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Is it normal for a tube to have brownish deposit rather than a silverish one? Anyone could share some technical knowledge and experience on this? Thx
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We often saw it in Sino 2A3s, after a few hundred hours. Used to joke it was second-hand smoke from the factory workers cigarettes. :^) Never seemed to have any sonic or lifetime effects. Now we're using the Sovteks so I don't know if it's still common.
you refer to I believe is the result of the manufacturer using sulfur based oil to lubricate the wiring when drawing them. I see them in Sino 807's, and many other tubes, and generally they are opposite the wire supports. They are bad, because in a good vacuum there should be no carbonizing evident. The deposits are an indication of faulty cleanliness in the manufacturing process.
Old RCA manuals are adamant about them causing contamination of the tube elements: cathodes in particular.On the other hand I always see brown deposits on Mullard manufactured EL84's. Even NOS tubes develop the brown spots opposite the plate inspection holes after a few minutes on a tube tester. I used to avoid them, but they seem to hold up quite well, even though the looks are appalling to me.
The Chinese tubes usually start to arc or act up after the brown spots start to appear, at least that has been my experience.
YMMV, of course.
If you're referring to the getter flashing inside the tubes, brown (often with a rainbow hue) is normal for some tubes, particularly older ones. Differences in getter deposits is a function of getter material, method of activation/vaporization, and ambient conditions (e.g. condition of the vacuum environment).
i had to
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