From: phil_a@tpg.com.au   
      
      
      
   I posted before while I was fixing up a couple of Quad II amps. After   
   working fine for a number of weeks, one of them just started oscillating   
   like mad before I even could put on a record. The "starting up a harley   
   davidson"-type of oscillation, ending with a squeal when I shut it off. Now,   
   the circuit is almost original, but a lot of things and wires have been   
   resoldered and I have replaced the OPT with another original one, which I   
   strapped for 9 ohms.   
   Swapping out all the tubes for known good ones did not help.   
      
   Last time I checked, all the original resistors were ok and less than 10%   
   from spec. I have raised the shared cathode R to 220 ohms to be able to run   
   6L6GC. The C's are all renewed: C1 is a Wima pp, C2/C3 are NOS Good-All   
   glass sealed PIO with metal case, C4/C6 are replaced with modern 105C 33 and   
   47uf. C5 is also modern, but raised to 100uF/100v.   
      
   I know I will soon have to fire up the scope and do my own work, but have   
   any of you had this happen before? Is the most likely cause a bad solder   
   joint, or something like a shorted resistor in the global negative feedback   
   loop?   
      
      
   ** If all the small components are OK  -   then suspect a broken wire, bad   
   solder joint, bad ground or an unintended  short to ground that affects the   
   feedback loop.   
      
   On ohm meter and your eyeballs are all you need.   
      
      
   ....  Phil   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)