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Have you recapped that set? Checked the tubes?
It would seem that you might have a leaky capacitor, or perhaps a resistor or two that have drifted in value.
Does the HOT plate get red?
I laid my hands on a Sams for a slightly later model Zenith using a 6SN7 for horiz osc/disch and a 6BQ6 for HOT. THe waveform they show is a reasonably pure sawtooth, and not one where the top is clipped the way yours is.
Checking the operating voltages against design figures may uncover the culprit. The screen voltage at the HOT could be low. Also the plate resistor in the preceding stage could have changed in value. Also check the amount of developed negative voltage there is on the HOT control grid. The Sams I have shows -15 volts for control and 150V for screen. I feel fairly certain that your 6BG6 should operate at voltages not far from these....
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