[Amps] Filament transformer question

[jonathan white]

Hi All.
Can someone clear up a point regarding centre tapped filament transformers.
I am having one wound for my 4x 813 amp,where I need 10v at 20amps.
I told the winder, and he wants clearing up does it mead 5-0-5v ,or 
10-0-10v and bias via a G3SEK board, to centre-tap of transformer.(I 
thought it was 10-0-10volt) so maybe its time for me to wear the dunces 
hat! Im just used to 6.3/12volt heaters.
Sorry for sounding dumb, but my epilepsy has been playing havoc with me 
today and just need some guidance.
Oh ,the tubes/valves  turned up today ..They look better than I could 
have imagined
cant wait to get building.
                                       73`s Jon G8CCL