No, it was voltage headroom I meant. 6.3v windings have a peak voltage of 8.9v, a normal silicon bridge will drop about 1.2v, so that leaves 7.7v into the regulator. If you want 6.3v out that only leaves 1.4v across the regulator. Most regulators need 2v across them to regulate. Its doable, and thats what the Tent Labs unit does, but you need to use either a discreet reg, or a LDO regulator and low voltage drop diodes.