Just how bad can an Ion burn be?
 

This bad.

Dumont 12JP4 installed in a Hoffman CT-800

Every 12JP4 I've had the misfortune of coming across has had this burn to some degree or another, this one is the worst.

It also had the worst emissions so probably a high hour tube, I cleaned it with the Beltron to get some brightness out of it, it really just made the spot look worse.
It's Green because it's a Hoffman.

http://videokarma.org/attachment.php...1&d=1491268776

A bit off topic, but...
...there is some strange bug in my brain that always causes me to, whenever I look quickly at a picture of a GE 801 TV, I always see it as having a big ion burn on the screen on the left.

https://s-media-cache-ak0.pinimg.com...2290efe8d4.jpg

Rip.

Thanks Dave, I needed a laugh.
:D

TV monster is bored. TV monster wonders what other living rooms are on. :D
http://www.videokarma.org/attachment...5&d=1491268752

Ion burn seems to be a recurring problem with DuMonts and Andreas.

For me, anyways.:smoke:

Ion burn is just a consequence of not having an ion trap or an aluminum backed screen. The ions are too heavy to be deflected along with the electrons, so you get a big nasty spot on any trap-less, non-aluminized magnetically deflected CRT. All 12JP4, 15AP4, 20BP4, 12AP4, and 9AP4s will burn sooner or later.

I seem to remember reading about an "X" shaped ion burn in a very good book by Robert G. Middleton, saying that it would happen on rectangular screen sets. I've never seen one nor ever heard anyone mention one before. Has anyone seen one?

Pretty, sure whoever you heard that from must have been mistaken. First off as Ben already said ions can't be deflected (too much mass), so the burn is limited by the scatter pattern of the electron gun. By the time rectangular CRTs were in consumer sets ion traps were standard, and aluminization was catching on...

Nope, nope.....had the book (sort of) handy. See?

Well, you can deflect an ion, just do it electrostatically. Charge predominates, and since an ion would have a magnitude of charge greater than or equal to the elementary charge, they'll zip right along with the electrons. This is why a great number of 7JP4s show decent emissions but look like garbage; they have a "whole-screen" ion burn.

Aluminization was becoming standard, the ion trap had been a standard for many years. In addition, I can't think of any mechanism that would lead to an X shape, aside from some element of the gun acting as a sort of crude, unwanted collimator for the ion beam.

That page from the book is puzzling.

Edit: If it is more likely on a rectangular tube, then it likely has something to do with electrostatic focusing or self focusing elements. Weren't the vast majority of the 1946 to circa 1950 round B/W tubes magnetic focus?

It's strange that Dumont, the Cadillac of Televisions, went with such an oddball CRT.

Not only is it not Aluminized, it's also made of thin glass and has a bulbous pre war shape, it's only advantages I can see is it's lighter, and they didn't seem too concerned about weight.

Others like Zenith and GE were using Aluminized tubes as least as far back as 1948. I wonder who had the Patent on that?

It's an oddball because Dumont made their own CRTs. My guess is Dumont was still using their pre-WWII (pyrex) tooling and methods for a while post war.

IIRC Dumont's CRT tech was state of the art pre-war (they had the biggest domestic production CRT 14"). Post war I think they were the first with a 20" CRT so they still lead, but as others made advances they were slow to latch on and mix in other's tech.

DuMont was building radar and CRO tubes for the war effort. Most of these tubes are just P4 versions of those same tubes. I have a collection of in-house datasheets with developmental numbers from '44 or '45. It's easy to match the 12JP4, 15AP4, and 20BP4 to the developmental types. They also pioneered the lackluster 7EP4 (rapidly displaced by the 7GP4/7JP4). Some of the developmental types died off however. It's interesting reading.

I must have had that book at one time. The page looks familiar and I too remember about "X" ion burns. I think it is because the ions are deflected some and they kind of lag behind the electron deflection, the sweep being rectangular. Maybe the rectangular tubes tended to have stronger fields since the deflection angles were greater because of the more recent design that the round.