Got a Zenith H2438 porthole with some sweep issues

[AmigaMan]

Hello all. I recently picked up this beauty for free from a video artist's basement. It's a 1951 Zenith H2438 with a 24H20 chassis, which uses the 16GP4 metal cone CRT. The tube is in beautiful shape, with amazing brightness and sharpness. It's labeled Sheldon, so I assume it was a replacement or rebuild.

I managed to work out most of its problems. I replaced the electrolytic and paper capacitors, replaced the weak vertical integrator, cleaned the tube sockets and pots, etc., but it has a slight blip in the horizontal sweep, visible in the image as a vertical light line, around which the image is distorted, as if the sweep slows at that point in its horizontal travel. The problem is intermittent, and it changes in severity abruptly. It can be seen clearly in the second photo.

One of the two 6BQ6 horizontal output tubes has a slight intermittent leak to the cathode as determined with my basic tube tester, and I believe that to be the fault. Any ideas on that? I'm off to a local electronic component store to search through their vast collection of unorganized tubes to find a replacement, so any simpler ideas would be wonderful.

Is there anything else I should know about these sets in general? I'm very knowledgeable about television broadcast/studio gear, but this is my oldest consumer TV.

Photos are attached.

Thanks!
–Tom