I couldn't resist messing with the set a little. Steve was right, it didn't take a ton of insulation. I cut a small notch in the retaining ring and covered the edges with a few layers of this tape:
The 21FJP22A tube survived its cross-country journey and it lights up OK. Strangely, I somehow lost signal since the last time I powered it up on the bench, when I had nice audio and I could see a video signal (with a scope) at the video amp.
No doubt it is something simple, but it's too late to investigate tonight. Anyhow, the CRT conversion was simpler than I expected.
Phil Nelson
P.S. No, I haven't yet played with purity adjustments or any of that other good stuff.