Oh dear, as if this stuff isn't hard enough I think I have been using a faulty multimeter...
I have retested voltage using a new meter at now the Voltage across the heaters (Vh) for all the valves all meet their specs at the 245V dial setting. Slightly higher when I use the 200V setting as you would expect.
AZ31: Vh = 3.8 VAC @245V ; 4.8 VAC @200V
EBL21: Vh = 6.2 VAC @245V ; 7.6 VAC @200V
ECH21: 6.2 VAC @245V ; 7.6 VAC @200V
However this time I am getting
NO AC voltage drop at the anode for AZ31 with the new meter (e.g. chassis to pin 4 or chassis to pin 6). The new meter is working because I can measure 240V AC at the mains input.
The old meter is still recording over 300V AC from chassis to anode!!
Interestingly, when I set either voltmeter to DC I measure about
-300V from chassis to anode on AZ31.
So perhaps my AZ31 isn't faulty. Its simply not had voltage applied properly. The transformer does appear faulty.