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:
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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
