This is exactly what concerns me although I thought that the alloys used in the pins were chosen to have a similar coefficient of thermal expansion as the glass. I do find it surprising that moisture can find its way past the glass-metal seal at all since they are perhaps 4mm deep. Is a copper brazing process used to join the internal part of the pin to the external part?
Only a guess, but I suspect that there was some contamination or corrosion on the leads before they were sealed on the 6V6GT's that have died. They were kept in a controlled environment and never used.
The 6AQ5's that I cleaned up about 2 years ago have been fine, right up to the melting point.
The 6AQ5's that I cleaned up about 2 years ago have been fine, right up to the melting point.
I am pleased to say that the seller has honourably offered full refund for destroyed tubes and 50% for the others. Under magnification the structure of the corrosion looks interesting. I am going to try to get a colleague to do an analysis of the material. I wouldn't be surprised if some acidic component of the cardboard was involved or that since the storage area was likely highly industrial perhaps oxides of sulphur or nitrogen.
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