On May 27, 6:22 am, Marcin <marcin.r.adam...@gmail.com> wrote: > Could somebody explain me how the discoloration around cathodes is > supposed to appear during the tube operation? I understand that the > common knowledge is that only used B7971 have it. The new ones should > have a uniform back-plate. > Marcin One the B7971s that I have broken, the back plates were white ceramic with the black coating. I always assumed that the lightening was caused by sputtering: some of the positive ions miss the cathode and hit the back plates, eroding the black material. If the discoloration came from the manufacturing process, like shadowing by the cathodes during deposition, I would expect it to be more loaclied under the wires.
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