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After the first gentle 20 min using, the 6N5PJ tube turns black... What is going on?
It's normal and was most likely there before you even turned it on, you might not've noticed. No rudeness implied in any way, tube amps are complicated beasts. Here's an example of another unused 6N5PJ from Ebay showing the same colouring:
My stock 6N5PJ also shows the same black coloration on the bottom. It doesn't effect its use. From my cursory understanding of tube audio it's likely the Getter for this particular tube:
" Flashed getter – The getter material is held inactive in a reservoir during assembly, then heated and evaporated after initial evacuation, usually by induction heating. The vaporized getter, usually a volatile metal, instantly reacts with any residual gas, then condenses on the cool walls of the tube in a thin coating, the getter spot or getter mirror, which continues to absorb gas. This is the most common type, used in low power vacuum tubes." (Wikipedia)
it turned dark to reflect its name
I noticed the same thing and a burning smell. Is the burning smell also normal for a brand new amp? I have other amps that don't smell or discolor tubes.
No smoke as I've seen on online videos. Also found a burn in procedure http://blog.thetubestore.com/burning-in-6sn7-tubes-for-the-dark-voice-336se-headphone-amp/#comments. I don't think this will affect the power tube though. Mine sounds great. I contacted MD anyways, and they said that they will take it back. If there is no issue, I'd rather not return it as it sounds great.