I have been having a very specific problem with my DT 50 112 combo. Basically, there are days when the amp works just fine without any problems. Then, all of the sudden you hear a single cracking sound similar to the one when changing channels, and then the amp stops working. What this means is that all lights are on and everything seems to be fine, except there is no output at all. I can be changing channels, topologies, stand-by, turning all knobs left and right and still there is no output. The only thing I can hear is this unnatural humming noise coming out of the speaker. Sometimes, it is enough to turn the amp off and on and then it works again, sometimes I need to wait much longer. I've taken the amp to an authorized service place in my country, but they couldn't find the issue. They changed the valves tough, but that did not really help. Has anyone experienced this kind of issue before?
Here's a link to a video showing the problem: https://drive.google.com/open?id=0ByoVl89Vpu7sLWcteDRKbk1YNGs