Jadis DA30: fuse keeps blowing

Since 8 months I own a 2nd hand Jadis integrated class A tube amplifier. It has worked fine until recently, when the left channel stopped playing, some 60 seconds after power-up. I opened the case, checked for any blown/ destroyed/ burned component, but the only thing blown was the left channel fuse. This I replaced and all was well again. Unfortunately, a few days later the left channel fuse blew again, within 60 seconds after power-up...

I have switched all the tube pairs (left pair to right and vice versa), but the left channel fuse keeps blowing, ruling out a faulty tube. Please note that after replacing the fuse, the amp is playing fine, but after some on/off cycles the fuse pops within a minute after power-up.

Has anyone experienced a similarly vague problem with a (Jadis) tube amp? What could be causing this?

Thanks for your assistance! Bernhard