I witnessed the recarb process on a big tube recently. It was essentially
as AG6K described it. The gas used and the exact technique are sort of
proprietary for each company but it was certainly an artform, watching the
controlled amount of gas while monitoring emission and stopping at the
correct moment for a particular tube.
By the way, I have been taught that the additional thoria makes the
filament MORE brittle. Pure tungsten is ductile, and as it is thoriated, it
gets brittle, especially towards the end of emission life. One company was
juggling the levels due to their source of Thoriated tungsten alloy, and
compared performance with 1, 1.5, and 2 % TT levels. The latter one was a
great emitter, but more fragile.
John
K5PRO
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