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In Reply to: RE: RCA cleartop 6cg7 question posted by Trouser Trout on July 02, 2016 at 19:09:00
It's quite safe to assume RCA "knew" what the tube type was going to be prior to actual mfg. Notice in your example that the top flash is most dense immediately above the open tops of the cathode tubes. Sometimes the "flash" above the filament(s) was the result of an extended aging process at time of mfg. or selection for specialized apps. More often for consumer grade tubes it's nothing more than the result extended use.
Pic below is of a new testing but used 6SN7GTB from an organ that was "on" for an extended period.
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I have several RCA 6CG7's that look similar to Onemug's that are supposed to be clear tops. No getter ring up top, only on the side. Bought them quite a while ago as NOS from a reputable dealer, and they test as new on my Hickok. No signs of use or extended use (like brown discoloration on the glass or rainbowing/darkening of the getter flash on the side). No guarantee they *haven't* been used, but the dealer I bought them from is honest and quite knowledgeable.
Maybe these these had a little too much barium when they were flashed, and some of it migrated elsewhere in the glass? Not even sure if that's possible...
That's how I always understood it.
A redistribution of the getter by the heaters.
I see it in 6sn7/6sl7 all the time.
I remember it being said that it happens fast in RCA 6sn7's.
Tre'
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