What is the theory behind low voltage focus CRTs?

[LukeSimon]

What is the theory behind how low voltage focus CRTs work, and how does that compare and contrast to the high voltage focus CRTs?

I know that in both low voltage focus and high voltage focus CRTs, the G1 control grid uses a low voltage (relative to the cathodes) to force the cathode ray to squeeze through the G1 control grid's aperture, which in a CRT is a metal cylinder surrounding the cathode with a small aperture at the exiting end. This is an example of how low voltage can be used to make the cathode ray a thinner, sharper beam. Do low voltage focus CRTs use a similar mechanism? What are good articles to read on this topic?

[Electronic M]

It's an electron beam optics question. The focusing electrode acts and a lense for electron beam.
You'd probably want to find a book on CRT design to read up on it (I'm not familiar with any).

The low focus voltage guns tend to loose the ability to reach optimal focus as they age.

[jr_tech]

Perhaps search for info on Einzel lens design such as this:

https://s3.cern.ch/inspire-prod-file...6fa56930f1de08

jr

[LukeSimon]

Originally Posted by jr_tech

Perhaps search for info on Einzel lens design such as this:

https://s3.cern.ch/inspire-prod-file...6fa56930f1de08

jr

Thanks! I did not realize the physical phenomenon used was called an Einzel lens. I will do my homework now and see what I can learn.

The reason why I am interested is to better understand how to tune a TV's sharpness and focus. I've experimented with modding a 1990s TV so that the G1 anode's DC bias can be adjusted. Based off of my experiments, I found that I was able to obtain a sharper and thinner cathode ray by adjusting the G1 DC bias, in addition to the DC bias of G2 and G3. Due to the G1 anode's shape, this is analogous to adjusting how open vs closed an optical aperture is, and can be used to make the cathode ray more sharp and thin. By separately adjusting the DC bias of the G1 anode, a sharper picture is possible than what can be obtained by adjusting the G3 focus voltage alone. Here are examples of 240p video content before/after the mod (left before, right after):





The GE Portacolor is my first color non-solid state TV, and unlike 1990s TVs, it has pots galore for controlling DC bias of: cathodes, G1 grids, G2 screens, and G3 focus... well, not a pot in my early Portacolor, but I can mod it to add a pot for controlling the DC bias of the G3 focus anode.



While many people understand that changing the DC bias of the G1 will change brightness, I've found that most people do not understand that it also changes the thickness and sharpness of the cathode ray. This is actually important information for TV collectors because most service manuals, Photofacts, and schematics just refer to the DC bias of these cathodes and anodes as "brightness" pots. It is important to distinguish between "brightness" as in "black levels" versus cathode ray "thickness/sharpness". It is possible to calibrate DC biases to have proper black levels and a thick and dull cathode ray, where the picture looks less sharp, but it is also possible to calibrate the DC biases to have proper black levels and a thin and sharp cathode ray, where the picture looks more sharp. Again, this is in addition to tuning the DC bias of the G3 focus anode.

I am going to experiment with modifying this GE Portacolor I got to add a pot for controlling the red cathode's DC bias (it already has pots for the other two cathodes), and also add a pot for controlling the G3 focus anode's DC bias (apparently the early Portacolors lack a pot for this, likely as a cost cutting measure). My hypothesis is that I will get the thinnest, sharpest cathode ray with the following calibration:

1. Calibrate cathode DC bias to 270 volts
2. Calibrate G1 DC bias to zero volts
3. Calibrate G2 voltage as high as it needs to achieve correct black levels. This is necessary because maxing out cathode DC bias voltage and minimizing G1 DC bias voltage will completely cut off the cathode ray, unless G2 voltage is high enough to pull electrons past the G1 anode.
4. Lower G3 focus voltage towards zero volts, stopping when the picture is maximally sharp.

The Portacolor's very bad mask dot pitch will definitely limit what is possible. If the cathode ray is too thin and sharp, the moire effect will be very bad.

[zeno]

As far as I know they work the same as Hi focus.
I would guess its just easier = cheaper that way.
On the US side most MFGS sold a 14" probably all with the RCA
jug. RCA also had a 19" low focus, Zenith 14" & 16". Most used taps & a few
a control to adjust. After a few years you usually didnt see a
difference adjusting them unless the CRT was VERY strong.
From Japan TONS of them were sold through Sears. Mostly
Toshibas & Sanyos. They looked even worse but on the test
jig not so bad at all after you put a hand full of low
level tubes in them.

73 Zeno
LFOD !

[LukeSimon]

Originally Posted by zeno

As far as I know they work the same as Hi focus.
I would guess its just easier = cheaper that way.
On the US side most MFGS sold a 14" probably all with the RCA
jug. RCA also had a 19" low focus, Zenith 14" & 16". Most used taps & a few
a control to adjust. After a few years you usually didnt see a
difference adjusting them unless the CRT was VERY strong.
From Japan TONS of them were sold through Sears. Mostly
Toshibas & Sanyos. They looked even worse but on the test
jig not so bad at all after you put a hand full of low
level tubes in them.

73 Zeno
LFOD !

As a CRT's cathodes age, the cathode ray emitting layer at the center tip of the cathode wears away, causing the center tip region of the cathode to emit fewer electrons. The image becomes dimmer because of this, and so people increase contrast and maybe brightness to make the picture less dim. These adjustments cause electrons to be pulled from a larger spot at the tip of the cathode, which makes the cathode ray slightly thicker and the picture less sharp.

I have a theory as to why high voltage focus became more popular than low voltage focus. The high voltage focus has both a lensing effect to focus the cathode ray into a tighter beam, AND it also has an acceleration effect due to a high increase in voltage in the direction leading towards the face of the CRT. The faster electrons move towards the face of the CRT, the less time they have to spread outwards from the center of the cathode ray. This, btw, is one reason why a low ultor voltage causes the picture to be blurry compared to a high ultor voltage. Less acceleration from the ultor anode gives the electrons more time to spread out.

The low voltage focus designs, assuming they are a true Einzel lens, only focus the cathode ray, they do not change its acceleration along the axis perpendicular to the face of the CRT like a high voltage focus design does.

This theory would help explain why low voltage focus CRTs had a reputation of being both less sharp than high voltage focus CRTs AND a reputation for not aging as well when it comes to losing ability to focus a sharp picture.

Side note: vintage television is fun because it gives you an excuse to play around with a particle accelerator. Yes, a CRT meets the scientific definition of a particle accelerator.

[LukeSimon]

I spent a few hours reading research papers, articles, and patents on electrostatic lensing and especially on Einzel lenses (commonly referred to as low voltage focus) and bi-potential lenses (commonly referred to as high voltage focus).

First, the National Valve museum has scanned a great series of articles from 1950s thru 1960s that explain the basic nature of electrostatic lensing. The physical phenomenon is explained in this figure which shows the cross section of two conductors shaped as hollow cylinders with a vacuum gap between the two cylinders:


Whenever two cylinders are arranged in this manner and the difference in voltage between the two is non-zero, an electrostatic field is created as depicted by the equipotential lines. The lens is formed in the gap between the two cylinders, and its effect on the path of a cathode ray (green line) is analogous to the effect that a glass lens has on the path of a ray of light. The power of the lens corresponds to the magnitude of the difference in the voltage of the two adjacent cylinders.

This patent from Intel has a great one page introduction on different CRT focusing designs. It also describes how to use the G1 anode as an iris:

The biasing grid effectively forms an iris, which the beam passes through. This iris can be opened or closed by varying the voltage on the biasing grid.
If the biasing Voltage is brought closer to the cathode Voltage then the cathode's active emitting surface becomes larger in diameter. This active area serves as the object in the total optical System. While this voltage change allows more current to escape from the cathode it increases the object size for the optical System.

The patent goes on to describe a standard low voltage Einzel lens using the following diagram. Cylinder 20 in the figure is what televisions connect to the "focus" voltage. Cylinders 16 and 22 are connected via jumper 24 so they both have the same voltage, and they are then connected to the ultor anode using snubber springs 30. The gap between cylinder 16 and 20 forms a lens, and the gap between cylinder 20 and 22 also forms an identical power and shaped lens.

This brings me back to the goal of maximizing the sharpness of the GE Portacolor. According to Einzel lens theory, the following DC bias settings will create the smallest iris opening and maximize the power of each electrostatic lens:

1. Set DC bias of cathodes to 270 volts (maximum possible for GE Portacolor)
2. Set DC bias of G1 to zero volts (minimum)
3. Set DC bias of G2 (screen) to 670 volts (maximum)
4. Set DC bias of G3 (focus) to 0 volts (minimum)

This voltage setting will likely have black levels that are incorrect. So starting from those "max lens power" voltages, if brightness is too high, I will lower G2 voltage until brightness is correct. Otherwise, if brightness is too low, I will raise G1 until brightness is correct. The electrostatic lens theory stats that as lens power is increased, spherical aberration is also increased. This is why increasing the G3 focus voltage above ground potential may be necessary for maximal sharpness. So the final step for calibrating the GE Portacolor's sharpness is to, starting at zero volts, increase the voltage of the G3 anode until the picture has maximal sharpness.

Doing this will require modifying the GE Portacolor to add 2 additional pots. One for controlling red G1 voltage, and the other for controlling the G3 focus voltage.

[Electronic M]

You may be trying to put lipstick on a pig going to those lengths for a portacolor...the first gen portacolor sets had a phosphor dot pitch too coarse for good NTSC.
The first gen portacolor CRTs instead of doing the right thing and making a new shadow mask with enough holes for the right number of phosphor triads to achieve full NTSC resolution, GE got lazy and simply cut the center 10" diagonal square out of a 21FBP22 roundy shadow mask and called it a day with something like 1/4 of the the triads it should have had...

Later with the IIRC 11WP22 in the second chassis generation portacolor they actually bothered to make a new shadow mask with a finer more appropriate dot pitch (I'd like to find one of those one day).

[LukeSimon]

Originally Posted by Electronic M

You may be trying to put lipstick on a pig going to those lengths for a portacolor...the first gen portacolor sets had a phosphor dot pitch too coarse for good NTSC.
The first gen portacolor CRTs instead of doing the right thing and making a new shadow mask with enough holes for the right number of phosphor triads to achieve full NTSC resolution, GE got lazy and simply cut the center 10" diagonal square out of a 21FBP22 roundy shadow mask and called it a day with something like 1/4 of the the triads it should have had...

Later with the IIRC 11WP22 in the second chassis generation portacolor they actually bothered to make a new shadow mask with a finer more appropriate dot pitch (I'd like to find one of those one day).

I am sure you are right, that is, moire effect will become very bad due to the bad dot pitch. My main goal is to learn new things and to see how far I can push things. I find CRT technology to be fascinating, and after getting into 1960s TVs… I am finding that non-solid state TVs are even more fascinating.

This is my first tube color TV. Sadly, in my area, vacuum tube TVs are very rare. This portacolor was a 6 hour drive round trip. A high end color tube TV like an early 1960s RCA… I may never get my hands on one. When life gives you pigs, you might as well put lipstick on them.

I am always accepting better color tube TVs, if anyone wants to donate one to me.

[AlanInSitges]

As soon as I saw who posted this I knew it would be an interesting read, and it was. ^

It will also be interesting to see how far you get with the 11SP22...I'm not sure they can even resolve differences in focus.

Where are yo located?

[LukeSimon]

Originally Posted by AlanInSitges

As soon as I saw who posted this I knew it would be an interesting read, and it was. ^

It will also be interesting to see how far you get with the 11SP22...I'm not sure they can even resolve differences in focus.

Where are yo located?

Your encouragement is always appreciated. I find that by pushing the boundaries of tech, I learn more... I break more things too because often historical boundaries in technology exist for good reason. However, sometimes the boundaries have no good reason for existing, and I find that people just copy what they see others doing, without ever understanding why. Modifying 1990s TVs to use a lower G1 voltage was "successful" in that a greater sharpness was achieved than with using the focus pot alone. Those pictures above are 240p content, where every other line of video should be not be illuminated. If the blank lines of video are not clear in 240p content, then in 480i content, adjacent lines of video will overlap, i.e., the vertical resolution is lower than it should be. I found that I could make the cathode ray much thinner and sharper than in those screenshots, but the moire effect became very visible. Some research I read stated that the "spot size", which is the smallest region of phosphor illuminated by the cathode ray, should be at least 1.5 times the dot pitch of the phosphor mask, otherwise moire effect becomes visible. Since many TVs use a spot size that is 2 to 2.5 times the dot pitch of the phosphor mask, an improvement in sharpness is possible, without suffering from moire effect.

I live in the San Francisco, California area. This area has a culture of modernity and "high tech". People upgrade their televisions very frequently here. Even late 1990s CRT TVs are becoming rare here. Non-solid state TVs are extinct in the wild here. I have to drive far to rural areas of California, where people are more slow to upgrade their TV to a newer model.

[Electronic M]

Actually that spot size 1.5x the phosphor dot size is a post 1970 thing. In 1970 Zenith introduced the Chromacolor black matrix CRT. In black matrix CRTs you can have dot size larger than the phosphor because there's a black guard band where the screen is unable to light...in the older CRTs you had to underscan the phosphor dots to keep from illuminating the space between the dots or adjacent dots since that space could have a random mix of phosphor from imperfections in the phosphor deposition.

[LukeSimon]

Originally Posted by Electronic M

Actually that spot size 1.5x the phosphor dot size is a post 1970 thing. In 1970 Zenith introduced the Chromacolor black matrix CRT. In black matrix CRTs you can have dot size larger than the phosphor because there's a black guard band where the screen is unable to light...in the older CRTs you had to underscan the phosphor dots to keep from illuminating the space between the dots or adjacent dots since that space could have a random mix of phosphor from imperfections in the phosphor deposition.

Interesting. I have been working backwards in time diving into CRT collecting and technology, so I just learned something new. How was moire effect minimized with such an arrangement? Is the space between the dots large enough that moire effect does not occur when the spot size is near the size of the phosphor dots?

This also brings up the one undeniable advantage of black and white CRTs. In terms of dot pitch, they have a dot pitch of zero, which is perfection. Everyone should own at least one black and white TV for this reason alone.

[old_tv_nut]

Originally Posted by Electronic M

Actually that spot size 1.5x the phosphor dot size is a post 1970 thing. In 1970 Zenith introduced the Chromacolor black matrix CRT. In black matrix CRTs you can have dot size larger than the phosphor because there's a black guard band where the screen is unable to light...in the older CRTs you had to underscan the phosphor dots to keep from illuminating the space between the dots or adjacent dots since that space could have a random mix of phosphor from imperfections in the phosphor deposition.

You are confusing two different things. The use of black matrix has nothing to do with line pitch/spot size.

Black matrix, also called "negative guard band" means that the visible phosphor dot area is smaller than the shadow mask hole. This makes white field purity more consistent since the whole visible phosphor dot is always fully illuminated even if the electrons coming through the hole are slightly off center. It also has the major effect of decreasing screen reflectance.

The older positive guardband tubes and the newer negative guardband tubes had illuminated dot sizes and triad spacings approximately equal. In the early tubes, the illuminated area was limited by the shadow mask hole, and in the later tubes it was limited by the black matrix opening.

The moire issue is one of line pitch vs. triad spacing. The older positive guardband tubes and the newer negative guardband tubes had illuminated dot sizes approximately equal. The ratio of line pitch to triad spacing should be about 1.5:1 so that the moire pattern is fine and therefore less visible. The strength of the moire (rather than its fineness) is dependent on the sharpness of the spot. The moire spatial frequency is a beat note between line spacing and triad spacing. If the line spacing and triad spacing are approximately the same, the moire spatial frequency is close to zero, that is, it makes a very coarse pattern. 1.5:1 makes the pattern much finer. Going to smaller triad spacing would make the tube much more expensive and more difficult for purity adjustment. 1.5:1 also is adequate for the triad spacing not to seriously limit video resolution.

[LukeSimon]

I applied the previously mentioned steps, which are based on the theory of using G1 minus cathode voltage as an iris, by maxing cathode DC bias, and minimizing G1 DC bias. Focus voltage is zero, and G2 DC bias is maxed out. I had to slightly increase the DC bias of G1 to get black levels calibrated and to keep the moire effect from being overly noticeable.

With a low voltage focus CRT, the lower the focus anode voltage, the more the cathode ray is squeezed into a thinner beam of electrons, as the negatively charged anode repels the negatively charged electrons. A high voltage focus CRT is a bit different in that it accelarates the electrons towards the center of the beam and forward towards the phosphor screen.

As you can see in the first picture, which is a test pattern of 240 alternating white and black horizontal lines. Before the tuning, the adjacent horizontal lines blurred together, making it impossible to distinguish between alternating lines. Now there is enough separation to be able to distinguish every line. The white lines are still too thick compared to the black lines, but if the cathode ray is made thinner, the moire effect becomes terrible.


This color bars pattern shows a subtle moire effect caused by the cathode ray’s spot size becoming too small relative to the phosphor mask dot pitch. If you zoom in to the horizontal white and black lines test pattern above, the white lines are one line of video, so the vertical thickness of the white line is the cathode ray spot size’s diameter. You can see that the spot size is just about 1.5x the dot pitch. This is why moire effect is becoming noticeable.