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It is widely known that intensity double peaking indicates a CRT is near the end of its life. Gas and low emission seem to be common factors.
However, I have been unable to find the actual mechanism in any references. Does anyone know how gas and low emission specifically cause double peaking?
Perhaps there is a Tek CRT expert in the group.
Curly
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At 12:20 PM 2/16/2019, Curlytronics wrote: It is widely known that intensity double peaking indicates a CRT is near the end of its life. Gas and low emission seem to be common factors.
However, I have been unable to find the actual mechanism in any references. Does anyone know how gas and low emission specifically cause double peaking? In my experience it is not caused by the condition of the CRT, but by the user turning up the brightness to see a weak trace and incidentally brightening the retrace to the point where it can be seen. Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
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Sometimes, a failure in the Z-axis circuitry will be masked
by the grid bias pot being set at a sweet spot... I think it
is due to non-linearity in the phosphor... but if you turn
the intensity up, the ordinarily blanked retrace beam will
be revealed.
But that is not double peaking.
I believe double peaking is caused by a gassy CRT, and is
due to the gas causing grid current in the intensity grid
and the focus grid. The grid circuits are very high impedance,
and any stray currents, going into or coming out of the
intensity grid, are going to bias the electrodes to the wrong
voltages for proper brightness and focus.
A low emission CRT will just be dim.
-Chuck Harris
Dale H. Cook wrote:
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At 12:20 PM 2/16/2019, Curlytronics wrote:
It is widely known that intensity double peaking indicates a CRT is near the end of its life. Gas and low emission seem to be common factors.
However, I have been unable to find the actual mechanism in any references. Does anyone know how gas and low emission specifically cause double peaking? In my experience it is not caused by the condition of the CRT, but by the user turning up the brightness to see a weak trace and incidentally brightening the retrace to the point where it can be seen.
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
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Hello,
My first Tek, I used to own until a few months ago, had a weak CRT since day one (when I got it from a local, Brazilian auction site).
One of the first thing that I noticed, besides the low intensity, was that it had this double-peaking thing (which I was unfamiliar with).
It was when I joined this group and the messages should still be there.
Back then David Hess (where's him? Haven't seen anything from him for a while) pointed me to Ed Breya (both are renowned members), as Ed had some experience in rejuvenating CRTs.
At the same time, I learnt about the extensive information available on the tekwiki site (google for TekWiki... you'll find it).
At TekWiki, there in the "Manuals, Catalogs and Other Publications", there's a section called "Concepts Series".
Between the "Concepts Series" publications, the very first one is the "Oscilloscope Cathode-Ray Tubes, 2nd ed.
LInk here for your convenience:
Between pages 10 and 14 there's a detailed explanation of how the electric field lines from the grid interact with that of the cathode, to create an electrical equivalent to a photo camera's "aperture" ring... which is actually what broadens or pinches how much of the Cathode surface is "opened" for emissions.
So,, follows what I conclude from the understanding:
When set to a lower intensity, the cathode effectively emits only from the center, and as we crank up the intensity, it opens up more and more area of the cathode to emit the beam. Therefore, the center of the cathode is the most used portion and is the one that wears out first.
As the CRT ages, the central spot of the cathode wears out and you can only obtain some intensity as you open the "aperture"more and more, exposing the next section (an emitting ring now, not anymore a circle).
That concentric wearing "pattern" ends up imprinted on the cathode and, in my opinion, is what causes the double peaking...
On a worn out cathode, when you start cranking up the brightness, initially the aperture is drawing electrons from the "worn out" area which, despite it's worn out, it responds to the intensity control...(more C.W. more brightness, only that at a lesser degree"...
As you keep opening up the "aperture", you expose more and more of the outer rings (which are also more worn out than the center, so the intensity decreases)...
Up until a point that you open the aperture so much that you expose the outermost ring of the cathode which is still not worn out... (thus, enters the second "peak").
At this point, although you managed to get more electrons and more beam intensity, the size of the spot is already too big and the Focusing anode can no longer focus the beam correctly... (or the Focus Tracking potentiometer can no longer track correctly the increase of the intensity control) and ultimately, the Focus and Astigmatism is ruined.
Part of my conclusion can be flawed, but overall, I think this is the mechanism.
Rgrds,
Fabio
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Thanks Fabio. That is the best explanation of the problem that I have seen. In addition, there is a case I would call single peaking where the intensity increases to a certain point and then dims as it is turned further. I assume from your explanation that this indicates that the outermost ring is also finally had the biscuit. I once thought that the cathode may be able to be rejuvenated as we did to old TV set crts back in the day. I don’t hold out much hope for that approach however.
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Dave Hess has been more active on eevblog.com lately, on the RF/microwave/ham radio forum.Come to think of it, we haven't heard from Ed Breya lately, either.Jim Ford?Sent from my Verizon, Samsung Galaxy smartphone
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-------- Original message --------From: Fabio Trevisan <fabio.tr3visan@...> Date: 2/19/19 1:11 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] Mechanism of CRT Double Peaking Hello,My first Tek, I used to own until a few months ago, had a weak CRT since day one (when I got it from a local, Brazilian auction site).One of the first thing that I noticed, besides the low intensity, was that it had this double-peaking thing (which I was unfamiliar with).It was when I joined this group and the messages should still be there.Back then David Hess (where's him? Haven't seen anything from him for a while) pointed me to Ed Breya (both are renowned members), as Ed had some experience in rejuvenating CRTs.At the same time, I learnt about the extensive information available on the tekwiki site (google for TekWiki... you'll find it).At TekWiki, there in the "Manuals, Catalogs and Other Publications", there's a section called "Concepts Series".Between the "Concepts Series" publications, the very first one is the "Oscilloscope Cathode-Ray Tubes, 2nd ed.LInk here for your convenience: pages 10 and 14 there's a detailed explanation of how the electric field lines from the grid interact with that of the cathode, to create an electrical equivalent to a photo camera's "aperture" ring... which is actually what broadens or pinches how much of the Cathode surface is "opened" for emissions.So,, follows what I conclude from the understanding:When set to a lower intensity, the cathode effectively emits only from the center, and as we crank up the intensity, it opens up more and more area of the cathode to emit the beam. Therefore, the center of the cathode is the most used portion and is the one that wears out first.As the CRT ages, the central spot of the cathode wears out and you can only obtain some intensity as you open the "aperture"more and more, exposing the next section (an emitting ring now, not anymore a circle).That concentric wearing "pattern" ends up imprinted on the cathode and, in my opinion, is what causes the double peaking...On a worn out cathode, when you start cranking up the brightness, initially the aperture is drawing electrons from the "worn out" area which, despite it's worn out, it responds to the intensity control...(more C.W. more brightness, only that at a lesser degree"...As you keep opening up the "aperture", you expose more and more of the outer rings (which are also more worn out than the center, so the intensity decreases)...Up until a point that you open the aperture so much that you expose the outermost ring of the cathode which is still not worn out... (thus, enters the second "peak").At this point, although you managed to get more electrons and more beam intensity, the size of the spot is already too big and the Focusing anode can no longer focus the beam correctly... (or the Focus Tracking potentiometer can no longer track correctly the increase of the intensity control) and ultimately, the Focus and Astigmatism is ruined.Part of my conclusion can be flawed, but overall, I think this is the mechanism.Rgrds,Fabio
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Dave Hess has been more active on eevblog.com lately, on the RF/microwave/ham radio forum.Come to think of it, we haven't heard from Ed Breya lately, either.Jim Ford?Sent from my Verizon, Samsung Galaxy smartphone
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-------- Original message --------From: Fabio Trevisan <fabio.tr3visan@...> Date: 2/19/19 1:11 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] Mechanism of CRT Double Peaking Hello,My first Tek, I used to own until a few months ago, had a weak CRT since day one (when I got it from a local, Brazilian auction site).One of the first thing that I noticed, besides the low intensity, was that it had this double-peaking thing (which I was unfamiliar with).It was when I joined this group and the messages should still be there.Back then David Hess (where's him? Haven't seen anything from him for a while) pointed me to Ed Breya (both are renowned members), as Ed had some experience in rejuvenating CRTs.At the same time, I learnt about the extensive information available on the tekwiki site (google for TekWiki... you'll find it).At TekWiki, there in the "Manuals, Catalogs and Other Publications", there's a section called "Concepts Series".Between the "Concepts Series" publications, the very first one is the "Oscilloscope Cathode-Ray Tubes, 2nd ed.LInk here for your convenience: pages 10 and 14 there's a detailed explanation of how the electric field lines from the grid interact with that of the cathode, to create an electrical equivalent to a photo camera's "aperture" ring... which is actually what broadens or pinches how much of the Cathode surface is "opened" for emissions.So,, follows what I conclude from the understanding:When set to a lower intensity, the cathode effectively emits only from the center, and as we crank up the intensity, it opens up more and more area of the cathode to emit the beam. Therefore, the center of the cathode is the most used portion and is the one that wears out first.As the CRT ages, the central spot of the cathode wears out and you can only obtain some intensity as you open the "aperture"more and more, exposing the next section (an emitting ring now, not anymore a circle).That concentric wearing "pattern" ends up imprinted on the cathode and, in my opinion, is what causes the double peaking...On a worn out cathode, when you start cranking up the brightness, initially the aperture is drawing electrons from the "worn out" area which, despite it's worn out, it responds to the intensity control...(more C.W. more brightness, only that at a lesser degree"...As you keep opening up the "aperture", you expose more and more of the outer rings (which are also more worn out than the center, so the intensity decreases)...Up until a point that you open the aperture so much that you expose the outermost ring of the cathode which is still not worn out... (thus, enters the second "peak").At this point, although you managed to get more electrons and more beam intensity, the size of the spot is already too big and the Focusing anode can no longer focus the beam correctly... (or the Focus Tracking potentiometer can no longer track correctly the increase of the intensity control) and ultimately, the Focus and Astigmatism is ruined.Part of my conclusion can be flawed, but overall, I think this is the mechanism.Rgrds,Fabio
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Hi Fabio,
"As you keep opening up the "aperture", you expose more and more of the outer rings (which are also more worn out than the center, so the intensity decreases)..."
I don't see the rationale behind this. With rising grid voltage larger rings on the cathode disk take part in the cathode current. Moreover, the increasing electric field strength in space just outside the already exposed smaller rings makes it easier for electrons there to escape from the cathode. So I would still expect a monotonically increasing intensity (or better, cathode current). BTW the potentials discussion in the CRT Circuit Concepts book seems to ignore space charge, while space charge is nearly always discussed in the context of ordinary tubes. I have no idea whether or not space charge plays a role here.
The Tekscopes issue , page 14, mentions this: Gassy CRT tubes may be identified by their "double peaking" characteristic. No mention of worn cathode material.
Albert
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Hello Albert, I don't see the rationale behind this. With rising grid voltage larger rings
on the cathode disk take part in the cathode current. Moreover, the increasing
electric field strength in space just outside the already exposed smaller
rings makes it easier for electrons there to escape from the cathode. So I
would still expect a monotonically increasing intensity (or better, cathode
current). Yep, as I mentioned... my rational may be (and probably is) flawed. Back when I was still struggling with that 464 and romancing the chance there could be some level of recovery to that CRT, I read a lot of things in a lot of places... In many of them I encountered widespread use of the term "Double Peaking" but in none of them I found explanation about the mechanism that originates the behavior. I agree that by my own rational, the intensity should always increase (maybe not at an even, or linear fashion, but always increase). BTW the potentials discussion in the CRT Circuit Concepts book seems
to ignore space charge, while space charge is nearly always discussed in the
context of ordinary tubes. I have no idea whether or not space charge plays a
role here. You may be right... I`m not familiar with the term "space charge". The Tekscopes issue
, page
14, mentions this: Gassy CRT tubes may be identified by their "double peaking"
characteristic. No mention of worn cathode material. Gassy tubes or worn cathode... Well... In my way of seeing it, apart from tubes (or CRTs) that have a manufacturing defect or a crack, I tend to generalize that gases develop on a tube as they wear out... so I think it's kind of difficult to tell apart which of the abnormal behaviors are due to the presence of gas, or from the cathode itself being worn out so, I plead guilty for using "Cathode Wear" for everything. A little side comment... with so many people in this group with access to things as exquisite as Electron Microscopes, high vacuum pumps and etc... I wonder how come that nobody has come with means to open the worn out tubes, replace or recoat the cathodes, and seal them back again. I see there are still some plants that do this kind of thing for specialized tubes... Rgrds, Fabio
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On Thu, Feb 21, 2019 at 07:45 PM, Fabio Trevisan wrote:
Hello Albert,
I don't see the rationale behind this. With rising grid voltage larger rings
on the cathode disk take part in the cathode current. Moreover, the increasing
electric field strength in space just outside the already exposed smaller
rings makes it easier for electrons there to escape from the cathode. So I
would still expect a monotonically increasing intensity (or better, cathode
current). Yep, as I mentioned... my rational may be (and probably is) flawed.
Back when I was still struggling with that 464 and romancing the chance there
could be some level of recovery to that CRT, I read a lot of things in a lot
of places... In many of them I encountered widespread use of the term "Double
Peaking" but in none of them I found explanation about the mechanism that
originates the behavior.
I agree that by my own rational, the intensity should always increase (maybe
not at an even, or linear fashion, but always increase).
Hi Fabio, I searched around here and via Google and must say that nearly always cathode degradation is mentioned as the cause of double peaking. I hink now the explanation could be as follows. The phenomena you describe lead to a quick increase of aperture when intensity is increased. Then the beam gets de-focused and that might produce a decreasing brightness for the observer, despite a monotonically increasing beam current. As soon as the aperture is at its maximum, the brightness might start to increase again. Albert
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I recall something Peter Keller said to me when I asked him about this phenomenon. He mentioned that he didn’t have an explanation for it but he felt it could be a manifestation of cathode interface similar to what happens in the distributed amplifier tubes.
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I once had an RM35 with double-peaking. The beam stayed focused across the initial peak and dip, and only broadened when I cranked Intensity to max. YMMV, Dave Wise ________________________________________ From: [email protected] < [email protected]> on behalf of Albert Otten <aodiversen@...> Sent: Thursday, February 21, 2019 1:48 PM To: [email protected]Subject: Re: [TekScopes] Mechanism of CRT Double Peaking On Thu, Feb 21, 2019 at 07:45 PM, Fabio Trevisan wrote:
Hello Albert,
I don't see the rationale behind this. With rising grid voltage larger rings
on the cathode disk take part in the cathode current. Moreover, the increasing
electric field strength in space just outside the already exposed smaller
rings makes it easier for electrons there to escape from the cathode. So I
would still expect a monotonically increasing intensity (or better, cathode
current). Yep, as I mentioned... my rational may be (and probably is) flawed.
Back when I was still struggling with that 464 and romancing the chance there
could be some level of recovery to that CRT, I read a lot of things in a lot
of places... In many of them I encountered widespread use of the term "Double
Peaking" but in none of them I found explanation about the mechanism that
originates the behavior.
I agree that by my own rational, the intensity should always increase (maybe
not at an even, or linear fashion, but always increase).
Hi Fabio, I searched around here and via Google and must say that nearly always cathode degradation is mentioned as the cause of double peaking. I hink now the explanation could be as follows. The phenomena you describe lead to a quick increase of aperture when intensity is increased. Then the beam gets de-focused and that might produce a decreasing brightness for the observer, despite a monotonically increasing beam current. As soon as the aperture is at its maximum, the brightness might start to increase again. Albert
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On Thu, Feb 21, 2019 at 10:45 AM, Fabio Trevisan wrote:
space charge
During thermionic emission... where electrons are "boiled off" the filament, or the heated cathode.. they form a cloud in the space around those elements. Literally, it is a cloud of charged particles (electrons), suspended in space, around the cathode, or filament. Hence, the term "space charge."
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On Thu, Feb 21, 2019 at 10:45 AM, Fabio Trevisan wrote:
open the worn out tubes
Hi Fabio: There is a guy, of maybe more than one, that fixes old irreplaceable television CRTs... but, I've never heard of anyone doing CRTs for scopes. Maybe part of it is because the phosphor, and the phosphor suspension, and the process for processing that in the tube face is not sufficiently known. Another thing is though people might have high vacuum technology, the sufficient skills for working with glass, and the equipment needed, seem to be almost non-existent. Recovering of CRTs...It's doable, I think; but, its not trivial in terms of skill, or investment in the equipment needed. Regards Roy
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Hi Fabio,
Your explanation is plausible except for one more bit of vacuum tube physics that has to be taken into account: Under ordinary circumstances a cloud of electrons boil away from the surface of the cathode as a result of the heater heating it to incandescence in a vacuum. This is sometimes called the Edison Effect because he noticed it first in light bulb filaments. It is more correctly called Thermionic Emission. This cloud forms around the cathode and is known as the space charge region.
The cathode is intentionally coated with a material that lowers the work function. The work function determines how much energy will be required for an electron to leave the surface of the cathode and join the cloud of electrons surrounding the cathode. A low work function means more emission.
In ordinary circumstances the space charge region is the source of the electron beam, not the cathode. The cathode is "filling" the space charge region with electrons that will be attracted to a positively charged surface and ultimately focused along the way into a tight beam.
As the cathode ages the chemical coating used to lower the work function wears out. The space charge cloud can no longer provide enough electrons to form a bright beam. The user turns the brightness up to compensate. This increases the positive voltage attracting the electrons. Eventually the user increases this positive voltage to the point where the space charge region is completely depleted and electrons are being stripped directly off the cathode increasing the brightness temporarily. This is not good.
There is a way to extend the life of the cathode in a CRT as most people who worked in a TV repair store are familiar with it. I recall it as "rejuvenation". It is a simple process. If you increase the voltage going to the heater this will heat the cathode to a very high temperature which will burn off the weakly emitting surface of the cathode exposing fresh chemical coating from underneath. There is the risk of burning out the filament that must be considered when you do this. There is another variation of this which boosted the filament voltage on a permanent basis. This allowed customers to temporarily get more life out of a weak filament. Since filament life decreases rapidly with increasing voltage this won't postpone the need for a new CRT for very long. As you mentioned Ed Breya has some experience with this process so maybe he can explain it in more detail.
For more on the subject of Space Charge you can go to:
Dennis Tillman W7PF
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-----Original Message-----
From: Fabio Trevisan
Sent: Tuesday, February 19, 2019 1:12 PM
Hello,
My first Tek, I used to own until a few months ago, had a weak CRT since day
one (when I got it from a local, Brazilian auction site).
One of the first thing that I noticed, besides the low intensity, was that it
had this double-peaking thing (which I was unfamiliar with).
It was when I joined this group and the messages should still be there.
Back then David Hess (where's him? Haven't seen anything from him for a
while) pointed me to Ed Breya (both are renowned members), as Ed had some
experience in rejuvenating CRTs.
At the same time, I learnt about the extensive information available on the
tekwiki site (google for TekWiki... you'll find it).
At TekWiki, there in the "Manuals, Catalogs and Other Publications", there's
a section called "Concepts Series".
Between the "Concepts Series" publications, the very first one is the
"Oscilloscope Cathode-Ray Tubes, 2nd ed.
LInk here for your convenience:
01.pdf
Between pages 10 and 14 there's a detailed explanation of how the electric
field lines from the grid interact with that of the cathode, to create an
electrical equivalent to a photo camera's "aperture" ring... which is
actually what broadens or pinches how much of the Cathode surface is "opened"
for emissions.
So,, follows what I conclude from the understanding:
When set to a lower intensity, the cathode effectively emits only from the
center, and as we crank up the intensity, it opens up more and more area of
the cathode to emit the beam. Therefore, the center of the cathode is the
most used portion and is the one that wears out first.
As the CRT ages, the central spot of the cathode wears out and you can only
obtain some intensity as you open the "aperture"more and more, exposing the
next section (an emitting ring now, not anymore a circle).
That concentric wearing "pattern" ends up imprinted on the cathode and, in my
opinion, is what causes the double peaking...
On a worn out cathode, when you start cranking up the brightness, initially
the aperture is drawing electrons from the "worn out" area which, despite
it's worn out, it responds to the intensity control...(more C.W. more
brightness, only that at a lesser degree"...
As you keep opening up the "aperture", you expose more and more of the outer
rings (which are also more worn out than the center, so the intensity
decreases)...
Up until a point that you open the aperture so much that you expose the
outermost ring of the cathode which is still not worn out... (thus, enters
the second "peak").
At this point, although you managed to get more electrons and more beam
intensity, the size of the spot is already too big and the Focusing anode can
no longer focus the beam correctly... (or the Focus Tracking potentiometer
can no longer track correctly the increase of the intensity control) and
ultimately, the Focus and Astigmatism is ruined.
Part of my conclusion can be flawed, but overall, I think this is the
mechanism.
Rgrds,
Fabio -- Dennis Tillman W7PF TekScopes Moderator
|
Hi:
On the subject of CRT rejuvenation...
The filament, of a vacuum tube... like a CRT... the filament is tungsten, operated at a high temperature (This is necessary for adequate thermionic emission). When the filament fails, and goes open, usually we say the filament has "burned out." But really, since there is no oxygen inside a vacuum tube, what happens is, that over time, the tungsten, comprising the filament, is slowly evaporated away (They used to say, it was boiled off.)... that is, the tungsten is evaporated away until there is a filament failure. Increasing the voltage to the filament, above the rated voltage, increases the filament's temperature, and significantly increases this tungsten evaporation process. That is one down side to rejuvenators that apply increased filament voltage: sometimes rejuvenators burned out the filament right away, or rejuvenators significantly weakened the filament... so that the CRT didn't last very long afterwards. Sencore, Eico, et. al. made rejuvenators.
Regards
Roy
|
In virtually all CRTs, the cathode is made up of a metal cup with one end coated with a mixture of barium and strontium carbonates. While the tube is being evacuated, the cathode is heated much hotter than usual, which oxidizes these carbonates. The heater voltage is often run at double the normal value for this step. The tube is still on the vacuum pump, so any evolved gasses are pumped away. The tube is sealed, the getter flashed, the base installed, the tube tested, and it is ready to go.
The use of an oxide-coated cathode permits operation at relatively low temperatures - about 1000 degrees K (reddish-orange). This compares with 1900 degrees K (yellowish-white) for thoriated tungsten and 2500 degrees K (bright white) with pure tungsten. However, the oxide (and thoriated tungsten) cathode can be damaged, usually by stray ions damaging the oxide. This is why it is important to keep any gas in the tube to a minimum (gas hit by an electron causes positive ions). In virtually all cases, wear-out of the tube is caused by poor emission from a damaged cathode. The concept of rejuvenation is to repeat the activation step to generate more barium and strontium oxides. However, this doesn't always work, since there may not be enough carbonates left, evolved gasses are not trapped, and the heater may burn open.
The heater in an oxide-coated cathode is run well below the temperature seen in an incandescent bulb, so vaporization during life is minimal. However, during activation, some tungsten is vaporized. Most "open heaters" in heater-cathode tubes are caused either by manufacturing defects or over-voltage (say in a series-string set), and are rare in modern (post World War II) tubes.
For more on tube theory, see:
For all the steps in making CRTs, see:
For Tektronix's take on CRTs, see:
- John Atwood
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On Sat, Feb 23, 2019 at 10:49 AM, Roy Thistle wrote: Hi:
On the subject of CRT rejuvenation...
The filament, of a vacuum tube... like a CRT... the filament is tungsten,
operated at a high temperature (This is necessary for adequate thermionic
emission). When the filament fails, and goes open, usually we say the filament
has "burned out." But really, since there is no oxygen inside a vacuum tube,
what happens is, that over time, the tungsten, comprising the filament, is
slowly evaporated away (They used to say, it was boiled off.)... that is, the
tungsten is evaporated away until there is a filament failure. Increasing the
voltage to the filament, above the rated voltage, increases the filament's
temperature, and significantly increases this tungsten evaporation process.
That is one down side to rejuvenators that apply increased filament voltage:
sometimes rejuvenators burned out the filament right away, or rejuvenators
significantly weakened the filament... so that the CRT didn't last very long
afterwards. Sencore, Eico, et. al. made rejuvenators.
Regards
Roy
|
On Sat, 23 Feb 2019 10:49:58 -0800, you wrote: Hi:
On the subject of CRT rejuvenation...
The filament, of a vacuum tube... like a CRT... the filament is tungsten, operated at a high temperature (This is necessary for adequate thermionic emission). When the filament fails, and goes open, usually we say the filament has "burned out." But really, since there is no oxygen inside a vacuum tube, what happens is, that over time, the tungsten, comprising the filament, is slowly evaporated away (They used to say, it was boiled off.)... And here's what I think they don't say a lot. The tungsten does not evaporate from each part equally. It does exactly what you say, it evaporates, and that leaves a thinner section. Thinner = high resistance = more voltage drop = more power dissipated. In short, the evaporated section, wherever it is, gets thinner. Now on a light bulb, think "inrush" current. The resistance of cold tungsten is less than hot tungsten, so the whole filament heats rapidly under the effect of the surge current, which is limited *only* when the tungsten gets white hot. And if a thinner section gets too hot? It breaks. One reason why incandandescent lamps only burn out at turn on.... Likely CRT filaments, too, bet it's the same mechanism. Not cathode depletion, the filament itself.... Carbon filament lights, if never turned off, generally won't burn out (likely they will, but the effect takes a long time). There are stories of carbon filament lights (run at a low temperature, too), that have lasted a good fifty years or more. that is, the tungsten is evaporated away until there is a filament failure. Increasing the voltage to the filament, above the rated voltage, increases the filament's temperature, and significantly increases this tungsten evaporation process. That is one down side to rejuvenators that apply increased filament voltage: sometimes rejuvenators burned out the filament right away, or rejuvenators significantly weakened the filament... so that the CRT didn't last very long afterwards. Sencore, Eico, et. al. made rejuvenators. Exactly. I think (check figures, please) that 10% overvoltage results in 10% of the life, and 10% less, 10 times the life. I have a microscope lamp that I have set on minimum voltage. Not sure how low that is. I think it's kept burning for a good three or four years of on/off life.... These may have been 50 to 150 hour lamps.... It's been on far longer than that. when it dies, I'm using LEDS BIG LEDS.... but LEDS..... Harvey Regards
Roy
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Yeah, Harvey, a lot more than 50 years for carbon-filament incandescent
lamps; at Thomas Edison's winter laboratory in (IIRC; it's been probably
about 40 years since I was there) Naples, Florida, they have some of his
original carbon bulbs that have been burning continuously since he
himself put them there! Now, they aren't very bright, and you can see
the thick carbon filaments, but they will last virtually forever.
I remember from electronic materials class somewhat more than 30 years
ago that the halogen lamps have chlorine or another halogen gas in them,
and it forms tungsten chloride preferentially at hotspots on the
filament, providing a negative feedback mechanism. So you can either
run the filament hotter, and thus whiter, with similar lifetime, or run
the filament at the same temperature as a non-halogen bulb and enjoy a
longer life.
Jim Ford
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------ Original Message ------ From: "Harvey White" <madyn@...> To: [email protected]Sent: 2/23/2019 5:56:53 PM Subject: Re: [TekScopes] Mechanism of CRT Double Peaking On Sat, 23 Feb 2019 10:49:58 -0800, you wrote:
Hi:
On the subject of CRT rejuvenation...
The filament, of a vacuum tube... like a CRT... the filament is tungsten, operated at a high temperature (This is necessary for adequate thermionic emission). When the filament fails, and goes open, usually we say the filament has "burned out." But really, since there is no oxygen inside a vacuum tube, what happens is, that over time, the tungsten, comprising the filament, is slowly evaporated away (They used to say, it was boiled off.)... And here's what I think they don't say a lot. The tungsten does not
evaporate from each part equally. It does exactly what you say, it
evaporates, and that leaves a thinner section.
Thinner = high resistance = more voltage drop = more power dissipated.
In short, the evaporated section, wherever it is, gets thinner.
Now on a light bulb, think "inrush" current.
The resistance of cold tungsten is less than hot tungsten, so the
whole filament heats rapidly under the effect of the surge current,
which is limited *only* when the tungsten gets white hot.
And if a thinner section gets too hot?
It breaks.
One reason why incandandescent lamps only burn out at turn on....
Likely CRT filaments, too, bet it's the same mechanism. Not cathode
depletion, the filament itself....
Carbon filament lights, if never turned off, generally won't burn out
(likely they will, but the effect takes a long time). There are
stories of carbon filament lights (run at a low temperature, too),
that have lasted a good fifty years or more.
that is, the tungsten is evaporated away until there is a filament failure. Increasing the voltage to the filament, above the rated voltage, increases the filament's temperature, and significantly increases this tungsten evaporation process. That is one down side to rejuvenators that apply increased filament voltage: sometimes rejuvenators burned out the filament right away, or rejuvenators significantly weakened the filament... so that the CRT didn't last very long afterwards. Sencore, Eico, et. al. made rejuvenators. Exactly. I think (check figures, please) that 10% overvoltage results
in 10% of the life, and 10% less, 10 times the life. I have a
microscope lamp that I have set on minimum voltage. Not sure how low
that is. I think it's kept burning for a good three or four years of
on/off life....
These may have been 50 to 150 hour lamps....
It's been on far longer than that.
when it dies, I'm using LEDS
BIG LEDS....
but LEDS.....
Harvey
Regards
Roy
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Dale H. Cook
Fabio Trevisan
John Williams