Advice on Repairs for Audio Research VT100 Mk II

ARC amps are built well and tend to be reliable. Until they're not. I believe most ARC experts and fans will admit that there were some clunkers along the way, in terms of sound, design and reliability. I had a VS110 (which was the lower-tier amp contemporary of the VT110 IIRC) that was remarkably reliable for ten years of constant usage. 

Most-but not all-tube amps are expensive to maintain. My ARC Ref 150SE uses eight KT150's. That means paying $1600 or more every three years more or less to re-tube just the power tubes, closer to $2800 if you opt to buy ARC's "specially screened and matched" tubes. 

There are 8 power tube 100 ohm 5% 3 watt screen resistors. (Part # 43100208) ARC uses the resistors as fuses.

Yes, in contrast to what Ralph said this was not an engineering goof (though some would maintain it is) but a conscious design choice by ARC, allegedly for the sake of sound quality. Finally, with ARC’s latest Ref series of amps, they feature fuses for the power tubes rather than resistors, soft-start with relays, and auto-bias. They threw in switchable pentode/triode to boot. They claim-and I have no reason to disagree-that they have simplified the circuitry enabling them to decrease the NFB. So FINALLY ARC has made a real effort to spare the customer of much of the previous ARC-induced pain. Now if they would only give up on the Tungsol KT150 as being inherently unreliable and unjustifiably expensive and model their top tier amps to accept more reliable power tubes :-)

I believe that ARC's long-time love and appreciation for nearby Magnepan and its loudspeakers has had an unfortunate influence on ARC's amp designs. ARC amps are designed with difficult load handling as a top criterion. Neutrality is now another top criterion. The KT150 is king of the heap for power, low end grunt, and neutrality in the tube world. 

Don’t get me wrong. I love ARC. I am in the "the preamp is the heart of a good system" camp and imho the Ref 6 and Ref 6 SE are as good as a preamp gets. There are different but equally good preamps, there may be some marginally better preamps at far greater cost, but not by much. I am not quite so bullish on ARC amps as I am on their preamp designs. I stick with ARC amps for the sake of optimum matching with my beloved Ref 6 pre. If the situation with the dreaded KT150 does not improve next time I need to re-tube, I may very well switch to a different brand of amp.

I agree with you Ralph and have a great Thanksgiving. As I said, ARC made some clunkers. Some of their amps were almost impossible to bias without taking to a good technician. And many dealers were forced to become proficient at replacing power tube resistors. I still maintain that for a large (by relative standards) tube amp specialist, their build quality is good and many have been reliable designs, again all things being relative. I know that your own tube amps are at the far spectrum of reliability. Kudos to you. At the end of the day, any electronic component will eventually need service unless pitched for something else, particularly tube amps.