66 zenith high voltage dies after 30 minutes

[drh4683]

Zenith reported a strange loss of high voltage symptom on the 26KC20 chassis to be caused by a gassy 6HL8, 1st color amp tube. I think the 25MC36 uses a 6KT8 for that function, but its worth a try anyway. I thought there was another case history where HV was lost caused by some other fault in the chorma circuit and it was a tube also if I remember correctly such as a demodulator. This is possible since your set has a color problem just moments before the HV is lost.
I'd swap out those tubes immediately after the bloom out and see if operation is restored. These Zenith tough dogs are the best ones to figure out.