[julie_m]

That's true enough. ISTR Ed advising that you can add an extra 6V3 voltmeter supply winding to one of his Sussex valve tester transformers with just 19 turns, poked around the bobbin and pulled tight against the core. Even if the laminations in the old core are only half as good as the modern steel alloys in the Sussex laminations, that would only be about an extra 40-odd turns to add in series with an existing 6.3V heater winding, so it's not unfeasible -- assuming you can find enough room.