[MrFixr55]

If the 2.25V winding is not being used, the center tap wire can be insulated by taping off.  If the 5V filament winding has a center tap, DO NOT GROUND THAT.  Remember that the Rectifier filament is at B+ level (180-300V) above ground.

Remember also that the 71A and 26 units must be powered by separate power supplies whose DC outputs are ISOLATED from chassis ground.  Ideally, each DC output should have a center tap by putting 2 20 Ohm 5 Watt resisters in series and placing this resistor pair across each of the 2 DC outputs.  The junction between these 2 resistors is the "center tap" to connect the cathode bypass resistor for the 71 (on the 5V supply) and the 26s (on the 1.5V supply).  This duplicates what was done on the original Radiola 18 Power Supply.  However, since these are DC supplies, the cathode bypass resistor can be connected to one leg of the supply.  Per the RCA Receiving Tube Manual Version RC10 (1932?), this should be the negative leg of the supply for both the '26 and the '71A. Refer to the diagram below.  This is from the Radiola 18.  The rest of the radio circuit was removed for clarity.

RCA Radiola 18 PS and Audio.pdf Size: 90.57 KB  Downloads: 74

The Radiola 18 transformer is likely a holdover from the Radiola 17.  The Radiola 17 used "hum balance" rheostats that were likely deemed to not be worth the small amount of hum that can be eliminated by this adjustment.  The fixed resistors were cheaper than either the rheostats or redesigning the transformer with center taps.  This is why there are no center taps on the filament windings of either the Radiola 17 or 18 transformers.