What does burned glass mean

[vanishingpoint]

Just saw some 6L6GC Sylvanias which have huge black blots on glass. What does this mean? THX

 

[mikey383]

I think what you are referring to is the coating on the inside of the tube. It's an applied substance (barium oxide...I think) that turns white when exposed to oxygen....that way you can tell if the tube has lost it's vacuum and air has been introduced into the tube.

Usually it's a silver color, but I've seen it look black on some tubes.

 

[Timbre Wolf]

I think what you are referring to is the coating on the inside of the tube. It's an applied substance (barium oxide...I think) that turns white when exposed to oxygen....that way you can tell if the tube has lost it's vacuum and air has been introduced into the tube.

Just adding a fine point: the coating inside is usually silvery barium metal, which forms white barium oxide upon exposure to oxygen. This "getter" material is used to bind free oxygen, so it does not degrade the energized metals inside the tube.

- Thom

 

[vanishingpoint]

I was told that the 'black' appearance may be a photographic thing-caused by camera flash. But if it IS in fact black, the tube's shot. Thanks

It's hard to figure tubes...I see them go dirt cheap or just sitting there on feebay. Others go for 100/tube and seem the same as those that don't seem to move/attract interest. I'm talking sylvies here.

 

[Timbre Wolf]

I was told that the 'black' appearance may be a photographic thing-caused by camera flash. But if it IS in fact black, the tube's shot. Thanks

Don't give up on a tube due to black spots. Do you have the photo available?

What appears black to you in a photo, may turn out to be a darker silver color upon in-person inspection. Black does not usually indicate a dead tube. White, clear, or light pink/reddish do often indicate a tube whose seal has been broken.

Wait until you've got the tube before you toss it in the trash!

As to why some tubes go for small $, and similar models go for huge $$$ - there are a lot of subtleties that, to an untrained eye, may seem insignificant. Two tubes may have the same labeling, but have different insides, and we all know - it is what is inside that counts :wink:

- Thom

 

[stokes]

Just to add,the getter is caused by a charge set off at the last stage of manufacture to complete the vacuum process,it burns up any oxygen that may be left.As well as indicating a "leak" as Wolf pointed out,when the tube is at the end of its life the silver flashing will turn brown,which can also be an indication a tube was overheated.I once put a pair of Tung-sol 5881's in a Boogie amp to see how long they could take the over voltage,in a matter of hours the getter turned brown and very soon after,the tubes fried.I know,a waste of a good pair of tubes,but I had 4 matched pairs that I got dirt cheap,just something to do when I was bored.

 

[Timbre Wolf]

I once put a pair of Tung-sol 5881's in a Boogie amp to see how long they could take the over voltage,in a matter of hours the getter turned brown and very soon after,the tubes fried.I know,a waste of a good pair of tubes,but I had 4 matched pairs that I got dirt cheap,just something to do when I was bored.

:shock: Oh, no you dinnit!

Only solid-state amps in heaven for you, when you die.

- t

 

[stokes]

Had a feeling that would get your attention,Wolf.Yeah,I dunnit.Would like to say I was young and foolish,but I cant hardly remember young and foolish anymore.Heaven?Who wants to go there,I wont know anybody,and all my friends will be in hell rockin' with the tube amps (tubes are too hot for heaven!).So if you ever happen to be wondering how much a Tung-sol can take,it aint much.About 440volts,idling at only 12watts,I'd say they got a good 3-4 hours sounding hellish,maybe another 8 hours till death.The other 3 pairs are still living the good life in some much better environs,they just dont know how close they came!

 

[Timbre Wolf]

Had a feeling that would get your attention,Wolf.Yeah,I dunnit.Would like to say I was young and foolish,but I cant hardly remember young and foolish anymore.Heaven?Who wants to go there,I wont know anybody,and all my friends will be in hell rockin' with the tube amps (tubes are too hot for heaven!).So if you ever happen to be wondering how much a Tung-sol can take,it aint much.About 440volts,idling at only 12watts,I'd say they got a good 3-4 hours sounding hellish,maybe another 8 hours till death.The other 3 pairs are still living the good life in some much better environs,they just dont know how close they came!

:lol:

Yer one of a kind!

I guess someone has to do these things, so the rest of us can learn. Reminds me of a friend of mine who, on a whim, ate a small red spider while we were camping together. The venom sack ruptured in his throat, causing his airways to swell till he nearly choked. He recovered, and passed on the important bit of wisdom as (one of) his contribution(s) to humanity: don't eat the red ones!

- T