Mod a CRT to increase its TVL

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

In order to help newcomers to this thread, I have edited the original post to make it easy to jump into the latest status. Expand the spoiler sections below to get the details. Many people tend to have a strong negative reaction to this mod at first, because many people have an incorrect understanding of how CRTs work, and so I'd encourage every one to first read the summarization of their contents in the original post in the spoilers section below, which contains citations for all controversial claims. The cited technical publications describe in detail how the electron gun works and how electron beam spot size is a key limiting factor for CRT resolution and these 20 year old publications also explain how electron beam width (aka "spot size") can be electronically controlled using cathode and anode voltage differentials within the "triode" section of the electron gun. If you aren't going to bother to read these citations, then consider avoiding posting any negative replies, as your negative reaction is likely due to your lack of understanding of CRT technology.

High-level Explanation Of How This Mod Works

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Original Post With Detailed Explanation (August 17 2020)

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Before & After Photo of Prototype (September 21 2020)
These before and after photos were taken on a hand held iphone. So the exact angle and position of the photos is a bit off because my hand moved between shots. Also, I did the prototyping work in my garage, so the CRT screen is dirty and there is sun glare on the glass in the photos. Sorry, I am not a photographer. The prototype in these photos is an old used 27-inch curved slot mask consumer Panasonic TV, model number CT-27L8G. This TV's un-modded TVL is roughly 450TVL.

2-chip SNES 240p Test Suite Displaying a Solid Green Screen Over Component Video (left before, right after):

Spoiler: show

PS2 Castlevania SoTN Over S-video (left before, right after):

Minimum CRT Requirements For Mod To Achieve 1000 TVL

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Step-by-step modding instructions

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How to make a 250v power supply using $1 of parts

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Josh128 · Joined: 16 Jan 2014 Posts: 1036

Sounds like a fun project. Good luck!

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

In theory, decreasing the size of the electromagnetic aperture at the G1 anode will also make it easier to tune the spot to be in focus simultaneously at both the center of the screen as well as the corners of the screen as the depth of field for focus will be larger, meaning the spot remains in focus for a larger variation in distance from the screen. This will address the other main shortcoming of many consumer CRT TVs: inability to bring both corners and center of the screen into sharp focus at the same time. This issue occurs because of the small depth of field in which the spot stays in focus when G1 anode is zero volts. The distance the electron beam travels changes as the beam scans the edges of the screen versus the center of the screen.

I am going to build a dual power supply by wiring two Maida high voltage regulated power supplies in series with the center 0VDC rail tied to the chassis ground, so that I have an adjustable 0VDC to +220VDC rail for driving the RGB cathodes and a 0VDC to -220VDC rail for setting the G1 voltage and in doing so, adjusting the spot size. The benefit of using a regulated voltage for the RGB drive will be better contrast and less blooming and breathing during bright scenes.

maxtherabbit · Joined: 05 Mar 2018 Posts: 1335

tag, very interesting

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

A deeper discussion of the interplay between mask pitch (sometimes called dot pitch) and spot size is in this reference in section "A Discussion of Issues Relating to Monitor and CRT Resolution". This means that the effective TVL of a CRT can be doubled before the mask pitch becomes the bottleneck. In fact, the aforementioned reference indicates that spot size can be smaller than the dot pitch and yield a pleasing sharp look.

The main goal of this project is to allow consumer CRT TVs to have a high TVL, RGB look, but with that huge fat boy 27-inch (or hell, 36-inch) consumer CRT goodness. Since the G1 voltage will be adjustable between 0 volts down to -220 volts, the spot size will be adjustable. For some people, the 1000 TVL look is too extreme, so having a dial will let people hit the sweet spot that works for them. I prefer the look of around 700 TVL.

SamIAm · Joined: 03 Mar 2005 Posts: 455

This doesn't make a great deal of sense to me.

No matter what kind of mask they're behind, the red, green and blue phosphors in a color CRT are arranged in clusters. Suppose we have a group of six clusters in a horizontal row like this:

R-G-B R-G-B R-G-B R-G-B R-G-B R-G-B

The maximum number of alternating white and black vertical lines that this group will be able to resolve - which is how TVL is defined - is six. Altering the way the beam is focusing on any particular phosphor element won't change the fact that each cluster as a whole can only be one color.

Yes, there are vagueries with CRTs and not everything works in neat little pixel-units. Nonetheless, I think that at best, all you're going to be able to do is make a low TVL set have a superficially high TVL set look, i.e. thin scanlines and fat black spaces. You're not going to make a 450 TVL set produce 700 distinct alternating white and black vertical lines (over a horizontal span equal to the height of the picture).

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

SamIAm wrote:

This doesn't make a great deal of sense to me.

No matter what kind of mask they're behind, the red, green and blue phosphors in a color CRT are arranged in clusters. Suppose we have a group of six clusters in a horizontal row like this:

R-G-B R-G-B R-G-B R-G-B R-G-B R-G-B

The maximum number of alternating white and black vertical lines that this group will be able to resolve - which is how TVL is defined - is six. Altering the way the beam is focusing on any particular phosphor element won't change the fact that each cluster as a whole can only be one color.

Yes, there are vagueries with CRTs and not everything works in neat little pixel-units. Nonetheless, I think that at best, all you're going to be able to do is make a low TVL set have a superficially high TVL set look, i.e. thin scanlines and fat black spaces. You're not going to make a 450 TVL set produce 700 distinct alternating white and black vertical lines (over a horizontal span equal to the height of the picture).

Read the references that I linked, which make it clear that spot size is typically 2 times the dot pitch, and a higher resolution is possible by decreasing spot size. I checked my CRTs and they clearly have at least 2 RGB triplets per illuminated scanline.

In your example a vertical line uses two triplets because spot size illuminates 2 RGB triplets. With a spot size of half the diameter, only one triplet is illuminated for a single horizontal line. This effectively doubles the TVL.

mikejmoffitt · Joined: 08 Jan 2016 Posts: 605 Location: Mountain View, CA

Improving beam focus will never overcome the phosphor pitch's limitations.

TVL has nothing to do with how "thick" or sharp the scanlines are. That correlation is simply a coincidence of higher-end monitors having superior tubes, regulation, condition, etc in general.

The evidence for this is empirical. Just get up close to the tube, and... look at it. Look at the phosphors! For medical imaging, they may be discussing monochrome CRTs, where this is a different matter.

If playing with G1 gives you any results, I'd be interested to see what you get, but I think the best you can expect is an improvement in beam focus. There are many other things you can do to make that better before you consider anything higher effort (adjust the focus knob, calibrate cutoff and gain levels correctly, replace capacitors to improve geometry, ensure the yoke is centered and mounted firmly, perform static convergence adjustment using the rings, fix dynamic convergence with yoke adjustments and/or strips...)

maxtherabbit · Joined: 05 Mar 2018 Posts: 1335

ok we've got 2, who's the next poster that's going to say the same thing without reading the OP?

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

maxtherabbit wrote:

ok we've got 2, who's the next poster that's going to say the same thing without reading the OP?

Every point they raised is addressed in the OP, and references are provided, which go into explanations of addressibility versus resolution, and that resolution is determined by spot size and dot pitch, with spot size being the bottleneck in most monitors because most monitors use a spot size of 2 RGB triples, with some using a smaller spot size.

The comments about phosphors shows they don’t understand what contributes to resolution too, and focus? The G3 anode for focus functions more like an electromagnetic lens, whereas the G1 control grid anode functions like an aperture.

My worry is convergence will be much harder as there are 3 spot sizes that get smaller: R, G, and B. A slight misconvergence of each beam will be much more noticeable with a smaller spot size.

Syntax · Joined: 09 Aug 2017 Posts: 1432 Location: Australia

Ill bite.

All you are doing is sharpening the image.
Reducing dot size reduces bleeding and increases the dark areas around each pixel. It reduces brightness somewhat though.

Would make a really poor set sharper sure, but it's not increasing tvl at all.

SamIAm · Joined: 03 Mar 2005 Posts: 455

LukeEvansSimon wrote:

maxtherabbit wrote:

ok we've got 2, who's the next poster that's going to say the same thing without reading the OP?

Every point they raised is addressed in the OP, and references are provided, which go into explanations of addressibility versus resolution, and that resolution is determined by spot size and dot pitch, with spot size being the bottleneck in most monitors because most monitors use a spot size of 2 RGB triples, with some using a smaller spot size.

If you modify a TV so that its smallest possible "spot" goes from using at least two RGB triplets across the horizontal axis to only one, that's great, but your title is misleading. If your tube is, under any circumstances, able to resolve 700 alternating white and black vertical lines over a horizontal span equal to the display height, then by definition it is at least a 700 TVL tube, and that's probably the spec the manufacturer would list.

One wonders why a manufacturer would bother putting a 700 TVL tube into a set where the surrounding electronics prevented it from resolving anywhere near that, but perhaps they had their reasons. I'd still be shocked if you managed to get 700 alternating white and black vertical lines to display on a TV where the spec sheet says it's 450 TVL.

Guspaz · Joined: 06 Oct 2015 Posts: 2368 Location: Montréal, Canada

I don't think a smaller dot would reduce brightness, not like you'd expect anyway: you're getting the same amount of energy in a smaller space, so the phosphors will glow brighter. The average brightness of the screen is the same.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

SamIAm wrote:

LukeEvansSimon wrote:

maxtherabbit wrote:

ok we've got 2, who's the next poster that's going to say the same thing without reading the OP?

Every point they raised is addressed in the OP, and references are provided, which go into explanations of addressibility versus resolution, and that resolution is determined by spot size and dot pitch, with spot size being the bottleneck in most monitors because most monitors use a spot size of 2 RGB triples, with some using a smaller spot size.

If you modify a TV so that its smallest possible "spot" goes from using at least two RGB triplets across the horizontal axis to only one, that's great, but your title is misleading. If your tube is, under any circumstances, able to resolve 700 alternating white and black vertical lines over a horizontal span equal to the display height, then by definition it is at least a 700 TVL tube, and that's probably the spec the manufacturer would list.

One wonders why a manufacturer would bother putting a 700 TVL tube into a set where the surrounding electronics prevented it from resolving anywhere near that, but perhaps they had their reasons. I'd still be shocked if you managed to get 700 alternating white and black vertical lines to display on a TV where the spec sheet says it's 450 TVL.

“TVL” is not a specification of a CRT. It is a specification for a display that is a combination of a chassis and a CRT. How the chassis drives the CRT determines TVL. I mentioned why consumer CRTs did not bias the G1 anode to -100 volts: it would increase costs because a negative power supply is required.

But hey, don’t read any of the references where biasing the G1 is described as a technique for creating a smaller spot size and a higher resolution. The slides I linked are from a pro CRT company that specialized in high resolution CRT monitors. I guess you think their detailed plots, schematics, explanations for developing high resolution CRT displays is made up?

Finally, you are confusing addressability with resolution. TVL measures resolution, not addressability. A 15khz, 240p display can only address 480 vertical lines that span the width equal to the display height on a 4:3 CRT. I spoke to this in the OP. When you play 240p content on a 700 TVL, 15khz display, you get a sharper image due to two factors: smaller spot size and smaller dot pitch. Your game is not actually able to alternate 700 white and black vertical lines in a horizontal width equal to display height. If you read my references, you’d know this.

I am not claiming that shrinking spot size will increase addressability. The CRT TV will still be a 15khz display. It can’t display 1080p content with a smaller spot size, but high TVL 15khz displays can’t display 1080p content either. Addressability is not the same thing as resolution. Read the references if you still do not understand.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

Syntax wrote:

Ill bite.

All you are doing is sharpening the image.
Reducing dot size reduces bleeding and increases the dark areas around each pixel. It reduces brightness somewhat though.

Would make a really poor set sharper sure, but it's not increasing tvl at all.

A smaller spot size doesn’t increase resolution? Please inform Texas Instruments and Display Labs Inc that their published research on CRT technology is wrong. See, I provided authoritative references for my statements, and you just made a statement without any references to back it up.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

Guspaz wrote:

I don't think a smaller dot would reduce brightness, not like you'd expect anyway: you're getting the same amount of energy in a smaller space, so the phosphors will glow brighter. The average brightness of the screen is the same.

The paper from TI and the slides from Display Labs Inc mentions that screen voltage does need to be increased to set black levels. The beam’s energy is more concentrated so the spot is smaller but brighter.

SamIAm · Joined: 03 Mar 2005 Posts: 455

There is a lot that I'd like to respond to, but this has to come first:

LukeEvansSimon wrote:

A 15khz, 240p display can only address 480 vertical lines that span the width equal to the display height on a 4:3 CRT. I spoke to this in the OP. When you play 240p content on a 700 TVL, 15khz display, you get a sharper image due to two factors: smaller spot size and smaller dot pitch. Your game is not actually able to alternate 700 white and black vertical lines in a horizontal width equal to display height. If you read my references, you’d know this.

I've read this a few times, and each time I'm left wondering if you really said what you seem to be saying.

You're not telling me that I could connect a pattern generator with an adjustable dot-clock to a pro monitor spec'd at 700 TVL and not be able to see, with my eyeballs, above 480 alternating vertical lines (in the relevant horizontal space) just because the monitor's horizontal refresh rate is 15khz, are you?

Because that would be a very extraordinary claim.

Syntax · Joined: 09 Aug 2017 Posts: 1432 Location: Australia

LukeEvansSimon wrote:

Syntax wrote:

Ill bite.

All you are doing is sharpening the image.
Reducing dot size reduces bleeding and increases the dark areas around each pixel. It reduces brightness somewhat though.

Would make a really poor set sharper sure, but it's not increasing tvl at all.

A smaller spot size doesn’t increase resolution? Please inform Texas Instruments and Display Labs Inc that their published research on CRT technology is wrong. See, I provided authoritative references for my statements, and you just made a statement without any references to back it up.

You will only see a resolution increase if you move the now smaller dots closer together, how will you do that?

All you've suggested is to make them smaller and increase the dead space around them.

Why would I need to give reference for something as simple as that?

Xer Xian · Joined: 06 Feb 2005 Posts: 859 Location: Italy

@SamIAm - I think what he wanted to say there was that 15kHz consoles had too low a dot clock to be able to render high resolutions.

I might be wrong, but it seems to me that all the objections raised here up to now are missing the mark. The OP already acknowledged that the mask pitch is a limiting factor, but provided some reference indicating that it may not be the actual bottleneck for some CRTs.

I think the thread is interesting and I'm eager to see the findings to come out of this. I would be very surprised if run-of-the-mill consumer CRTs turn out to be able to resolve 700 TVL with such a basic mod. At least in the case of very cheap sets, I would expect the quality of the electronic components and of the optics * to be other relevant factors in limiting the TVL count, so that reducing the spot size will surely achieve thinner scanlines (thicker blanklines) but not increase the TVL count substantially.

* this is brought up on the first article linked in the OP:

Spoiler: show

This small excerpt is also worth a read for its clarity (taken from 'Image Performance in CRT Displays', page 47):

Spoiler: show

An article that goes into more detail about optic-related factors than the one cited above can be found here.

Edit: incidentally, but mostly unrelated to the thread, I've just bumped into this article on reducing the spot size by acting on the yoke rather than the anodes (unfortunately, it is in japanese)

Josh128 · Joined: 16 Jan 2014 Posts: 1036

Syntax wrote:

LukeEvansSimon wrote:

Syntax wrote:

Ill bite.

All you are doing is sharpening the image.
Reducing dot size reduces bleeding and increases the dark areas around each pixel. It reduces brightness somewhat though.

Would make a really poor set sharper sure, but it's not increasing tvl at all.

A smaller spot size doesn’t increase resolution? Please inform Texas Instruments and Display Labs Inc that their published research on CRT technology is wrong. See, I provided authoritative references for my statements, and you just made a statement without any references to back it up.

You will only see a resolution increase if you move the now smaller dots closer together, how will you do that?

All you've suggested is to make them smaller and increase the dead space around them.

Why would I need to give reference for something as simple as that?

This is an interesting question, and perhaps either OP or you could shed some light on what makes the difference between the granularity and spacing of horizontal dots or pixels on a line between say, a consumer Trinitron and a BVM Trinitron. Is it not simply the pitch of the aperture grille and its distance from the phosphor coating of the screen (for focus)? Or is it the phosphor coating itself? Its certainly not the input horizontal resolution because feeding each of the sets with say a standard SNES 256x224 yields vastly different results in terms of horizontal granularity, which appears to be different altogether from focus.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

SamIAm wrote:

There is a lot that I'd like to respond to, but this has to come first:

LukeEvansSimon wrote:

A 15khz, 240p display can only address 480 vertical lines that span the width equal to the display height on a 4:3 CRT. I spoke to this in the OP. When you play 240p content on a 700 TVL, 15khz display, you get a sharper image due to two factors: smaller spot size and smaller dot pitch. Your game is not actually able to alternate 700 white and black vertical lines in a horizontal width equal to display height. If you read my references, you’d know this.

I've read this a few times, and each time I'm left wondering if you really said what you seem to be saying.

You're not telling me that I could connect a pattern generator with an adjustable dot-clock to a pro monitor spec'd at 700 TVL and not be able to see, with my eyeballs, above 480 alternating vertical lines (in the relevant horizontal space) just because the monitor's horizontal refresh rate is 15khz, are you?

Because that would be a very extraordinary claim.

Is your pattern generator running on a 240p/480i gaming console/arcade? Again, you are confusing addressability with resolution. The OP said that addressability is not increased, and since the OP is only scoping to 240p and 480i gaming content, addressability is off topic. None of the 15khz gaming hardware in consideration can address 700 TV lines. The high TVL look for 240p content is influenced by spot focus and dot pitch.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

Syntax wrote:

LukeEvansSimon wrote:

Syntax wrote:

Ill bite.

All you are doing is sharpening the image.
Reducing dot size reduces bleeding and increases the dark areas around each pixel. It reduces brightness somewhat though.

Would make a really poor set sharper sure, but it's not increasing tvl at all.

A smaller spot size doesn’t increase resolution? Please inform Texas Instruments and Display Labs Inc that their published research on CRT technology is wrong. See, I provided authoritative references for my statements, and you just made a statement without any references to back it up.

You will only see a resolution increase if you move the now smaller dots closer together, how will you do that?

All you've suggested is to make them smaller and increase the dead space around them.

Why would I need to give reference for something as simple as that?

Being able to move the TV lines closer together is determined by addressability, not resolution. The term”resolution” has a specific definition in CRT literature and it is not the same thing as the addressable resolution or input resolution. None of the retrogaming 15khz retrogaming consoles are capable of addressing more than 480 TV lines.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

The research paper on using yoke voltages to decrease spot size is interesting, but the entire paper is in Japanese. Can somebody get an English translation? I am trying to compile and read as much CRT literature as possible. It is a shame if we lose all of this tech AND the knowledge that goes with it, because that means we will never manufacture this stuff again if the knowledge is lost. Also, there are going to be cool mods and projects that come from better community understanding of the tech.

I own a BK 490b CRT analyzer, and I have used it to analyze and rejuvenate old CRTs. It has a setting that allows for testing the CRT with different G1 voltages applied. In vacuum tube literature, this is often referred to as the "bias" of the tube, and the bias is typically ground (zero volts) or it is negative.
Sencore made the highest end CRT analyzers and rejuvenators, and their literature here provides more explanation of how the G1 anode's negative voltage is used as an iris.

Spoiler: show

This article, on the CR7000, remarks that high resolution CRTs use a negative bias (that is a negative G1 voltage):

Spoiler: show

Xer Xian · Joined: 06 Feb 2005 Posts: 859 Location: Italy

LukeEvansSimon wrote:

Being able to move the TV lines closer together is determined by addressability, not resolution.

I'm no expert but I would say it's the other way around - TV lines measure resolution. Unless you're trying to say something else here?

Also, it somehow slipped my mind but upon rereading I realized that my post pretty much reiterated what SamIAm said, which makes me think some of the confusion expressed here was warranted. What you're trying to do should be clear by now, but the thread title should probably be ".. to make it high TVL-looking".

maxtherabbit · Joined: 05 Mar 2018 Posts: 1335

if you visualise a beam sweeping continuously across each horizontal line at a fixed speed, and then reduce the diameter of the beam, you should easily be able to understand how that would accommodate more horizontal detail if the source dot clock is sufficiently high and the mask pitch is sufficiently small

SamIAm · Joined: 03 Mar 2005 Posts: 455

Well, I think I get what you want to do, and I don't think it would be productive to sit here and argue about wording, so best of luck to you. It would indeed be neat if what you're theorizing turns out to be doable.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

SamIAm wrote:

Well, I think I get what you want to do, and I don't think it would be productive to sit here and argue about wording, so best of luck to you. It would indeed be neat if what you're theorizing turns out to be doable.

There is value in reading and discussing all of the published research on CRT technology and then designing mods. This is how the RGB mods came about. This ideation process is how other mods will come into existence. Also, the CRT gaming community would benefit from using the standard terminology from the published research literature when talking about CRTs. It is clear that many in the community have no idea that their high TVL CRT's addressability is not what gives the pleasing high TVL aesthetic for 240p and 480i gaming. Your Genesis and SNES cannot address more than 480 TV lines. So spot size and dot pitch are what gives the high TVL aesthetic.

The OP mod can be applied to professional broadcast CRTs too to increase TVL for those that want a little more sharpness and thicker unilluminated scanlines. I checked my Panasonic BT-H1350Y's schematic and the G1 anode is tied to chassis ground (zero volts). I then hooked its M34JEF037X picture tube to my BK 490b and tested the cutt-off emissions of each electron gun for various voltages between -100 volts and 0 volts. The most catastrophic thing that could happen if the G1 anode is made too negative, such as -500 volts, would be electrical arcing between the cathode K and the G1 or arcing from the G1 to G2. But a quick test with my BK 490b shows that these tubes can easily handle -100 volts.

I have kids and a job, so getting the time to put together the mod is restricted to a few hours block on Saturdays and Sundays. I've placed a mouser order for the parts: transformer, rectifier, filter capacitor, and high voltage regulator. I will try it with and without a voltage regulator. The benefit of the regulator version will be a very smooth voltage to the G1. The unregulated version will be a cheaper mod, but may cause the spot size to jitter, which will look horrible.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

I finally bought a bench power supply. I should have done this a long time ago. Would speed up experimenting with so many projects. The power supply has adjustable voltage up to 120 volts and current limiting up to 3 amps. I will use a 1N5402 diode to tie the positive output of the supply to the chassis ground that the entire CRT TV's circuit that sits on secondary side of the TV's isolation transformer. I will then experiment with low current and low absolute value negative voltages, slowly dialing them up until spot focus no longer decreases... or I blow up the electron gun

The voltage range I should experiment with is clear to me, but the current range is not clear. My understanding is that the max current should be very low, less than 10mA because the G1 anode is not connected to the cathode or the other anodes. Anybody have a better understanding of the electrical characteristics of vacuum tubes? What is the best way to determine the max current that I should allow to run through the G1 anode?

buttersoft · Joined: 24 Jul 2016 Posts: 295

Current ranges for later, very high-end rejuvenators that had a super low "re-activate" setting (super-low rejuv current for up to 30s) were still only using about 1mA. Not sure if that helps.

I'm not sure i grasp how you're going to reduce spot size. My understanding is that lowering the screen grid voltages will lower the number of electrons and reduce the brightness. This reduces spot size, everything else being equal, because the focus grid is now capable of delivering more energy per electron vs time, keeping the beam tighter. Simply adjusting the brightness or even the Screen control, the latter normally on the flyback of a set, has this effect (and it implies such in that CRT's for medical imaging paper you linked). If in a given set you bypass the HV regulation and simply make beam focus tighter, but keep everything else the same, you'd be lasering x-rays through the leadglass - or having that effect. Note that i said, in a a given set. Different sets are designed with different parameters.

Addressability would simply seem to be a way of measuring the dot-pitch against the chassis bandwidth with spot-size thrown in, i'm not sure i see the big deal there. You might be able to put slightly faster, slightly more expensive transistors onto the neckboard and improve things very slightly, but i don't get how else you'd attack the issue. My feel is that to make any real gains in a particular set using a fixed tube, you'd need to redesign the entire chassis. There's a reason old NANAO/Sega MC-2000S arcade monitors from the early 80's were like 8 times the price of their nearest rivals, eve ntough they were only slightly more reliable and giving a slightly nicer picture, albeit noticeably so. And the same goes for BVMs vs PVMs vs Prosumer vs consumer CRT's.

Don't mistake me, i'd love to be proved wrong, and if you could find a way to make this happen it would be completely awesome.

EDIT: I think the lion's share of what i'm really trying to say here is that surely every consumer set is designed to run up against its limits in terms of HV regulation and x-ray production. If you dial up the brightness you increase the grid voltages, if you increase the grid voltages you increase the beam current, and if you increase the beam current the focus voltages drop to spread the beam wider and reduce x-ray production. Though now i read what i've written, i want someone to help me examine that last part. Does spreading the electrons out reduce x-ray production? Perhaps it's simply that the focus winding in the flyback is fixed (at any given time) and as you increase beam current it's just not able to deliver as much energy? Any thoughts on that? Surely it can't just be phosphor halation? No matter how you dial the knob around, a very bright image is always in poorer focus than a dim one. But why not simply add more energy? There'd be a reason why, and that reason is usually to do with regulation. Along with which there must physical limitations to focus imposed by the quality of the neck assembly in CRT tubes. And how exactly do the bias and drive voltages figure into this? Either way you look at it, changing the focus alone, or the G1 or G2 alone, or any other important part of the regulated circuitry, is likely to have secondary effects.

I should add that i'm using an unusual chassis in one of my arcade machines, and slightly overdriving a consumer tube to about 105% brightness. The resulting x-ray production isn't geometric/much increased, but going any higher, to say 110% brightness, spikes x-ray production to about 200%. So it might be worth measuring for any side effects like that, if you do achieve a noticeable gain in one parameter.

LukeEvansSimon · Joined: 17 Aug 2020 Posts: 58

Each grid in a CRT serves a different purpose. G2 is a particle accelerator that also determines how many particles get pulled off of the cathode. G4 is a particle accelerator. G1 acts like an iris, and G3 acts like a lens. So while an iris does increase and decrease the amount of particles that pass through it, it also filters the particles down to those that are more uniform in their motion along the beam. This causes the spot size to be smaller AND it causes the depth of field for spot focus to be larger. G2 does not have this iris effect because of both the shape of the anode and its positive charge.

Reading published literature for high resolution CRTs, it looks like it was common to have G1 set to a constant -70 up to around -100 volts so as to keep spot size small.

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Mod a CRT to increase its TVL

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