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 Post subject: What is this white deposit inside a 12L6 tube?
PostPosted: May Sat 25, 2019 1:12 am 
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Usually when encountering a white deposit inside a vacuum tube, it is an oxidized getter flash indicating of loss of vacuum or "up to air" rendering the tube useless. Today I encountered a 12L6 tube with a white deposit inside the envelope but located away from the still robust getter flash. The tube tests strong and the white deposit is in line with a seam in the plate structure. A slightly smaller white deposit is present on the opposite side of the tube, also in line with a seam on the opposite side of the plate structure. Has anyone here observed a similar deposit? What is it?

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Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sat 25, 2019 7:02 pm 
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Electrons hitting the glass. An aged tube with slight air leak inside. That white color is okay for now. Suspect the tubes days are numbered.


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sat 25, 2019 7:41 pm 
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While I do believe the deposit is emission related because of the relationship between the deposit and the plate seams, I have never seen an emission stain that did not resemble a coffee stain. Furthermore, I do not believe an emission stain can oxidize or turn white upon exposure to oxygen. Being emission related, the deposit had to form during operation or possibly before when heating during the evacuation process. The tube has been sitting idle in a box for 30 or more years. If any oxygen were in the tube, enough would have entered to render the tube nonfunctional.

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sat 25, 2019 8:31 pm 
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Could it be that at some point the tube was subject to a very severe but short-lived overload (bright red or white hot plate)?

P.S. I recognize what the tube is lying on, lol.


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sat 25, 2019 9:48 pm 
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I took another, closer look at the tube today. First, it is a type 12V6 tube rather than a 12L6 tube. I was testing both types yesterday afternoon. Second, there is no seam in the plate structure behind the less prominent deposit. Also the alignment of the deposits varies slightly from the angle of the plate structure leading one to believe the deposit is not related to the internal structure of the tube. However, there is less deposit at the top mica with slightly more deposit above leading one to believe the deposit was formed after the internals were in the envelope. It appears to be paint but if so, for what purpose? Lime getter?

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Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 1:38 pm 
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It might be the cathode coating. This can happen when the tube is overloaded, especially when the cathode hasn’t sufficiently warmed up.

Dennis


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 2:02 pm 
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It is not redeposited cathode material because 1) there is no seam in the plate structure on the side with the less prominent deposit and 2) if that much cathode material were available to be redeposited, the tube would certainly test bad due to cathode material redeposited on the control grid.

Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 2:23 pm 
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Interesting. I've never seen anything like this. Have you tested the tube for
gas? In some cases a tube can have good emissions and still have some gas.
My guess would be there were some contaminants inside the tube from
the time it was made and electron bombardment from normal
use deposited them on the inside of the glass much in the way
getter flash is formed.

Can't think of any other way to explain this.
Steve

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 2:30 pm 
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There's only one answer.

Someone at the plant spilled their cocaine into one of the envelopes before it got sealed.


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 3:16 pm 
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PhilF wrote:
There's only one answer.

Someone at the plant spilled their cocaine into one of the envelopes before it got sealed.



PhilF has a pretty good point, i mean really just imagine the stress working a the tube plant, knowing that it's about to close soon, that would stress out the person and there's why the cocaine spillage

Cheers Don


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 4:29 pm 
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Cocaine in the tube? That is like the story someone posted on ARF the other day about the guy repairing the vintage music synth that had LSD inside it and he got "infected".

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 4:38 pm 
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I think it’s more likely that it’s an audiophile tube, and the white substance absorbs excess distortion.....

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 4:58 pm 
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Location: Monte Vista, CO. USA
Barry H Bennett wrote:
I think it’s more likely that it’s an audiophile tube, and the white substance absorbs excess distortion.....


That's not incompatible with the cocaine theory. I thought everyone knew cocaine absorbs distortion.

I'd put that tube on eBay if I were you...


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Sun 26, 2019 10:36 pm 
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The tube was made by GE in 1955. No threat of closure at that time. I'm going to stick with lime getter in a rather unique location. Some tubes from that era had lime getters in addition to flashed getters to reduce the manufacturing reject rate.

Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 1:36 am 
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I'm sticking by my temporary but severe overload hypothesis.

Looking at the pic where the white line is at the top, you can see the line corresponds with where the plate is closest to the glass. It is thickest in the middle, right where the plate under severe overload would be at its hottest. Notice how the line thins out as it reaches the cooler edges of the plate, and then thickens up again right where the gap is between the plate and the mica insulator. Looking closely at that part of the plate, I think I can detect a slight discoloration in the middle, although that could be just a reflection on the glass.

I think the plate got so hot so fast that the combination of out-gassing along with the vaporization of a small amount of plate material was more than the getter could handle, which resulted in the deposit on the glass, and that the overload condition was caught in time to prevent the tube from completely melting down.


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 1:54 am 
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But the deposit lines are not located where the plate is closest to the glass envelope. The plate is closest to the glass at the corners of the plate. The white deposits are located in the middle of the flat surfaces between the corners. I have not observed any typical evidence of overheating.

Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 2:01 am 
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Well, it can't be coincidence that the line is thickest in the middle, thins at the plate edges, and thickens again at both the upper and lower gap. Maybe it wasn't caused by heat, but it is certainly related to the plate structure.

EDIT: Now that I think about it, I believe that the middle of the surface between the corners would have been hotter than the corners themselves. So even though the corners might be slightly closer to the glass, the glass would have experienced maximum heat right where the line is.


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 2:23 am 
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Ok, why would the deposit line not be parallel to the plate? There is a small angle between the two.

Norman

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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 2:33 am 
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Hmmm. The thick white line in the pic where it is shown at the top does look parallel. Even if it isn't, its relationship to the plate structure can't be ignored. The line on the other side doesn't look parallel, but in the presence of that much heat (I am picturing a white-hot plate that is less than a minute from self-destruction) it's hard to say what focuses it.

In the picture I can see a very slight darkening of the plate, right in the middle under the thick part of the white line. It that really there, or is it just an artifact of the photo?


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 Post subject: Re: What is this white deposit inside a 12L6 tube?
PostPosted: May Mon 27, 2019 2:49 am 
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The dark patch on the plate is the shadow of the white deposit. The photo was taken in direct sunlight with the sun pretty much directly overhead with respect to the plane of the plate.

Also, the white specks on the envelope are inside and resemble overspray.

Norman

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