I ordered a pair of Mullard 12AT7s from Upscale Audio. One of them (left channel) would crackle intermittently and get worse after a few hours. Easily audible from the listening position, and over the music. I swapped the 2 and the problem followed the tube, so I sent it back. They said it tested fine, but sent me a new one anyway. The new one is doing exactly the same thing...
That really makes no logical sense, other than they sent me 2 tubes that test fine on their end, with the exact same problem. Any ideas?
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