[Herald1360]

Hopefully not adding any further confusion, my understanding of this set is that the original indirectly heated ouput valve has been replaced by a directly heated one- the AZ 1. Looking at the schematics, it looks like some attempt has been made to avoid potential hum problems by adding a resistive centre tap to ground on the AZ 1 filament since the usual centre tapped heater winding used with this type of valve is not present. This may have been done incorrectly.

If the output valve has its own heater winding on the transformer, it should be possible to sort it out fairly easily but if it's commoned with the other valves' heaters and they have one side grounded, they will need to be connected via individual twisted pair wires to the output valve heater pins and not grounded. It might end up easier to fit another indirectly heated valve for the output and go back to something closer to how it may originally have been built!