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.
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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.
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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



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Dennis Tillman W7pF
TekScopes Moderator

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