Jensen Passive guitar splitter hum problem

Hi, I've just built this passive guitar splitter:

http://www.jensen-transformers.com/as/as013.pdf  

I have a perfect hum-free signal when the clean channel is on but when I switch to rythm or lead channel I get a massive loud LF hum.

I've tried different amps, - same thing happens

I've checked the relative simple circuit and nothing seems wrong (I've also checked the insulation of "out 2")

The splitter is placed right after the guitar.

I use the old version of jensen 11-P1 transformer.

What could be wrong?

Best regards

Thomas