From: info@turneraudio.com.au   
      
   Just a followup to my original question in case others have the same problem.   
   I managed to localize an intermittent contact somewhere around the 680k   
   resistors (R7, R9) by lightly tapping on every R with a stick while the Quad   
   II was in operation.    
   Replaced both resistors plus the 2.7k (R8) inbetween, and resoldered   
   everything else on the tag board. I am not sure how this fault could produce   
   an oscillating squeal, but now things are stable inside the amp.    
      
   Martin    
      
   I can't answer exactly why your amp squealed.    
   The minimum improvement to Quad-II is to    
   1. Replace all old caps and resistors installed in Quad-II in 1950s-60s.   
   2. Use Welwyn 1.2W metal film for most R.   
   3. Use at least use 390r 5W wire wound bypassed with 470uF at each KT66   
   cathode to CFB winding end.   
   4. Ground CT of CFB winding.    
   5. Coupling caps all should be Wima MKP 630Vdc rated.   
   6. Electros in PSU can be 33uF.   
   7. Use GZ34 instead of GZ32.    
   8. While resoldering new R&C parts to bent brass lugs on old Quad terminal   
   board, be prepared to find that lugs just break off, because the bend in the   
   lug is so sharp, and this fatigues the metal over 50 years. Sometimes I have   
   deliberately broken off    
   all lugs, and relied on soldering to the riveted part in the board.   
   9. Install an RCA input socket.   
   10. Make sure the chasssis of all units used are connected to EARTH via   
   green/yellow wire amoung 3 wired from wall socket cabble.   
   11. Replace all R&C in Quad-22 control unit, if used, especially those under   
   one switch bank which relate to RIAA eq.     
      
   The cathode biasing of each KT66 ensures much better Iadc balance so OPT Ia is   
   balanced so iron related distortions and hum is less.   
      
   For best class A performance, don't use less than 16 ohm speakers when OPT is   
   strapped for 9 ohms. This seems strange, but with 9 ohms used the RLa-a is   
   about only 4k, way too low for best class A.    
   These old amps were best driving sensitive 1950s - 60s speakers, many of which   
   were 16 ohms nominally, eg, Tannoys et all.    
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
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