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-   -   grey fogging inside 6sn7 (http://www.diyaudio.com/forums/tubes-valves/225154-grey-fogging-inside-6sn7.html)

p.dow 7th December 2012 12:12 AM

grey fogging inside 6sn7
 
i have a sylvania 6sn7 chrome top that has a grey like halo inside glass just at edge where chrome stops and clear glass begins.
i have seen this grey like appearence in tubes that crack or fail !
is this normal in a old failing tube ?
should i expect this tube to fail soon?
it is in a 300B integrated and so far it still works ok.
just wondering about this because i have been watching 6sn7's on auction site and saw a tube that had the same hazey ring near the top of the tube like mine !
thanks for response.

Einric 7th December 2012 12:32 AM

I had a 6CG7 that happened to, but I actually cracked the tube at the bottom.
The grey cloud appeared right where the crack was and migrated throughout the tube.

tomchr 7th December 2012 07:21 PM

Sounds like the getter has starting to oxidize as oxygen is being introduced into the tube.

The "chrome" is actually magnesium. It'll turn white as it oxidizes to magnesium oxide when oxygen (air) gets into the tube.

As long as there's getter material left, I suspect the tube will probably work OK, but it's likely reaching the end of its service life.

~Tom

p.dow 7th December 2012 11:39 PM

safe to assume....
 
Quote:

Originally Posted by tomchr (Post 3275015)
Sounds like the getter has starting to oxidize as oxygen is being introduced into the tube.

The "chrome" is actually magnesium. It'll turn white as it oxidizes to magnesium oxide when oxygen (air) gets into the tube.

As long as there's getter material left, I suspect the tube will probably work OK, but it's likely reaching the end of its service life.

~Tom

thanks for the explanation... it then seems safe to assume that any tube with that white -grey haze creep is then bad news-oxidation occuring in the tube vacuum ?
.... thanks
paul

tomchr 8th December 2012 01:39 AM

Quote:

Originally Posted by p.dow (Post 3275368)
it then seems safe to assume that any tube with that white -grey haze creep is then bad news-oxidation occuring in the tube vacuum ?

Obviously, if the the tube has a good vacuum, there wouldn't be any oxygen in the glass envelope. The fact that the getter has started to oxidize indicates that oxygen has gotten in, i.e. a seal is leaking. If this has happened suddenly, I'd consider the tube dead. If it took 50 years to get to a thin white line of oxidized getter, you can probably get a bit more life out of the tube. But unless we're talking some rare version of the 6SN7, you should be able to find a new replacement for not that much money. AES has plenty. Starting as low as $6. At those prices, I'd just replace it. The Sovtek tubes are pretty good IMO, though, I haven't tried their 6SN7.

~Tom

kevinkr 8th December 2012 01:53 AM

I have a number of very old Sylvania 6SN7 with that very slight whitish fringing on the edge of the getter - none have failed due to this issue in the 20 or so years I have been buying and using these tubes. (None tested gassy either) I've had one or two fail for other reasons - usually mechanical.

freax 8th December 2012 03:38 AM

if thats the case then it could be just a manufacturing issue and the OP's tube is fine.

Would be nice to see a photo.

tomchr 8th December 2012 08:43 AM

Quote:

Originally Posted by kevinkr (Post 3275495)
I have a number of very old Sylvania 6SN7 with that very slight whitish fringing on the edge of the getter

Back in the day, they probably didn't have the vacuum pumps we have today. Getting a good vacuum is surprisingly hard. The getter has reacted with what little oxygen was left after the bottle was sealed. Unless the white band of oxidized getter gets bigger, the tube is probably OK.

~Tom

Miles Prower 8th December 2012 08:21 PM

Quote:

Originally Posted by p.dow (Post 3273894)
i have a sylvania 6sn7 chrome top that has a grey like halo inside glass just at edge where chrome stops and clear glass begins.
i have seen this grey like appearence in tubes that crack or fail !
is this normal in a old failing tube ?
should i expect this tube to fail soon?
it is in a 300B integrated and so far it still works ok.
just wondering about this because i have been watching 6sn7's on auction site and saw a tube that had the same hazey ring near the top of the tube like mine !
thanks for response.

Maybe yes; maybe no. I've seen the same thing myself with a 5U4GB -- that hazy halo surrounding the getter. This could be due to a slow leak, or it's a sign that the getter was flashed too soon during pump-down. As for the 5U4GB, it still tests strong, and has not failed in the five years it's been in service.

I also had a defective 807 that lit up like a thyratron, and yet showed no haze around the getter. If the 6SN7 is still working, chances are that there's no problem here.

In this case, it's a "Realistic Lifetime" that Rat Shack used to sell. Realistic never made their own VTs. They were a rebrander that bought factory seconds that had cosmetic flaws, and attached bases, stamped their logo on them and sold 'em.

I also had a couple of 6BQ6GTBs that were of the same brand that looked weird due to overheating that caused the glass to soften and begin to implode. These proved to be slightly gassy, as the bias would not settle down. Though there was no visible glow, they did test out as being gassy. So they don't last a lifetime after all.

p.dow 9th December 2012 01:32 PM

Perfect Quad 1954 Sylvania Chrome Top 6SN7GTA Amp Tubes Black Triangular Plate | eBay

here is a link to pic of what i am seeing in my tubes.


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