Audio Research REF 750 pre July 4 FIRE WORKS. TUBE ARC.

Did you check to see how many hours the tubes had on them before you fired up your amp? Did you ask the previous owner if the tubes were original (ALL of them) or had been replaced and if replaced, from where were they sourced?

KT 150's are problematic. They are great when they are great but the QC coming out of the New Sensor factory is terrible. Few middlemen can supply reliable matched sets of them.

I apologize for being critical, but will say that as an owner of four ARC amps past and present (present are the Ref 150 SE and Ref 80S), I would have warned you that the Ref 750SE's are NOT the amps to start out with as your very first tube amps. Add the fact that they are used and your choice is even more unfortunate. The more power tubes you have, the more likely one will arc and fail. Add KT150's to the mix and well, this was not unforeseeable. 

I would have taken your amps to John Rutan to check out before you fired them up for the first time. If you had done this, there is a good chance (but not necessarily) that John would have seen something amiss with bias. This concerns me; "Tube bias is controlled by two KT150 tubes, each driving a bank of eight KT150 output tubes" (from the website). With my Ref 150 SE there are four KT 150's per side but there are two bias pots per side so that one KT 150 is "slaved" to the other. When an output tube is failing or faulty, it can most often be detected by checking bias on my Ref 150 SE. Perhaps with your Ref 750 SE's the best way to check all those tubes is to pull them and test them on a good tube tester. 

What you evidently did by unboxing them and firing them up right away is perform a stress test and the stress test failed. I am not trying to beat you up over this, nor should you beat yourself up about it-it is a good learning lesson. 

I truly hope John can guide you on the path of getting reliable service with 32 KT 150's running in those monsters. 

Not trying to be negative here either (like other posters have also mentioned), but I sometimes wonder where ARC come up with these designs???

There has been a lot of negativity towards ARC for years on this Board, long before it's recent troubles, and as an ARC fan I have little trouble ignoring the naysayers. All that said, on this I agree. I bought my first ARC amp, a VS110 from what I gathered was a very wealthy and knowledgeable guy who had a collection of ARC amps. Years later after buying the VS110 from him I contacted him to say hello and he mentioned he had another ARC amp to sell. I lost the email and can't remember the model name/number but what I will never forget is looking it up and it looked like a welding machine. It was similarly vertical to the Ref 750 but far more industrial looking. Those of us that follow ARC closely recall some real duds, regardless of looks, in the VT range. They were almost universally panned sound-wise. They also were very difficult to bias without being an expert. 

I do wish ARC would not mount tube sockets directly to circuit boards. The implementation, finally, of fuses on the tubes as in my Ref 80S is welcome. 

VAC and others have built and sold monster tube amps with vertical designs but power is useless without reliability and user-serviceability. 

@mulveling You are one of the most knowledgeable regulars on this Board. When you post, particularly in the analogue section, I read what you have to say. I appreciate your input here.