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Mechanism of CRT Double Peaking (UPDATED)


 

Peter Keller just provided me with a revision to the 4th paragraph of my original explanation for why the brightness of a well-used CRT first brightens then dims before finally brightening again as the intensity control is increased (known as "Double Peaking"). Here is the updated explanation.
-----------
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. After depletion of the space cloud, the current is being drawn from the damaged center area of the cathode coating which decreases available current. Further increasing the intensity control (lowering the negative grid bias) begins to draw current from the undamaged periphery of the cathode and increases beam current again but with a resulting larger spot size.
-----------
Dennis Tillman W7pF

-----Original Message-----
Sent: Saturday, February 23, 2019 9:28 AM
Subject: RE: [TekScopes] Mechanism of CRT Double Peaking

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:
https://en.wikipedia.org/wiki/Space_charge
https://en.wikipedia.org/wiki/Thermionic_emission
https://en.wikipedia.org/wiki/Grid-leak_detector
https://en.wikipedia.org/wiki/Vacuum_tube

Dennis Tillman W7PF

-----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:
http://w140.com/tekwiki/images/6/62/062-0852-
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


Nenad Filipovic
 

Hi Everybody,

I still believe that we're missing the proper explanation. A credible theory must precisely describe the mechanism behind the negative resistance which obviously occurs here. Negative resistance is most likely related to a process with positive feedback, and none of the proposed explanations describe such a process. By looking at the aging cathode alone I cannot see any process that can change the sign of its first derivative on its own, therefore the solution will most certainly include other components of the aged tube and the control circuit as well.

I have hints, but not enough data for a credible theory (all my CRTs are healthy). I would be interested in the plot of the following against the observed screen intensity:
1. Vk-Vg1 (first grid bias)
2. Vk-Vg2 (screen grid voltage with respect to cathode)
3. Subjective evaluation of the beam focus on the screen
Someone willing to make these measurements would greatly help crack this mystery. I believe approx. 10 measurement points would be enough to sample the two peaks.

I have only one clear encounter of the title phenomenon. As a kid I experimented with a badly worn out B/W TV CRT, which could not form any visible picture on the screen. I subjected it to voltages significantly higher than its specs and the picture appeared, but it was "negative". I cannot forget my astonishment and it kind of haunts me, hence my curiosity for solving this.

Best regards,
Nenad Filipovic


SCMenasian
 

This is a very complex subject and the correct explanation of what is going on depends critically on what type of cathode is involved. I do not know exactly what types of cathode Tektronix used. They probably used several in various generations of tubes. Two types, in addition to coated cathodes come to mind. Both can respond to higher than normal heater current.

The first is the dispenser cathode, familiar to many experimental atomic physicists. These cathodes (which are extended in nature and probably not suitable for CRTs) are consist of a pellet of sintered material in which the active electron emitting oxides are actually in the body of the cathode. The must be "activated" by raising the temperature high enough to diffuse the active material to the surface. If, for example, Tektronix developed a dispenser cathode in which, initially, the activated surface was only a tiny point and, in which, subsequent overheating would cause active material to diffuse, not only to that point, but to the entire anode facing surface, the behavior might be as described.

Another cathode material, often used in vacuum tube filaments, is thoriated tungsten. In this material, thorium is, initially, distributed through the body of the material and initial (and subsequent) heatings serve to diffuse the thorium to the surface, with, possibly, similar results.

Stephen Menasian


Chuck Harris
 

The CRT cathodes are not thoriated tungsten, but
rather a dispenser cathode.

Think of a tiny thimble cup that has its outer
bottom dipped in a cathode emitter material, and
has a heater element inside.

-Chuck Harris

SCMenasian wrote:

This is a very complex subject and the correct explanation of what is going on depends critically on what type of cathode is involved. I do not know exactly what types of cathode Tektronix used. They probably used several in various generations of tubes. Two types, in addition to coated cathodes come to mind. Both can respond to higher than normal heater current.

The first is the dispenser cathode, familiar to many experimental atomic physicists. These cathodes (which are extended in nature and probably not suitable for CRTs) are consist of a pellet of sintered material in which the active electron emitting oxides are actually in the body of the cathode. The must be "activated" by raising the temperature high enough to diffuse the active material to the surface. If, for example, Tektronix developed a dispenser cathode in which, initially, the activated surface was only a tiny point and, in which, subsequent overheating would cause active material to diffuse, not only to that point, but to the entire anode facing surface, the behavior might be as described.

Another cathode material, often used in vacuum tube filaments, is thoriated tungsten. In this material, thorium is, initially, distributed through the body of the material and initial (and subsequent) heatings serve to diffuse the thorium to the surface, with, possibly, similar results.

Stephen Menasian






Nenad Filipovic
 

I kind of confirmed my initial belief, Tektronix document 062-0852-01 (Cathode Ray Tubes by Chuck DeVere) clearly states on page 9 that CRT cathode is indirectly heated oxide based type.

However, a Tektronix submitted dispenser cathode patent exists indeed:
https://patents.google.com/patent/US4954745
but it appears to date 1989-1990, therefore the CRTs we're discussing are older than that.

Nenad FIlipovic


greenboxmaven
 

Some years ago, I had a very fine conversation with a picture tube designer at General Electric here in Syracuse. One phenomena that could explain the double peak you see is the electrical interface between the cathode material and the metal cup it is applied to. Apparently, a contaminated layer can build up that carries current well up to a point, then passes it poorly until there is enough current demand to develope a voltage across the layer and cause it to break down. This effect is far more detrimental for television than it would be for an oscilloscope where the beam intensity is usually on or off. Decades ago, there was a rejuvinator called the Raytronic Beamer that was supposedly superior in breaking down this layer. They were so effective they raised a great deal of controversy because people would zap dim jugs and sell them as excellent used or even rebuilt. I have had mixed success zapping scope jugs, it made a small improvement for a very dim 453, but worked well for 5UP1s and other older ones. One thing about zapping a jug must be kept in mind- it usually enlarges the aperture in the first grid, which enlarges the beam spot. That does help brightness, and is usually OK for black and white television, but would certainly reduce clarity and resolution on a scope or color television.

Bruce Gentry, KA2IVY

On 12/25/20 8:53, SCMenasian wrote:
This is a very complex subject and the correct explanation of what is going on depends critically on what type of cathode is involved. I do not know exactly what types of cathode Tektronix used. They probably used several in various generations of tubes. Two types, in addition to coated cathodes come to mind. Both can respond to higher than normal heater current.

The first is the dispenser cathode, familiar to many experimental atomic physicists. These cathodes (which are extended in nature and probably not suitable for CRTs) are consist of a pellet of sintered material in which the active electron emitting oxides are actually in the body of the cathode. The must be "activated" by raising the temperature high enough to diffuse the active material to the surface. If, for example, Tektronix developed a dispenser cathode in which, initially, the activated surface was only a tiny point and, in which, subsequent overheating would cause active material to diffuse, not only to that point, but to the entire anode facing surface, the behavior might be as described.

Another cathode material, often used in vacuum tube filaments, is thoriated tungsten. In this material, thorium is, initially, distributed through the body of the material and initial (and subsequent) heatings serve to diffuse the thorium to the surface, with, possibly, similar results.

Stephen Menasian





Nenad Filipovic
 

If the "interface layer" theory was true then the same negative resistance phenomenon would also be observable in conventional tubes. Personally I've never seen one behave like that, although I curve traced a considerably large and diverse tube graveyard out of curiosity. I witnessed a lot of funny things, but never a negative resistance. Perhaps some more experienced group members have?

For the reason above I believe the phenomenon is related to CRT gun specifics, coupled with the control circuits designed for high impedance on most electrodes. Until someone performs the measurements I proposed previously we'd hardly know in which direction to go.

Nenad Filipovic


Chuck Harris
 

In receiving tubes, that "interface layer" is called Cathode Interface,
and is very true. Look it up.

-Chuck Harris

Nenad Filipovic wrote:

If the "interface layer" theory was true then the same negative resistance phenomenon would also be observable in conventional tubes. Personally I've never seen one behave like that, although I curve traced a considerably large and diverse tube graveyard out of curiosity. I witnessed a lot of funny things, but never a negative resistance. Perhaps some more experienced group members have?

For the reason above I believe the phenomenon is related to CRT gun specifics, coupled with the control circuits designed for high impedance on most electrodes. Until someone performs the measurements I proposed previously we'd hardly know in which direction to go.

Nenad Filipovic






Chuck Harris
 

That is probably a miss-memory.

What the designer in GE was talking about is a layer of contamination
that boils up to the surface of the normally highly emissive cathode
material. When the layer covers the cathode material, the emission
drops.

There is good evidence that overheating the cathode, and using the
first grid as the anode of a diode to draw much higher than usual
current from the cathode (using much higher voltages than usual)
will break up the contamination layer, and restore some activity to
the cathode... Indeed, all CRT manufacturers do this to activate the
cathodes of new CRT guns, during manufacture.

Zapping is not really agressive to anything but the cathode layer,
certainly not to the first grid, which is a heavy piece of stainless
steel... way more rugged than the cathode.

Raytronic Beamer, Sencore Rejuvinator, B&K ... there were hundreds
of CRT restorers available to the TV repair shops that all worked the
same way. Each was so magnificent that it was head and shoulders
above all of the rest... Each allowed the repairman to go from gently
(and ineffectually) caressing the cathode to kill'em or cure'em levels
of zapping.

-Chuck Harris

greenboxmaven via groups.io wrote:

Some years ago, I had a very fine conversation with a picture tube designer at
General Electric here in Syracuse. One phenomena that could explain the double peak
you see is the electrical interface between the cathode material and the metal cup it
is applied to.  Apparently, a contaminated layer  can  build up that carries current
well up to a point, then passes it poorly until there is enough current demand to
develope a voltage across the layer and cause it to break down. This effect is far
more detrimental for television than it would be for an oscilloscope where the beam
intensity is usually on or off. Decades ago, there was a rejuvinator called the
Raytronic Beamer that was supposedly superior in breaking down this layer.  They were
so effective they raised a great deal of controversy because people would zap dim
jugs and sell them as excellent used or even rebuilt.  I have had mixed success
zapping scope jugs, it made a small improvement for a very dim 453, but worked well
for 5UP1s and other older ones. One thing about zapping a jug must be kept in mind-
it usually enlarges the aperture in the first grid, which enlarges the beam spot. 
That does help brightness, and is usually OK for black and white television, but
would certainly reduce clarity and resolution on a scope or color television.

       Bruce Gentry, KA2IVY


On 12/25/20 8:53, SCMenasian wrote:
This is a very complex subject and the correct explanation of what is going on
depends critically on what type of cathode is involved. I do not know exactly what
types of cathode Tektronix used. They probably used several in various generations
of tubes. Two types, in addition to coated cathodes come to mind. Both can respond
to higher than normal heater current.

The first is the dispenser cathode, familiar to many experimental atomic
physicists. These cathodes (which are extended in nature and probably not suitable
for CRTs) are consist of a pellet of sintered material in which the active electron
emitting oxides are actually in the body of the cathode. The must be "activated" by
raising the temperature high enough to diffuse the active material to the surface.
If, for example, Tektronix developed a dispenser cathode in which, initially, the
activated surface was only a tiny point and, in which, subsequent overheating would
cause active material to diffuse, not only to that point, but to the entire anode
facing surface, the behavior might be as described.

Another cathode material, often used in vacuum tube filaments, is thoriated
tungsten. In this material, thorium is, initially, distributed through the body of
the material and initial (and subsequent) heatings serve to diffuse the thorium to
the surface, with, possibly, similar results.

Stephen Menasian











Nenad Filipovic
 

On Tue, Dec 29, 2020 at 06:43 AM, Chuck Harris wrote:
In receiving tubes, that "interface layer" is called Cathode Interface,
and is very true. Look it up.
Perhaps my statement wasn't precise enough, I meant to say "if the interface layer was behind the double peaking phenomenon...", not question the interface layer existence per se.

What the designer in GE was talking about is a layer of contamination
that boils up to the surface of the normally highly emissive cathode
material. When the layer covers the cathode material, the emission
drops.
Oxide cathode is so complex (not entirely understood even to the day) that any plausible mechanism one can think of - probably occurs in it. The question is which of these mechanisms has higher significance for the particular case study. The predominant deterioration mechanism in CRT cathode is sputtering by positive ion bombardment, due to high voltages and concentrated electron beam shape[1, page 116].

There is good evidence that overheating the cathode, and using the
first grid as the anode of a diode to draw much higher than usual
current from the cathode (using much higher voltages than usual)
will break up the contamination layer, and restore some activity to
the cathode... Indeed, all CRT manufacturers do this to activate the
cathodes of new CRT guns, during manufacture.
Yes in general, but the exact mechanism is slightly different. To the demise of some audiophiles the (activated) oxide cathode is an n-type semiconductor, and a quite sensitive one. To enhance its conductivity and emission, oxide cathodes are doped with impurities in form of reducing activators which are alloyed with the cathode metal carrier[2, page 557]. This metal carrier (e.g. nickel tube in case of indirectly heated tubes) acts as a reservoir of spare activator throughout the life of the tube. Contamination (aka cathode poisoning) consists mostly of oxidizing agents that act as traps[3, page 8][1, page 112], and counters the effects of activation. Funny thing is that both the activator and most contaminants are mobile at the operating temperatures (e.g. can diffuse by electrolysis)[3, page 16]. Some contaminants also become embedded in the crystalline structure of the oxide layer. But contamination is not the only deteriorating effect, there's also sputtering erosion, evaporation and sintering[1, page 115]. The high efficiency of the oxide cathode is in good part due to its high porosity, which significantly increases the effective surface area which acts as an emitter. Sintering solidifies the oxide surface and therefore reduces emission.
During operation of the tube, cathode current forms an electric field in the oxide layer which balances the diffusion of activator, barium (directly related to donors) and contaminants. A natural equilibrium forms which depends on the structural recipe of the cathode and its health state[3, page 35]. The above mentioned deteriorating effects effectively change the structure of the cathode rather permanently, resulting in general conductivity drop[3, page 18]. One artificial way to restore the conductivity is to pump the oxide layer with additional activator. This is accomplished by combination of overheating the filament so that metal base can release the activator, and by running pulse current to stimulate diffusion. Unfortunately the excess barium (its vapor pressure) created in this way is not in equilibrium with the aged oxide layer structure, and will subsequently be lost due to evaporation[1, page 67][3, page 30] in a process known as deactivation. This explains why rejuvenation is short-lived.

In my experience I witnessed rejuvenated CRTs which were bright, but certainly not sharp. This supports the argument that the center of the cathode is mostly lost, and the (temporary) increase of brightness is due to the artificially increased emission of the surrounding area.

Negative resistance and double peaking still remain without clear explanation.

References:
[1] THE OXIDE-COATED CATHODE Vol 1/2; Dr. Ing. G. HERRMANN, Dr. Phil. S. WAGENER; CHAPMAN & HALL LTD. London 1951
[2] Materials and Techniques for Electron Tubes; Walter H. Kohl; Reinhold Publishing Corporation 1960
[3] Electron Tube Design; RCA 1962

Nenad Filipovic


Chuck Harris
 

Hi Nenad,

I don't dispute any of that, but rather add one thing,
and that is, ion sputtering can only occur if the CRT is
gassy. I have spent a good bit of time playing with
sputterers used to coat samples for imaging with an
SEM.

My expectation is that double peaking is the result of a
gassy CRT, and not a worn out cathode, per se.

In my playing with double peaking CRT's, I note that you
cannot achieve any sort of sharp focus after the initial
low intensity sharp focus. It only gets worse. Even though
the focus control has a best focus setting that is within its
range... that tends to indicate that the focus grid is getting
the proper bias voltage.

If double peaking was a function of lost emission in the center
of the cathode, you should still be able to "print" a sharp
image of the cathode's emissivity on the screen at any intensity.

The affect of defocusing caused by gas in the CRT is one of
dispersion of the beam as the electrons are deflected by
the gas molecules floating around in the vacuum.

The greater the intensity, the higher the quantity of electrons
streaming through the CRT, stirring up the residual gas.

I doubt we will ever know how the phenomenon works, as it is
undoubtedly a complicated interaction of multiple causes.

And, it is unlikely that anyone with sufficient resources to
fund such research cares about CRT's anymore. They are so last
century...

-Chuck Harris

Nenad Filipovic wrote:

On Tue, Dec 29, 2020 at 06:43 AM, Chuck Harris wrote:
In receiving tubes, that "interface layer" is called Cathode Interface,
and is very true. Look it up.
Perhaps my statement wasn't precise enough, I meant to say "if the interface layer was behind the double peaking phenomenon...", not question the interface layer existence per se.

What the designer in GE was talking about is a layer of contamination
that boils up to the surface of the normally highly emissive cathode
material. When the layer covers the cathode material, the emission
drops.
Oxide cathode is so complex (not entirely understood even to the day) that any plausible mechanism one can think of - probably occurs in it. The question is which of these mechanisms has higher significance for the particular case study. The predominant deterioration mechanism in CRT cathode is sputtering by positive ion bombardment, due to high voltages and concentrated electron beam shape[1, page 116].

There is good evidence that overheating the cathode, and using the
first grid as the anode of a diode to draw much higher than usual
current from the cathode (using much higher voltages than usual)
will break up the contamination layer, and restore some activity to
the cathode... Indeed, all CRT manufacturers do this to activate the
cathodes of new CRT guns, during manufacture.
Yes in general, but the exact mechanism is slightly different. To the demise of some audiophiles the (activated) oxide cathode is an n-type semiconductor, and a quite sensitive one. To enhance its conductivity and emission, oxide cathodes are doped with impurities in form of reducing activators which are alloyed with the cathode metal carrier[2, page 557]. This metal carrier (e.g. nickel tube in case of indirectly heated tubes) acts as a reservoir of spare activator throughout the life of the tube. Contamination (aka cathode poisoning) consists mostly of oxidizing agents that act as traps[3, page 8][1, page 112], and counters the effects of activation. Funny thing is that both the activator and most contaminants are mobile at the operating temperatures (e.g. can diffuse by electrolysis)[3, page 16]. Some contaminants also become embedded in the crystalline structure of the oxide layer. But contamination is not the only deteriorating effect, there's also sputtering erosion, evaporation and sintering[1, page 115]. The high efficiency of the oxide cathode is in good part due to its high porosity, which significantly increases the effective surface area which acts as an emitter. Sintering solidifies the oxide surface and therefore reduces emission.
During operation of the tube, cathode current forms an electric field in the oxide layer which balances the diffusion of activator, barium (directly related to donors) and contaminants. A natural equilibrium forms which depends on the structural recipe of the cathode and its health state[3, page 35]. The above mentioned deteriorating effects effectively change the structure of the cathode rather permanently, resulting in general conductivity drop[3, page 18]. One artificial way to restore the conductivity is to pump the oxide layer with additional activator. This is accomplished by combination of overheating the filament so that metal base can release the activator, and by running pulse current to stimulate diffusion. Unfortunately the excess barium (its vapor pressure) created in this way is not in equilibrium with the aged oxide layer structure, and will subsequently be lost due to evaporation[1, page 67][3, page 30] in a process known as deactivation. This explains why rejuvenation is short-lived.

In my experience I witnessed rejuvenated CRTs which were bright, but certainly not sharp. This supports the argument that the center of the cathode is mostly lost, and the (temporary) increase of brightness is due to the artificially increased emission of the surrounding area.

Negative resistance and double peaking still remain without clear explanation.

References:
[1] THE OXIDE-COATED CATHODE Vol 1/2; Dr. Ing. G. HERRMANN, Dr. Phil. S. WAGENER; CHAPMAN & HALL LTD. London 1951
[2] Materials and Techniques for Electron Tubes; Walter H. Kohl; Reinhold Publishing Corporation 1960
[3] Electron Tube Design; RCA 1962

Nenad Filipovic






Nenad Filipovic
 

Hi Chuck,

Yes, I agree we derailed the subject by going deep into the cathode issues, but I still instinctively believe that double peaking explanation may be within grasp (without major research). Until someone does the measurements I proposed and triggers the lovely "experiment - check your theory" loop, we cannot say we even tried.

The fact you mentioned about focus is interesting. I believe the solution is related to contamination of g1 and g2 with cathode material, where interaction deforms the field distribution which manifests also as a visible change (loss) of focus. Stray gas is very likely involved, as its presence directly influences the electrode currents. But for any credible theory we need the sufficiently sampled plot of the electrode voltages against the observed intensity and focus.

Nenad Filipovic


 

Nenad,

I have a 5103N that appears to be showing double peaking in its CRT, and I'm willing to try making these measurements.

Am I correct in assuming that I can do this with nothing more than a multimeter?

-- Jeff Dutky


snapdiode
 

The only "pure" tube-based negative resistance effect I can think of is the tetrode kink.


Nenad Filipovic
 

On Wed, Dec 30, 2020 at 03:37 PM, Jeff Dutky wrote:
I have a 5103N that appears to be showing double peaking in its CRT, and I'm
willing to try making these measurements.

Am I correct in assuming that I can do this with nothing more than a
multimeter?
Hi Jeff, 5103N appears to be only the module mainframe, separate from the CRT unit. However I found the CRT circuit to be similar (if not identical) among the 5000 series models, so no problem there.

However during examination of the CRT schematic I learned that I overlooked some facts (e.g. the g2 to cathode voltage being approx. 2.5kV to 3.5kV on most 5000/7000 series), making the measurements "not so simple" any more. I guess I am too much used to B/W TV CRTs where this voltage is usually below 1kV. To avoid wasting any one's time and effort I think it's best I first try the measurements myself (e.g. on one of my Teks with a healthy tube). If I manage to develop an acceptably simple and safe procedure, then it would make sense you try it on your double peaking tube. Anyway thanks for your offer to help, please stay tuned.

Nenad Filipovic