Zenith 24H20 help please

[PaulOF]

Kind people, I'm out of my depth.
I finally had my Zenith porthole (chassis 24H20) working just dandy on the bench with a little 5" test CRT. Nice picture, good hold, not too much buzz.

So I reinstalled the big tube, slid it into the cabinet, popped on the ion trap that banderson so kindly provided me with and fired it up.

As soon as the tubes warmed, I saw sparks dancing in the neck of the CRT and heard a loud whine at what I would guess is around horizonal retrace frequency (yeah, I can still hear 15 Kcps if it's loud enough). Turned it quickly off but still got the smell of hot resistor. As I say, I'm out of my depth. My experience prior to this project is all radios and phonos.

So, any idea where I should look? Could this be a symptom of a dead 16GP4?
Inquiring minds want to know.
Thanks,
Paul