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PostPosted: 07 May 2014, 16:52 
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After much research I have yet to find the definitive answer so,,, Hello. I have one hybrid, one trans coupled (6dj8), and, one cap coupled HP-amp (6as7), the last two of which are from this form and both are excellent in performance and quality topology. I thought next i would look at some of the higher voltage ones with mercury/argon tubes and neon gasses.
That led me to wondering about the glass used in construction of the tube casing. So far it seems clear glass aka. sand glass, aka pure glass is the one due to quantity, price, and ease of use in the forming proses, also strength in end use,, is the glass of choice. I know from my experience with neon signs that leaded glass, which they use, filters out the deadly short wave UV. But, clear, pure, sand glass does not filter any waves. (hence germicidal lamps etc) Then i see pics of many units displaying and being stared at, due to the prominent, pretty blue glowing tubes.
Does anyone here know if these tubes are really encased in leaded glass or sand glass.
Jokes about getting a suntan while one enjoys music aside,--- an hour a day even, would have ones rods and cones looking like oven burnt cookies in no time. My common sense tells me leaded glass but, that is just my common sense and I can not speak for all.
Secondly, wouldn't the gas inherently make the amp noisier? Especially for HP use?


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PostPosted: 07 May 2014, 20:17 
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Hi, An interesting question that I can't answer directly. But my background in electronics (primarily the part about testing for health issues) indicates that there is no problem. It seems that harmful X Rays do not occur in equipment that has under about 11,000 volts potential. While this voltage (actually up to about 25KV) was found in earlier tube tvs it was always shielded. This included a leaded tv screen. I don't know of any audio gear that even comes close to the minimum for producing X Rays. Therefore since lead is a problem in itself I strongly suspect that it is not used in producing new tubes. This would not include some early specialized electron tubes that actually contained small amounts of actually radioactive material (like thorium), often as a portion of the cathode coating. Such tubes are very well marked and now rather scarce.

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PostPosted: 07 May 2014, 22:12 
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Yes i realize that it would not be producing X-rays. But the "argon and mercury" combination does produce mostly UV light same as in florescent lamps. Ultra-violet region has long and short waves in it. The long waves are not so bad for eyes and is used for such things as BLACK LIGHTS etc. The short wave radiation however is also present and is very harmful to the eyes.
In NEON signs the glass used is leaded glass, and it's the" lead" which filters the short waves out.
In florescent lights it is the phosphor coating on the tube that filters out the short waves.
In florescent lights that have no phosphor coating the short waves are aloud to get out and so these lamps are used as germicidal lamps for killing microbes and erasing EEPROMs etc.
Even the small MR16 lamps so popular, produce short wave UV and that is what fogs up the mirror reflector in time. I know when they first came into use most lamp fixtures had UV glass filters in front of the lamps. But, they kept fogging up so manufactures quit supplying them and just made it the user's prob.
Short wave UV is what store windows with coatings and screens are trying to stop.
Short wave UV is what kills plastic, fabric, discolors your car's paint job, etc etc.
Since these tubes have no phosphor coating, and, as i am reading, are mostly made of pure glass which allows all frequencies of light to pass threw makes these tubes very dangerous to the eyes. On the other hand if "leaded glass" is used then the lead content in the glass would filter the short waves out and no prob. (although with no germs, we are in one of the cleanest hobbies)
I find it interesting that all kinds of warnings are posted about UV in so many products (radio, papers, and TV have UV index readings now to tell us when it is safe outside and when to slather up with sunscreen to protect from short UV rays), and yet nothing is said about tubes. I can understand from days gone by views, but i would have thought it should be mentioned somewhere.


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PostPosted: 08 May 2014, 19:22 
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Just a point voltage over 5KV will produce X Rays. The threshold is not 11kv the old electro static TV sets from the 30's and 40's did produce X rays the main problem was the 7JP4 as this tube was hade from hard glass and color sets did produce X rays and had a shield. The color sets that had tubes had HV reg to keep the X rays to a minimum. Also the first sand sets had problems with run away Hv they had a lot of safety cap problems and all the sets had metal around the crt's to stop the X rays. it was not to the later 80's that the CRT's used leaded glass


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PostPosted: 08 May 2014, 23:03 
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laurie54 wrote:
Does anyone here know if these tubes are really encased in leaded glass or sand glass.
...
Secondly, wouldn't the gas inherently make the amp noisier? Especially for HP use?

The second question is an easy one. Not to a level that you would notice. The secondary emission current in the tube is very uniform for equipotential points in the glow field. As such, the noise is purely white and relatively low. Only at very low bias currents (~ tens of µA) would the noise level rise appreciably.

Your first question is much more interesting however. So, I have reviewed both the 1940 and 1962 versions of the RCA Electron Tube Design manuals, as well as the seminal 1948 text by Karl Spangenberg "Vacuum Tubes" looking for answers. It is important to note that with the exception of some very special purpose tubes, most tube envelopes are either "lime bulb" glass or "lead bulb" glass. The short answer to your question is that it doesn't appear that the transmissibility of the glass was an issue of concern in glow tube design. J Gallup of RCA in the 1962 paper says the following in the section on Light Transmission, Luminescence, and Index of Refraction.
Quote:
The light transmission of receiving-tube envelopes and stems is of value in that it allows nondestructive visual inspection of completed tubes. The transmission of clear glass is approximately 91 percent in the visible spectrum (wavelengths between 4000 and 7000 angstroms). The major portion of the loss in the visible spectrum results from a loss of about four per cent by reflection at each surface. However, the so-called clear glasses start to absorb energy in noticeable amounts in the ultraviolet region below 4000 angstroms. Light transmission through ordinary lead and lime glasses is down to nearly zero at 3000 angstroms. A similar absorption occurs in the far infrared region, and complete cutoff occurs between 45,000 and 50,000 angstroms for all normal glasses. As a consequence of these effects, special ultraviolet transmitting glasses must be used for ultraviolet lamps, and electron tubes and electric lamps run hot because of the absorption of infrared, or heat, rays in their envelopes.

The luminescence of commercial glasses is also of little importance with respect to electron tubes, save as a method of identification. Under short-wave ultraviolet illumination (2537 angstroms), all high lead glasses glow a bright blue. This effect permits ready discrimination between lead and lime bulbs because 0080 lime glass fluoresces very weakly in comparison with lead glass.

... [Emphasis added.]

So given this information, I would suggest that, whereas the general properties of lead glass did lead to it's use in some tubes, the limitation of ultraviolet radiation was not among those properties. However, when the leaded glass was used in gas tubes where short-wave ultraviolet was produced (i.e. glow discharge tubes) the bright blue color observed is not due to the ionized glow region within the tube but rather the luminescence of the lead glass under bombardment of the ultraviolet radiation from within the tube.

For these reasons I strongly suspect that the emission of such tubes does not present a significant risk to vision or retinal health.

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PostPosted: 08 May 2014, 23:37 
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gofar99 wrote:
This would not include some early specialized electron tubes that actually contained small amounts of actually radioactive material (like thorium), often as a portion of the cathode coating. Such tubes are very well marked and now rather scarce.
Actually not so well marked or rare as one could assume.

I recently picked up a Westinghouse KX-642 from someone who didn't know any better. The gas in the tube is doped to control the breakdown voltage. There is no labeling and no reference to radiation anywhere on the unit. When I had the tube tested, the radiation level was very low so as to not be a health concern (no I won't give the exact number, however I will say that it is on the same order of magnitude as the background radiation in Denver Colorado).

So, radioactive? Yes, technically. Dangerous? Certainly not. As for thorium cathodes, I wouldn't worry unless you plan on eating the silly thing. ;)

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PostPosted: 09 May 2014, 01:03 
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Suncalc thank you. I have my answer. Not that it would have stopped me from building anyway i must confess. One thing i did not know and found very interesting is the fact that the perceived blue glow is not from the gas itself but from the inner wall of the tube. I would think the perceived glow would be from the gas and it's path be from the tubes inside wall. As is the case of a plasma ball which also have argon mixed with neon and others to produce diff colors.
But made from leaded glass is the answer i was looking for as i know the lead would filter out (or more properly put, NOT LET THE SHORT RAYS OUT) as is the case in using argon and mercury for signs and decorative lighting.
Scary that the googled sights i picked suggested Pyrex is most often used in making vacuum tubes. That is what started me going in the first place. Pyrex is pure glass which defiantly pass all harmful waves that are produced in the tube. The visible blue glow is of no worries at all. It's the high freq. waves that can not be seen that are the problem.

My second question comes from knowing that more than once a neon lamp used as a power on indicator especially in "pwr bars" is a great source of audio noise. But that would be more a from RF transmission caused when the lamp is reaching end of life and flickering on and off causing spikes. Conduction in panel lamps usually around 90v which also makes them good for triggering triacs etc. before diacs. Too often the series R's are too high value and lamps are guarantee long life but easily/often fail to conduct properly.

Thank you all for the info. Now i can dream about little blue tubes.


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PostPosted: 25 Feb 2018, 22:50 
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Well, it finally happened. I have heard about this phenomenon but had never witnessed it personally. Evidently a couple of the NOS GE tubes in my stash have high lead concentrations in the glass. As such I was rewarded by this view as I looked at the amp in very low light...
Attachment:
6V6 Tube Glow.jpg
Have to admit, it gave me quite a start when I first saw it. :eek:


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PostPosted: 26 Feb 2018, 14:56 
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I've only seen green when making X-rays with a 1B3 :idea:

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PostPosted: 26 Feb 2018, 15:35 
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I've seen that same blue glow (shown in Suncalc's picture) in NOS 12V6's that I have in an amp. At least now I know the cause.
It goes well with the blue Edcor transformers :)


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