CurtPalme.com Home Theater Forum

Melifluonze · Posted: Fri Apr 03, 2015 12:57 pm Post subject:

I think the blooming effect is because when you increase contrast, you are actually placing a higher voltage difference on the plates and accelerating the electrons faster towards the phosphor. Because they are going faster, it takes a higher magnetic field to keep the beam focused. If the focus and astig magnets are not strong enough, the electrons will start leaving the nice, focused trajectory and cause bloom... Just a theory...

Either that, or the change in velocity is out of the range of the profile of the magnetic lens, so it's like the wrong lens is in place... but I think it's probably the magnetics are too weak to keep the beam well formed.

The phosphor burn resulting from heat makes me think that a lower refresh rate is a good thing (less beam passes per second and potentially running at a lower contrast)...

I tried to find out how phosphor wears out, but all I could find was generic statements like "the phosphor looses its ability to emit over time"... That's not very helpful...

redfox001 · Joined: 16 Mar 2009 Posts: 2251 Location: The Netherlands

Well take a look at your hypotheses. Put on a focus pattern focus it and ramp up the contrast focus it again and see if that takes a different value Wink

I never noticed it.

ronaldus · Joined: 25 Dec 2010 Posts: 183 Location: france

Hi everybody,

I thought I could chime in and give my point of view,

I work myself on a particle accelerator so I have some experience with that. I think it's correct that the required focussing strength from the coil on a beam of particles (in this case electrons) changes with the energy they have. It's harder to bend particles with a higher energy so it would need more focus current. But the electrons get their energy from the HV (34KV in the marquee) That's the particle accelerator. So for me the energy of the particles is always the same and it't 34KeV. To my knowledge with the plates you only steer the intensity of the particle beam (you pull more electrons out of the cathode) Correct me if I'm wrong. The electrons however also interact with each other and the more you have of them in a beam the more interaction and the wider the beams gets and hence you would need some more focusing power. I could also be that the focus amplifier is distorting.

Anyway my two cents.

regards,

Ron.

Melifluonze · Posted: Sat Apr 04, 2015 2:06 am Post subject:

Ronaldus, your two cents is right on (but you knew that). What I said was not quite right, and what you said changes things a bit!

The cathode in the tube is heated and the compounds on it create an electron cloud. An accelerating anode down the gun has the 34.5 KV on it. The electrons boiled off the cathode accelerate towards the anode. There's a control grid (or two?) between the cathode and anode that control how many electrons accelerate towards the anode based on their potentials. There's a hole in the anode that some electrons go through (a tube, I read) instead of hitting the anode itself, and those accelerated electrons form the beam.

The electrons are always accelerated with the same potential (the constant HV established by the good power supply in the projector), so when they leave through the hole in the anode, they have a constant velocity you can calculate. The brightness is changed by how many electrons are allowed to accelerate to the anode which is controlled by controlling the potential on the grids. I wonder if "G2" stands for grid 2...

So, what I said wasn't totally right. The HV doesn't vary, but the density of electrons in the beam does. The energy imparted to the phosphor by the collision of an electron is a constant, because the velocity is set by the 34.5 KV. That supports what you guys said. And now I think I get what "Beam Current Density" is now... When you turn up the contrast, you're making the beam more dense.

So that makes the whole phosphor burn thing a bit more simple, but it's still probably all about how much total energy is hitting the phosphor... and the explanation of less electrons hitting the phosphor per pass = lower brightness at higher frame rates still checks out.

Thanks Ronaldus!!!

ronaldus · Joined: 25 Dec 2010 Posts: 183 Location: france

Hi Melifluonzo,

So one question is still there. What is causing the blooming when you increase the contrast?

I don't know.

regards,

Ron.

km987654 · Posted: Sat Apr 04, 2015 3:56 am Post subject:

Have you looked here?

http://www.curtpalme.com/forum/viewtopic.php?t=2498&highlight=frankenyoke

redfox001 · Joined: 16 Mar 2009 Posts: 2251 Location: The Netherlands

Just some information. Yesterday I tried 72 Hz on the Marquee 9500 to compare with 60 Hz and i was kind of tired after some time comapring a 909 with a 9500 but it seemed the Marquee had more punch at 72 Hz. I achieve something the same when I take the 909 out of economic and set it to light boost mode. So 72 Hz and I might be fooling myself because I was tired but it seemed to have a different kind of punch than hightening the contrast could achieve. Anyone else seen that?

noos@xp37+ · Joined: 17 Jun 2008 Posts: 464 Location: Berlin/Munich

redfox001 · Joined: 16 Mar 2009 Posts: 2251 Location: The Netherlands

redfox001 · Joined: 16 Mar 2009 Posts: 2251 Location: The Netherlands

He it is a small world backslap got that 8500 from me! Did it still have the hd145 lenses?

barclay66 · Posted: Sun Apr 05, 2015 4:15 pm Post subject:

noos@xp37+ · Joined: 17 Jun 2008 Posts: 464 Location: Berlin/Munich

@redfox: So you are the one who also posted the huuuge measurements of that machine in the Dutch forum? To my knowledge, this was the first comprehensive test on P43 phosphor. I remember it was equipped with HD144s, but I would have to ask if I want to find out. The reason for that is that I never picked it up in person, but a friend of mine did (Gerjon aka the "Barco 701 man"). He stored it for about a year (yes, very friendly), before he bought it from me for his new house. Not sure if the project is done. The next time I rent an M3 at Herz, I planned to drive to the North Sea (and maybe buy it back). Late 2002 Ultra model....

@Barclay66: Nice!
Needed some time to understand "Frankenlenses" - but that description is a bulls-eye!
I will not have the same room as last time, but the one beside it. Some work to do, but higher wall for higher/wider picture and for Dolby Atmos. And I did receive a huge schematics catalog with the HDIH!

redfox001 · Joined: 16 Mar 2009 Posts: 2251 Location: The Netherlands

noos@xp37+ · Joined: 17 Jun 2008 Posts: 464 Location: Berlin/Munich

When my friend Gerjon asked if I would sell the 8500 Ultra that I had not picked up for a year, I said yes because I was specializing on Marquees modified by Helmut Tan for non planar surfaces. As I fully returned to flat screens after having faced physical contrast issues of deep curved screens, I slightly regret any normal machine I dropped.

Always when there is CRT time, I in fact modify my TAN 8500 Ultra for HD144! Adapters are close to be ready, but they needed 6 weeks to dry so far. I used a mod published in a dutch forum, where the plastic housing of the 7" lenses is inserted in a 8" lens plastic housing. The 7" housing gets centered and the 8" housing is filled up with a silicone derivative. My derivative is still not perfectly dry....

I will post pictures when done. And I will do a resolution comparison of water instead of glycol on that HD144 machine. I will try to capture the differences, but I am confident as I expect them to be huge.