, one set at a time (input, output - I'm assuming there are more than 2 tubes in your pre....) and see it the problem moves after you power up again - that isolates the problem so you can figure out which tube to replace.