Audio Research I/50

Yes. All of these factors figure into the total lifespan. Some more than other. At the time, I was playing music 4-5 hours a day (more on weekends) and got about two years from ARC factory tubes. You can probably compare tube bias to the idle speed on a car. If your idle is set to 4000 RPM, your gas won't last as long.

although its looks like there is an issue with the bias adjusting, i still think that it’s something to check with ARC
i don't think i'll buy an tube amp without this option

@dweller  your calculation is right.  

I have  vt100 mkiii and the tube is from period owner, 

Recently I changed all 6550 tube and 6h30 tube, but the old one is not dead. I am wondering how we can found the tubes are almost end and got swapped out?

If I do not prepare in advance,does that damage the amplifier?

I owned the VSI-55 years ago. This looks like a redesign of that or the newer VSI-60. Entry level, they say, so likely cheaper parts to keep the price down. Not an ARC basher, I love the products. I wish they would put their best technologies into a low powered amp. 

@runwell - I too owned the VT-100 III. Also the Mk-I and a Classic 60 (the good old days!). I never used them with a preamp that matched their quality, however so never heard them at their best. If you are worried about tube failure, just replace the 6550s after two years. You can send the old set to someone who does testing (like Upscale Audio in L.A. California) and keep the good ones for spares. The 6H30s last a lot longer than 6550 (ask ARC about lifespan). BTW, I had a power tube go out on my ARC Classic 60 and didn’t even notice it (I listen at conservative levels and use a subwoofer). Discovered when I traded in for the VT-100 Mk I.