Straight Beam Guns, Ion Trap?

[RitchieMars]

My rebuilt 17LP4 arrived this week. It's an old rebuilt tube that was never used after rebuild. I believe it was likely rebuilt around 1970. The odd thing is that this tube calls for a bent gun and the use of a ion trap whereas this one was rebuilt using a straight gun. It appears that bent guns were becoming hard to come by during that time. What I'm wondering is this: Since the tube doesn't appear to be aluminized, wouldn't that mean there's a good chance that the straight gun will eventually create some ion burn on the phosphor screen?

If so, is there a way I can avoid this? It's been suggested to me that using a pair of ion traps might work by positioning the first to bend the beam to scatter the ions, and the next to bend the beam back into place. I only have one and it's the standard one that came with my Zenith's original 17LP4.

[jr_tech]

Originally Posted by RitchieMars

It's been suggested to me that using a pair of ion traps might work by positioning the first to bend the beam to scatter the ions, and the next to bend the beam back into place. I only have one and it's the standard one that came with my Zenith's original 17LP4.

Clever idea, but it will not work that way. You can bend the electron beam with a magnet, and perhaps bend it back with a second magnet to get the beam to travel straight down the gun structure again, but the ions (being much heaver than the electrons) will be mostly unaffected by the magnets and will just go straight down the gun anyway.
The good news is that more modern processing, better getters, cleaner gun parts has improved the vacuum in CRTs, so that the need for an ion trap is reduced, even though the tube is not aluminized,
jr

[RitchieMars]

Ah, so were these advancements around when my CRT was rebuilt, assuming that was roughly 1971? I just wanted to be sure that my tube was rebuilt properly with all of these concerns accounted for, as I did not know if perhaps there was a chance that the straight gun was a "last resort" for a tube that was considered to be in limited demand by that time. These types of tubes were sold to the guy I got this from because he was moving his business and no longer wanted to drag around all these older style tubes.

But yeah, if they knew what they were doing at that time and had found a way to avoid using ion traps, then I guess I have nothing to worry about.

Another small question; there's two small scratches on the face of my CRT. Can these be filled in with something or polished out? They aren't too noticeable. I was also curious to if you can paint over the old aquadag coating, perhaps with aerodag? The coating is in fine condition, but I was wondering if I could even it out and expand it to the same surface coverage at my original tube.

[eberts]

You will get an ion burn on that tube.
Why don't you look for an aluminized tube ?

[yagosaga]

The 17LP4 is well operated with 14,000 Volts anode current. With such a high plate voltage, you should not see any ion burns, even after a longer time interval. My experience with vintage crts is that those with low EHT have strong ion burns and with higher EHT (above 7 KV) have dim or no ion burns. I did test it with a crt with a strong ion burn at 5 KV. When I increased EHT, the ion burn faded away.

- Eckhard

[jeyurkon]

Originally Posted by yagosaga

The 17LP4 is well operated with 14,000 Volts anode current. With such a high plate voltage, you should not see any ion burns, even after a longer time interval. My experience with vintage crts is that those with low EHT have strong ion burns and with higher EHT (above 7 KV) have dim or no ion burns. I did test it with a crt with a strong ion burn at 5 KV. When I increased EHT, the ion burn faded away.

- Eckhard

That makes sense. Ions will have a very short range in the phosphor. By running at a higher EHT the electrons will pass through the damaged layer of the phosphor and excite the remaining good portion.

I do have ion burns on CRTs that ran at 11KV though. They are for the most part minor.

John

[RitchieMars]

Well, since my tube was rebuilt 15 years before I was ever born ( and the place went out of business just before I was born ) it's not like I'll have anyone to complain to if it does eventually develop a burn for some reason. From the sounds of it, it's not a major concern and even if it does develop an ion burn eventually, I'll just consider it an idiosyncrasy of early television!

[jr_tech]

I found one reference that shows that Superior Electronics (a major supplier of guns to rebuilders) was H2 firing the gun parts "for high vacuum after tip off" in 1960, and perhaps before.
jr