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I would ignore the 1B3 HV rect. It's heater is powered by a winding on the HV osc coil, and thus not part of the series heater chain you're trying to troubleshoot.
If you are using an auto ranging meter try manually setting the range...I've found if there's capacitance on a circuit that I want to measure resistance on then that capacitance and the auto range form a sort of oscillator circuit that prevents any kind of meaningful stable reading.
Another troubleshooting option is to hook the set to a variac at 60V, wait 2 minutes for stable heater operation then check voltages along the chain... Voltage drops across heaters should be roughly half of their normal values at half of normal line voltage...
When I have an open heater in a series set you can flip a coin whether I troubleshoot it using voltage or resistance.
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