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My ARC Ref 5SE has developed a low level hiss and crackle in the right channel. It is volume dependent, but cannot be perceived at the listening position at normal listening level, only with an ear close to the midrange driver. I have removed the tubes and cleaned the pins and sockets with no change in the noise. So, I have a couple of old 6H30 tubes on hand that I can swap in and out of the four sockets in the preamp to try to identify a possible noisy tube. The four 6H30s installed are labeled V1, V2, V3, and V4 on the main circuit board. Does anybody know which pairs of tubes serve the right and left channels? I can easily change one tube at a time, but knowing the general circuit design would help with interpreting the results. Thanks in advance.