that tube would be happy with a 2 k transformer primary which we could step down easily to 500 ohms, (easy math)

2000/500= 4 impedance ratio

turns ratio is sq root of 4 which is.....

2,

so we need a 2:1 transformer with 2k pri z, which handles 20 ma dc,

good news is we generate low ac flux because the output voltage can only be about 20 volts unless somebody overdrives the tube.

you could even go 1:1 as now a days, most stuff could be driven with a 2k secondary,

freq response -high end is cake walk with 2:1 turns,

bottom end needs scrutiny as we have a dc bias,

pick a number between 1 and 20, say 10.

design for 10 hz, 2K z input, 10 ma dc, 20 volts ac, 500 ohms out,

typical 500 ohm output winding is 5 to 20 henries, lets use 20 henries for good bass,

so now we figure pri Z, 4 to 1 ratio, so pri Z = 4*20=80 henries,

we can guess at the size, a 12at7 takes a small core for the reverb trans, so a 625 EI would be overkill, but we do have 20 ma dc, so 312 EI might be to small, so maybe start with a 1/2 inch tongue and see how many turns we need for 80 henries,

50 EI - look up the specs in the Mag Met catalog, all we need is 3 things to compute a SE transformer,

cross section, Henries coeef, and MPL-mag. path length

ok i will be right back...