DOA tube from Audio Research

Thanks for the responses guys. I have a compu-bias meter. I didn't change the bias at all from where it was which by the way was on the low side of what the manual recommends. There is no recommendation to reduce bias before tube installation anywhere in the manual. The amp comes from the factory already biased. And yes I did swap sockets and confirm that it was the tube and not a blown resistor. I got a call back yesterday from one of the other ARC guys and he had me send an email to their service department. Th tube was definitely DOA as you can easily see the slow turn on and ramp of the bias current and output power with the comps-bias meter. It didn't arc, pop or any other telltale of failing on installation.

I've been wondering about the use of the KT120s. Thanks for the info BIF. I will definitely inquire with the service guy when we communicate. My local ARC dealer actually suggested the same thing last year. What kind of difference did you hear?

ARC service (Kal) says "no problem" using the computer-bias. It measures plate V, cathode I and dissipation power on a pair of tubes simultaneously. Per my original post, I had replaced the original tubes last year with KT 88s and set the bias at 60 mv / 60 ma. I ordered the new set of ARC 6550s in December because I thought the KT88s were a little too warm sounding. I haven't had the time until last week to sit down at length and listen to the 88s and then do the switch while trying to maintain some aural memory. Some one suggested I should have turned the bias down prior to installation and copied the ARC manual in the post. The implication being that I had somehow made a known mistake in installation and that may have caused the tube to blow. There's no reference in the manual to doing such a thing. In contrast, the amp ships with the tubes in a separate box and requires the owner to install them in a factory biased amp. The turn on circuit has some amount of current ramp up delay in it which can be easily observed using the compu-bias. I had not re biased the 88s since installation but when I initially installed the new 6550s, I noticed the bias was on the low side (for the ones that worked). When I reinstalled the 88s I realized that they in fact we're running quite low (only 17 watts). So I re biased them to (25 watts) and proceeded to listen. WOW. You are right. The difference was dramatic. Everything sounded better. So in the meantime, ARC is sending me a replacement tube at a discount. Bad on me for not noticing the warranty. I realize tubes are somewhat fragile but I seriously doubt that the overall failure rate of all tubed products is 25% due to shipping damage. Therefore my conclusion that this tube somehow escaped the factory in a faulty condition and my ensuing frustration. But as you said, $..t happens.

As for using KT120s in this amp, Kal says it will work but the power supply transformer will be substantially over worked and will likely fail early due to significant overheating.

Thanks for all the responses. This is my first tubed amp and I'm getting initiated to all the issues involved. That said, I still think tubed power is far preferable to the alternative when it comes to music.