12LP4 post mortem

[jeyurkon]

Originally Posted by jr_tech

Bill...

I have posted a picture of the gun in a 10FP4 that is in a GE 10T4 set that I am working on. The Grid cup and G2 show plenty of yellow to blue color... does your gun resemble this? This tube is not gassy and is a strong performer.

I have always thought that the colors that I see here were developed during pumping due to the RF heating of the gun before it was sealed off. I believe that in those days, gun parts were NOT hydrogen fired before processing. But this is only a guess.

jr

jr_tech, the vacuum would haft to have been very bad for the electrodes to end up looking like that after induction heating. It wouldn't explain the graduation in color that we see either.

I've done a lot of evaporations using boats made of various metals in vacuums as bad as 10**-5 Torr and have never noticed anything like this.

Maybe when they activate the cathode and convert the carbonates to oxides the excess oxygen that is released isn't pumped away quickly enough. It would require a lot of material to do that though.

But now that I see your photo I think my explanation was wrong. It certainly still looks like oxidation, but did they silver solder the leads to the gun assembly rather than spot weld them? Doing that in air would account for the coloration.

What do you think?

John

[jr_tech]

Originally Posted by jeyurkon

jr_tech, the vacuum would haft to have been very bad for the electrodes to end up looking like that after induction heating. It wouldn't explain the graduation in color that we see either.

I've done a lot of evaporations using boats made of various metals in vacuums as bad as 10**-5 Torr and have never noticed anything like this.

Maybe when they activate the cathode and convert the carbonates to oxides the excess oxygen that is released isn't pumped away quickly enough. It would require a lot of material to do that though.

How about during cathode "breakdown" on the pumps, when the binder is baked out?... I have seen manifold pressure rise almost to 5 scale during that part of the process, and assume that near the cathode , the presssure would be much worse.

Originally Posted by jeyurkon

But now that I see your photo I think my explanation was wrong. It certainly still looks like oxidation, but did they silver solder the leads to the gun assembly rather than spot weld them? Doing that in air would account for the coloration.

What do you think?

John

An odd part about my picture is the very shiny metal on the grid cup where a lead is attached... was the cup "cleaned" by the welding process?... or perhaps the lead acted as a heat sink and limited the temperature rise in that area, be it by processing or operation of the crt.
What is the melting point of silver solder, these parts usually become red during RF... 700-750 degrees or so, perhaps.

Just for grins, I rechecked the tube with my trusty old B&K 400. It was in the middle of the green "good" scale, after about a minute of warm up... no "hot shot" or "rejuvenate" was used.

jr

[jeyurkon]

It's that shiny spot that made me think it might be soldered. The melting point of BT alloy, which is a eutectic, is 780C. Other alloys are higher.

There must be a clue in the fact that the end nearest the base is darkest and gradually get lighter as you move away from it. I wouldn't expect that temperature distribution from induction heating or activating the cathode.

Maybe another possibility. I forget what you call the glass disk feed-through portion. It's basically a disk until after they mount the gun. Then it's attached to a tube that will later form the neck. Or perhaps done in one step.

When they do that joint that end of the gun will get hot. It could oxidize during that step, depending on how they do it. If they join it to the end of a tube then it would see some flame and get hot. If they shrink a tube down around it and then remove the excess, then it would stay fairly cool.

Does that make any sense?

John

[jr_tech]

Originally Posted by jeyurkon

It's that shiny spot that made me think it might be soldered. The melting point of BT alloy, which is a eutectic, is 780C. Other alloys are higher.

There must be a clue in the fact that the end nearest the base is darkest and gradually get lighter as you move away from it. I wouldn't expect that temperature distribution from induction heating or activating the cathode.

In the processing that I have seen, the RF coil is positioned to heat the cathode/grid cup area the most, and elements further up the gun receive less heating. The color gradient that I observe on my 10FP4 would not seem to be inconsistent with that sort of gradient.

Originally Posted by jeyurkon

Maybe another possibility. I forget what you call the glass disk feed-through portion. It's basically a disk until after they mount the gun. Then it's attached to a tube that will later form the neck. Or perhaps done in one step.

When they do that joint that end of the gun will get hot. It could oxidize during that step, depending on how they do it. If they join it to the end of a tube then it would see some flame and get hot. If they shrink a tube down around it and then remove the excess, then it would stay fairly cool.

Does that make any sense?

John

Yes! that makes a lot of sense, perhaps the gun coloring was produced during the flame seal of the stem/header to the neck glass.

I am now starting to wonder if these early guns perhaps did NOT use stainless steel for gun elements.

jr

[jeyurkon]

I don't know if they did use stainless, but stainless will behave that way.

In making fixtures for brazing in a hydrogen furnace I'd take a piece of stainless that was machined to hold the pieces and then heat it in a muffle oven to "green" it. If I got it too hot it would have the dark color. Just right and it would come out green. You could make it almost any color by picking the right temperature. The oxide layer kept the BT alloy from wetting the stainless during the brazing.

John

[jr_tech]

Interesting, What temp and atmosphere does it take to "green" ss? Is that chromium oxide?

jr

[jeyurkon]

I'm not sure which oxides are involved, but it's an interference film producing the color.

Straw yellow is about 340C and blue/green about 500C, when produced by heating in air.

John