Check emissions to ensure it isn't substantially lower than the green gun. If emissions looks fine, then I'm positive you have damage from arcing. Unfortunately, either way the long term prognosis isn't good. I strongly recommend you really use the hell out of that set while you can. No sense in "saving" cathode material in a dying tube.

I really do hope it's not the tube, but if everything tied to the various elements of the red gun is fine, then that only leaves the CRT.

I have no way to measure the green gun without cutting wires. But I
measured the cathode resistor of the red gun and got 600 microamps ..
which isn't great but that was average with a mostly white screen.

I'm actually beginning to think I'm being paranoid. I will know for sure
after the next ETF convention when I will take several pictures of all their
15GP22s under the exact same conditions as mine ... same camera
settings, same picture of Dorothy.

Just keep in mind, a 15G can go at any time. Ed Reitan lost at least one in a restored set, and both my Uncles lost a couple, seemingly at random....
I'd still want to know what the three guns look like relative to one another on a tester. If Steve said all three guns tested ~90%, and then you find the red gun much weaker relative to green and blue, then I think paranoia is out.

How about posting a photo of Dorothy on your CT 100?

I've already long ago posted a picture of Dorothy that look just fine.
She looks just as good now.

That's not the question, which is "how bright is she now compared to then?"

I just realized I have the old .cr2 file ... I'll compare to a new
one adjusted to look the same!

Answer: a factor of five. The red gun max is down by five.

I am now convinced that an internal arc did it, at the time the
first fuse blew.

So I will just rarely demonstrate my set in a dark room, and
await and pray for rebuilding.

I can relate to that sorrow
 

I am always amazed at all you felas do for those very first color sets. I just hope to see one operate in person one day. It must be wild to see one perform as new.

Mine did. Well, I have no idea about "like new".
"Perfect" was more like it. Not perfect on color bars (ringing) but on
ordinary test static pictures, yes, especially near perfect color.

My avatar is a very easy scene, however.

No chance of getting to Hilliard?

Don't hold your breath on 15G rebuilding. Even of it does become possible, I doubt the yield of usable tubes is going to be terribly high. Subjecting an under vacuum tube to the rebuilding process would be an enormous risk. That said, buying a "good" 15G is a massive risk anyway...

Enjoy that 15G while it's still possible to do so. If the arcing is a gas issue, and I strongly suspect it is, you're saving cathode material in order to light up a purple neck. If you run the S.O.B into the ground, at least you'll have enjoyed the set more. Either way, the outcome is the same: you sit with a dark set and await the rather unlikely rebuild process.

Running a gassy tube also may slow the advance of the gas issue by exciting the gas into moving more and thus being more likely to reach and be absorbed by the gether.

The tube shows no sign of gas. No blue glow and no problem with focus
whatsoever, sometimes I see the scan lines. Nor do the blue and green guns
show gas signs.

I'm in no hurry to get it rebuilt. I'll just watch the super bowl on my
55 inch TV. I just wanted to own such a set .. and I do note that it does
display pictures quite nicely, just dim (with blue and green throttled
back to match red).

Going gassy is a process, not a single event. It's possible to have a tube that's going gassy with no blue neck, no focus problems, etc. Just periodic arcing as the first symptom. Remember, every gassy 15G out there (and there are a lot of them) was good at some point.

I slowly lost a 16AP4 to a leak. The first symptom was (probably, looking back on it) arcing. It became more frequent, then it got very dim and lost the ability to focus. Now that said, it's been a SLOW process. A few years minimum...

Ivery much hope to go to hillard!
 

Thanks for reminding me.