Thanks guys, but now the problem is GONE. All I could find was that the first   
   EF86 socket seems slightly wiggly, perhaps it lost contact with one of the   
   pins.    
      
   The other possibility is that I had a longer speaker cable to the channel that   
   oscillated, combined with the fact that the home studio monitors I was using   
   recently were *6 OHMS* and not 8 as I stupidly assumed. When I swapped the   
   speakers between    
   channels it was OK again. An issue with capacitance vs impedance? An issue   
   with one of the second hand irons? I have only played at low to moderate   
   living room levels so an impedance mismatch should AFAIK not cause harm. I   
   hope.   
      
   Thanks Phil for checking what a shorted GNFB R does. Once I put trimpots in an   
   old amp to learn different effects of feedback. Past a certain pont it would   
   start growling like an old engine.   
   And thanks Patrick for posting your thoughtful advice. Yes I see the point of   
   using hassle free components:) I see you updated your site? Great reading.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)