Fryette 2:90:2 Hum Ground noise

Hi
I've just scored a mint Fryette 2:90:2 with JJ kt88 in it
(I already own one and wanted a backup for shows)
I've just noticed a weird hum noise problem when I switch both channel A and B on at the same time
If I only play through channel A or channel B alone (other one in standby) , there's no noise at all.
I'm not sure if it's a tube problem or something worst
Here's a video of the problem

What do you have it hooked up to? Sounds like a ground loop.

absolutely nothing
no input cables
just output to different cabs
same noise happening whatever the cab is and my other poweramp plugged the same way isn't making any noise with the same cables.

I've tried swapping power tubes and the noise seems to be alot less with my other amp matched sovtek kt88 tubes.
I think I'll need to order new matched power tubes