[miniman82]

Sagging there indicates the second plate on your 6CG7 is being dragged down, in other words the tube is being turned on hard which lowers plate voltage. You have no waveform, the oscillator is not running. I would expect grid voltage on the same triode section to be high, confirming the stuck 'on' diagnosis. You need to have a close look at the components around that tube; drifted resistors, leaking caps, bad grounds, the works. Synchroguide circuits are fiendishly simple, which paradoxically makes them difficult to troubleshoot at times because there is so little to go wrong.