JTM45: hums when jumping channels

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hi forum,

this is also my first post here so: hi!

anyway, i recently finished my first JTM45 build and am having a weird issue when jumping channels, as described in the title. when using one input, the amp doesn't produce any notable hum. not even with all controls at 10.

the only noise coming from the amp in this case, is some hiss in the treble channel (as i would have expected anyway).

now as soon as i jumper two channels (any combination of I and II), the amp produced a loud hum. even with no guitar plugged in. by lowering the volume in either channel you can tell it's just as loud with either one.

i've checked all the wiring and connections, especially the input section. since i can't find a mistake myself, i'm figured that somebody with your experience might point me in the right direction.

thanks in advance!

cheers,
christian