Black pollution

[boyofwax]

I just had an argument with a friend of mine who is also into tubes. Can someone explain where the black pollution on the inside of the glass tube comes from? Thanks in advance!

 

[TheGimp]

That depends on what you are referring to.

(1) Shiny black to silver film on the inside which is a barium compound deposited during the manufacture of the tube. This is a "Getter" which functions to help remove traces of gas left from the vacuum process.

(2) Smoky deposit at openings in the plate structure are ion bombardment of the glass during operation of the tube.

 

[boyofwax]

Thanks Gimp, i was wondering about the smoky deposit. I have some tubes which have gotten so black there is no way you can see through. Don't know if these still work OK. Does this pollution degrade the working of a tube? Maybe someone can tell what kind of chemistry is going on? I have always thought it was burnt off kathode/heater material and by that way some measure of the tube's life.

 

[SY]

Some tubes are made with a carbonization on the inside of the envelope. This can be useful in distortion reduction (cf. Deketh).

 

[M Gregg]

Just for interest.

https://www.tube-town.net/info/doc/dark-getter.html

The only other discoloration I know of is caused by the blue glow around an area of glass like this..

www.thetubestore.com - Blue Glow in Tubes, Tube Getters, and other Info

This can cause a dark burnt look to the glass and looks like staining..this is normally because a tube has been run overcurrent or high bias for some time.
mentioned in 2. post 2 (The Gimp) The blue glow doesn't normally create the staining..unless its run hot.

However you can also get what looks like an extra deposit of silver getter in a ring on some tubes that happens after a couple of months of normal running but the tube is perfectly OK. Its common between the top two mica's on these type tubes..

http://www.richtonemusic.co.uk/prod...amp_tube.asp?gclid=CP-g6YLG88QCFWvKtAodZEYA1A

Dark getter normally means a good tube.

Here is an example of a tube run high bias..

https://www.google.co.uk/search?q=old+el84+tubes&biw=1455&bih=665&source=lnms&tbm=isch&sa=X&ei=eN0rVfTSDIfB7AbBlYDICA&ved=0CAcQ_AUoAg#imgrc=1UHCE5MaBLeB_M%253A%3BJOmTAGuSs1FuKM%3Bhttp%253A%252F%252Fthumbs4.ebaystatic.com%252Fd%252Fl225%252Fm%252FmNvs4rqKMt_q-PWTj4rEF4A.jpg%3Bhttp%253A%252F%252Fwww.ebay.co.uk%252Fbhp%252Fmullard-el84-tubes%3B225%3B169

Regards
M. Gregg

 

[DF96]

Another source of dark colouration is electrolysis of the glass. Likely to be seen in valves which get very hot. In Europe this is often seen on well-used EL41 (radio audio output) and PL81 (TV line output i.e. 'sweep tube'), and some rectifiers.

I have always thought it was burnt off kathode/heater material and by that way some measure of the tube's life.

There is no measure of a tube's life i.e. how much longer it will last.

 

[M Gregg]

There is no measure of a tube's life i.e. how much longer it will last.

+1

That's the trouble with tubes on show, I have seen tubes in old Tv's that should have been in the scrap bin years ago still going strong, trouble is they look dire.😀

tilted to the side, black as bag, sooty, roasted, burnt Circuit board hanging on by melted solder and bits of bare wire...the list goes on..😀 but the tubes are still working with black tar melted and dripped over them and crusty anti corona spray peeling off the hot glass..

NB that was the good stuff...😀


Regards
M. Gregg

 

[JohnAtwood]

I think the original poster meant what some people call "smoked glass", i.e. a uniform grey or black coating inside the glass, usually seen on tubes from roughly the 1930s to the mid 1950s. I have not found a definitive reference on this, but here is what I think is going on:

When a tube is operating, excess electrons escape the plate structure and land on the glass, where they can build up a significant negative charge. This electrostatic field can then, in turn, affect the operation of the tube. The grey or black coating is made of graphite and equalizes the charge on the inside of the glass, reducing this effect.

I saw direct evidence of this in some Chinese 2A3 tubes that would generate odd squeaking and squealing sounds out of the speaker. When I turned out the lights, I noticed that near the top of the tube was a blue glow on the inside surface of the glass that was dancing around in exact synchronism to the squealing sound. Apparently the electron charge (which was causing the blue glow) was building up and discharging chaotically in a way that affected the plate current. When the amp was turned off, the spots on the glass where the blue glow appeared had a brown discoloration, another evidence of being bombarded by electrons.

My question is why tube manufacturers stopped coating their tubes in graphite around the mid 1950s? This was before gross cost-cutting came into play (the 1970s), so there must be another reason. I suspect that a different kind of glass was used that allowed the electrons to discharge. The reason my Chinese 2A3s made noise is that they just used conventional lime glass. Any ideas?

- John Atwood

 

[M Gregg]

I think the original poster meant what some people call "smoked glass", i.e. a uniform grey or black coating inside the glass, usually seen on tubes from roughly the 1930s to the mid 1950s.

I guess you mean like this..some 6L6 I have also on some 6V6..etc

Quick search gives this:
http://www.diyaudio.com/forums/tubes-valves/152600-black-6v6gt-originals.html

Regards
M. Gregg

 

[SY]

I think the original poster meant what some people call "smoked glass", i.e. a uniform grey or black coating inside the glass, usually seen on tubes from roughly the 1930s to the mid 1950s. I have not found a definitive reference on this, but here is what I think is going on:

Hi, John! See post 4- the reference that I didn't have to hand was Deketh's Fundamentals of Radio Valve Technique, page 439 onward.

 

[M Gregg]

SY,

Whats the answer?
I don't have access to the book..

My question is why tube manufacturers stopped coating their tubes in graphite around the mid 1950s? This was before gross cost-cutting came into play (the 1970s), so there must be another reason. I suspect that a different kind of glass was used that allowed the electrons to discharge. The reason my Chinese 2A3s made noise is that they just used conventional lime glass. Any ideas?

Regards
M. Gregg

 

[SY]

Reduced secondary emission from electrons striking the (nonconductive) glass.

 

[M Gregg]

Reduced secondary emission from electrons striking the (nonconductive) glass.

How did they get around it? to remove the coating..
I remember grey coatings as well..

Regards
M. Gregg

 

[M Gregg]

Off topic,

Remember the blue Groove tubes..😀
They did deep purple ones as well..

Regards
M. Gregg

 

[Merlinb]

My question is why tube manufacturers stopped coating their tubes in graphite around the mid 1950s?

I have often wondered that too. And why don't we see carbonizing mentioned and explained much in the literature? I don't think I've seen it discussed in any sort of detail anywhere but in Deketh, which is unusual compared to the endless articles about cathodes, glass, mica etc.

 

[Refugee1]

I have a very grotty EL41 that slowly went to air that is absolutely filthy inside the glass.
Also I have an example of a tube with the graphite coating on the inside also gone to air.
The third is an NOS example of the conductive band near the base that was added to later production to replace the graphite.

 

[FullRangeMan]

Some tubes with metal plate as GM70 copper plate develop a black cloud with use time due copper oxidation cause by eletricity.

This dont happen with 6C33 due this tube have lots of a black dust in the plate in the 60,70 and mid 80 years.

An externally hosted image should be here but it was not working when we last tested it.