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Have a tube that gets really bright when turned on and then dims quickly

I have two 6922 tubes on the top of my preamp, and one of the tubes gets very bright at the base of the tube when I turn on the preamp. It doesn't light up the whole tube just the lower part of it.  It dims fairly quickly and then it looks just like the other tube.  I switch the position and that sudden brightness follows the tube.

The tube sounds OK and it's fairly new. 

It just seems kind of weird, wonder why this is happening. Anyone have any ideas?

jumia
bigshutterbug
378 posts
 

It is known as the "Mullard flash", and does not mean anything is wrong with the tube. Is it a Mullard 6922?

newbee
4,751 posts
 

Quite common for small tubes made in Eastern Europe, not just Mullards. Not to worry.:-)

sfar's avatar
sfar
1,756 posts
 

I have a pair of Telefunken ECC81 tubes that test the same and have the same construction. One flashes brightly on startup and the other not at all. 

invalid
1,073 posts
 

 

mulveling's avatar
mulveling
1,774 posts
 

Very common for vintage Euro and British small tubes. Doesn’t indicate a problem or wear. If it’s a Russian, Chinese, or power tube, then you have cause for concern. I have a particularly wonderful pair of British long plate square getter 12AX7, where one does it and the other doesn't, lol. Also have some Euro 6922 / 7308 that do it. 

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