Life span of tubes?

Depends on the tube manufacturer and your equipment. My Jadis amps seem to eat up every 6550 (ARC Phillips, which lasted 5 years in my ARC M300s, and Svetlanas) I've put in them within 12-15 months, but I've been using NOS GEC Gold Lions for the last 3-1/2 years with no problems (the Jadis have a fuse for each tube that protects the amp if the tube goes bad), and I know of a JA200 owner who's had his Gold Lions in his amps for about 7 years. Don't know if the Mac is as hard on tubes as the Jadis, but I suspect not--I recently bought some Gold Lions used from a McIntosh owner who'd used them for a year, and they tested as strong as unused tubes, and I also know of another Mac owner who kept his amps on all the time with 6550s and no problems for years! More recently manufactured KT88s are reputed not to be as long-lived as the old stuff, but I haven't tried them. As far as tubes wearing out, I usually go by the fuses in my amps, but I've got a good tube tester coming from Andy Bouwman soon and you might want to consider getting one as well. Otherwise, I've found worn out tubes seem to lose a bit of their high frequency extension, dynamics and life when they started to get worn out (and before they started glowing cherry-red); I'm sure others will chime in here with their experience. The change is usually so gradual, though, that you often don't really notice it till you put in new tubes, as you become accustomed to the sound.

Life span depends upon amp design/application. Originally tubes were designed with a 10000 hour life span. if the tubes are used 'nicely' then they should last. I know that the old quicksilver 8417 monoblocks would eat tubes ~ a set every 10~18 months, but a Music reference RM-9 can keep the same tubes in place for 10 years. THis I also know to be true as I just replaced the tubes on my RM-9, and have had the amp since 1990.
The mac should be on the longer tube life end of the scale.
Happy listening

how much does tubes costs when replacing ?
i can think that there are lower and higher cost tubes but..how much in general?

thanks:)

Depends on the brand (I'm assuming you mean KT88s here). The Svetlanas probably are in the $30-40 range, I think, from what I've seen on the 'Gon classifieds. Not sure about the Teslas and Chinese tubes, but probably similar, maybe a little less for the Golden Dragons. NOS Gold Lions/Gold Monarchs from Europe (not the Chinese ones from late in their history) are out of sight ($250-350 each, depending on their vintage and condition); if you can get used ones that test good, they are less expensive and can be a good buy.

Rcprince is right of course. Haven't seen any NOS Gold Lions or Monarchs for quite a while. Occasionly you can find one or two used at Billington's in the UK and they cost lots of Sterling. To get a feel for prices of other tubes, I suggest you check Ebay regularly to get a feel how the market is.