Audio Research I/50

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.

@dweller Good Point. 

It is really trick things to handle how the power tube is dead or not. 

The most difficult things is the handover amplifier from the prior owner, as I even do not know how long they used it. I want to replace 6550 set this time, as the sound is way beautiful as some one tell me the status pro of the tube will be best just before they die.

For 6h30 tube, Audio research official one is not good for the  sound,I will change it to EH 6h30PI GOLD even the machine is new like my REF 75se.

By the way,I had Classic 120 many years ago, beautiful  sound, just have maintain problem as I was in China that time, so I have to sell to a Engineer who can handle it  by himself.