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Old 2nd Aug 2022, 9:51 am   #21
Heatercathodeshort
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Just to add. The picture is from a PYE LV30 [1950] The tube is a Mullard 9" MW22-18 non aluminised with a straight gun. The stain is not so prominent in this shot usually forming a perfect circle in the dead centre of the tube most easily seen in the Mazda CRM121B series. Tiny molecules of air within the tube tends to hasten the appearance of the stain hence the advantage of a very hard vacuum.
The circle is formed due to the damaging ions being much less deflected by the electron stream.
The higher the EHT the less you will see the 'stain' due to punch through.

The second picture shows a GEC BT1748 fitted with an aluminised triode tube that is 65 years old! No stain. John.
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Old 2nd Aug 2022, 9:56 am   #22
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Pictures.
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Old 2nd Aug 2022, 10:19 am   #23
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Quote:
Originally Posted by Heatercathodeshort View Post
The higher the EHT the less you will see the 'stain' due to punch through.
Hello John

By "punch through" do you mean that the ions pass straight through the phosphor due to greater velocity and hence energy than when accelerated by a lower EHT voltage?

I'm intrigued by this.

John
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Old 2nd Aug 2022, 10:43 am   #24
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Not absolutely certain about the exact happening but I would guess that a more intense beam current may bring some of the 'dead' phosphor back into action, rather like being hit by a brick rather than a tennis ball.
Just a pure guess but it is rare to see heavy staining on a PYE D16T with 6kv of mains derived EHT but very noticeable with a receiver operating on around 5KV particularly the Mazda series.
The stain tends to take on a bluish tinge due to what could be described as 'poisoning'.

Just a point but it is very important that the ion trap is always adjusted for maximum brightness and not used to correct focus, corner cutting etc.
Incorrect adjustment can lead to permanent astigmatism and a 'splat' shaped ion burn if the tube is not aluminised.

I have no technical proof for this but can only guess that the intense off beam electron stream being pulled off line damages the tiny hole that the beam has to pass through in the grid of the gun assembly, making it slightly oval after a long period of time. I have seen this several times with the MW22-16 in Bush TV22s. In every case the ion trap has been offset to quite a large degree. Correcting this produces a very watchable picture but it is difficult to focus both vertically and horizontally together. Just my thoughts.

Don't confuse the tiny ring magnet that is fitted to mainly early GEC triode tubes. This is not an ion trap and is usually fitted to tubes that require added correction for corner cutting and overall focus. John.
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Old 2nd Aug 2022, 11:06 am   #25
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

I am sure I have seen photos of damage to the tiny aperture due to incorrect ion trap adjustment. Of course I can’t find them now.
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Old 3rd Aug 2022, 10:31 am   #26
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Yes I know the problem. You file away for safety items in ring binders but forget to update the index.
If you do find the pic, it would be interesting to see it. Regards, John.
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Old 7th Aug 2022, 4:38 pm   #27
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Default Re: Ion burn, When did aluminised CRT's start and become the norm?

Certainly amongst the first tubes to have aluminised screens were the tiny ones used in projection tv sets. These had a straight gun and no ion trap. As they operate at 25KV they would have been ruined by ion burn in 5 minutes without the aluminisation. With aluminisation they made it to at least 6 minutes
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