[Mondial]

Impedance is a function of the transformer turns ratio squared. For example if you have an output transformer and apply 1 VAC to the secondary and measure 40 VAC on the primary, then the turns ratio is 1 to 40. The impedance ratio will be 40 squared or 1600.

So if you connect a 4 ohm speaker to the secondary, the impedance seen at the primary will be 4 X 1600= 6400 Ohms.

Typically, the output transformers for small tube radios were designed for speakers of 3.2 to 4 Ohms, so pretty much any used one can be measured as described above.