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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

5 volt filament equivalent to GXU1

Unsure have thought of that....the schematic i have uses 4 in a fullwave bridge....filament supplies are 2.5 volt...no ct...1 for each device would like to use the 4 my solution is to fullwave bridge with solid state diodes at the negative end of the bridge and parallel up the 4 5 volt secondary's ....they have ct to each half seccy should still give the 2.5 volts at 5 amps to a pair of the gxu1,s
 
You would have to draw what you suggest including the CT on the secondaries. For the positive side yes you could do that. For the negative side SS diodes are a good solution and just use 2 tubes. I was thinking you had a CT on the HT part of the transformer. Because the positive side the cathodes are commoned you have options.