[morzh]

Eric

Fom the sch it is not clear what your voce coil impedance is.
But the simple way to measure it (not exact but more or less) is disconnect your transformer alogether (it is bad anyway) and simply measure the resistance with an ohmmeter.
Then you may if you choose to multiply it by maybe the factor of say 1.2 for a good measure.

For demonstration let us have voice coil impedance equal 8 Ohms.

Then you go here
http://www.radiomuseum.org/tubes/tube_6k6.html
or to any place that lists the tube's parameters and see what your load is.
In this case your load impedance is 7.6kOhms.


Then you go here

http://www.hammondmfg.com/125SE.htm

And see the matching combination for 125Cse series.
The closest you get is BLK-GRN (10kOhms to 8 Ohms).

And when you know your output power (say 3.4W for 6K6) you go here
http://www.radiodaze.com/category/570.aspx

and buy 125Bse.

You will have to run your measurements but still 125BSE is I think a decent match.

Now those are expensive so you might give it a search to see what else is out there.