WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
You would find spec-sheets on interstage transformers,not circuit diagrams.
Most of the time they serve as impedance matching devices but many different designs and purposes exist.
On the other hand were you to have a circuit diagram of the amp it would be a lot easier to "guess" what the transformer is doing.
Another suggestion is to carefully dismount the thing,jotting down the way is hooked up,then have it measured by a professional.
Preamp 12AX7, Driver 6c4 then into trans, out of trans to the grids of the tubes. the CT is the biasing. I was wondering of there are any schmatics out there thats an amp design with interstage/coupling transformer.