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Well for starters you need to have around 6.3 VAC on pin 3 or the tubes not going to work properly. Pin #3 is going through a choke (L12) to the other tubes in the set. So I'd check that choke and also measure AC voltage in the heaters of other tubes in the set.
If they're all low, could be a problem with the power transformer. Or possibly something is shorted somewhere and really dragging down the filament supply.
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