Atwater Kent 20 was produced in two versions : with a big case, like this
one, and with a small case as in the compact version.
It works with direct amplification coupled cascade stage circuits,
The third tube is a detector and you may use a 01A, a 00A or 12A.
The horn speaker is an Atwater Kent mod. M with 37,5 cm diameter bell and
63,5 cm overall tall.