Arc Ref 5se owners

Audio Research definitely misjudged how its products are used when it put the plastic cover on its Ref 5 preamps. My cover has never deformed, but I keep my preamp on the top of my rack, so it is well ventilated. But many users allow only a few inches of ventilation - that can make the cover prone to warp as well as introduce other problems.

I also use an ARC Ref Phono 2SE as a phono preamp that is in a rack just below my turntable. It has only about 4 inches of ventilation above the unit, so I use an AC Infinity fan on it. That has prevented any damage to the plastic top and keeps the unit cool. The fan can't be heard from my listening position.

I had a Reference 6 twice. The first one (maybe an early model? it would have been made ~2016), came with a flimsy-ish acrylic cover that quickly deformed (sagged) from the 6550’s heat even freestanding in open air (nothing above nor around) in a cool room. That was a bad cover design. It looked awful, but did not cause any issues of concern other than that. ARC shipped a metal cover for free, but it was extremely thin gauge steel and rang like a bell. 

The 2nd Ref 6 had a much thicker cover with beveled edges that did not demonstrate this problem. 

Now this is off topic but I also wondered what their logic was for putting this same cover on the phono stage. Their Ref 3SE phono made demonic noises if you had a plasma TV on (even in a next room) - of note this only affected High Gain mode. Surely a thick metal cover (with damping applied because...tube phono stage) would have helped shield those JFETs? IIRC Fremer mentioned this in his review, so it wasn't just a one-off issue. 

OP,

Audio Research has experimented with the covers for their preamps for a long time. Originally they were metal... they found out that the plastic (acrylic?) sounded better... and that is why they made the change. They probably experimented with different thicknesses as well. This is common after a new model is nearly complet, or even... to experiment with the fit and form to optimize it. 

Sounds like your 6550s are redplating.  I had a the same issue. Replace ALL tubes. 

I have purchased tubes from Upscale Audio, and from my ARC dealer.  I can say that over years I have learned that the matched and tested and re-tested ARC tubes are worth the money. As one English car company used to say "The quality remains long after the price is forgotten"   

Do you know why the British don't make stereo preamps?  They can't get them to leak oil! 

@yesiam_a_pirate 

+1 Well put.