• 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.

Kapton tape for transformers

In an output transformer modern plastic isolation foils indeed will increase intra and inter winding capacitances. That's the reason why some manufactures of OT's for tube powered guitar amplifiers still prefer paper, despite of it's poorer technical specifications.
Best regards!
 
In my big 100W+ SE OPT projects, I'm using a lot of mesh cloths sandwiched with Nomex between high capacitance layers, the whole has a lot of air and epsilon varies between 1.3. Together with careful capacitance distribution, it gets a good recipe to achieve high power transformers when one needs to use a lot of turns and smaller cores.