TV works great for a few minutes then the damper tube plate gets red and sparks fly in horizontal output tube 6LB6, of which I have lost 2. Hoping it is not a flyback issue, I will start replacing the disc capacitors in the damper circuit. Shorting capacitors on the boost line are suspect as well. I have R.L Goodman's Zenith service manual volume 2. If you have any ideas, please reply.

make sure you replace the damper tube along with Ho tube when you first noticed it. The two work in tandem.

Hope you can find what's loading down the boost. I hate the kind of problems where you have to risk destroying a hard to find part in order to solve a problem.

This set saw 14 solid years of use, then sat for 20+, turned on only a few times.
That is when funny stuff seems to happen to our "museum pieces".

The problem is intermittent and I wish I could fuse something before I would lose another tube. The original problem was a bad 6CJ3 damper tube, I have several of them, new and old. First suspect is the 2kV 120pf C298. Also the 6HV5A may be to blame.

At least with my RCA CTC11 and CTC12 sets, I have 10 of any tube and multiple service manuals.

I had a zenith do that It was an open 1meg resistor in the regulator circuit causing the regulator to clamp super hard and load the HOT to death. I would try and fuse the B+ to the damper at about 300ma. Its better to fuse the HOT cathode but that may be almost impossible on a Zenith.