I agree with Jjrenman. I would do a bit more isolating first. One of your other components could be at fault. Have you made any changes to your system lately? (New components, fixed something, moved the system, etc)
Schematic for Melos hc 180
I have a Melos hc 180 that breaks up at about 40 watts. Prior to that it seems to do fine. I have checked the tubes and they test ok. If anyone has papers on this amp or has suggestions please let me know. Thanks. Tom