An ordinary filament transformer with a 120 volt primary and 12.6 volt
secondary will work well. The turns ratio is about 10 to 1 and that
yields an impedance ratio of 100 to 1. That will match a 600-ohm
output to a 6-ohm load and will provide a decent match to an 8-ohm or
4-ohm speaker. A small 1 amp. transformer is sufficient. --Ed