[morzh]

700 Ohm has nothing to do with what you need. Or almost nothing.
You size up your transformer by the allowed wattage and then see if at your load impedance (say 4 Ohm) the input of the transformer gives you your 10 - 12kOhm.

So,
http://www.hammondmfg.com/pdf/5C_125SE.pdf


Step 1: get your speaker voice coil impedance (what is it?). Say it is 4 Ohm.
Step 2. 4 Ohm will yield 10,000 Ohms primary impedance if connected to BLK/ORG leads.

If your Voice coil impedance is different, see the chart for what you need. Basically the PRI impedance = VC impedance times square of the turn ratio.