New to tubes

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.

Marco, who said anything about prostitutes? I think my girlfriend wouldn't be to happy.... :)
And Musicslug, actually there are only two tubes in the pre, but I still can switch them, so your idea is valid. Although, with two tubes probably the same age, it would be easier just to replace them both I suppose. And TJ, since tubes ar more expensive then oil, I don't think I'll replace them that much, besides, I just got 'em, so I'm still rediscovering my music at the moment..
Just one other remark: first thing I'm going to change is the cheap, plastic offboard psu (12V/500 mA) for something homebuild and beefier: 12V/5A. I guess that will improve the pre a good deal.