From: apogosso@tpg.com.au   
      
   Hello,   
      
   You know those Japanese tuner-amplifiers of the 60's which have dual AM   
   receivers, one possibly with FM and a stereo amplifier, typically powered by   
   6BM8 tubes...   
      
   I got one of those, Pioneer SM-B200A. It is not working -- the tubes get hot   
   and "burning". Most likely dead electrolytic and/or leaky interstage paper   
   caps which bring positive bias to grids. I need to fix the beast before   
   getting rid of it.   
      
   I have no schematic, but tracing the wiring, realised that 6BM8s have fixed   
   bias originating from some selenium rectifier. Cathodes of 6BM8s are   
   grounded. More over, there are flinsy open (to dust) carbon trimpots to   
   "balance" (!) bias of the output tubes. At the same time there is no way of   
   knowing values of the currents in the tubes. There are no even small current   
   sensing (measuring) resistors in the cathodes. That design does not make   
   much sense.   
      
   I would get rid of the fixed bias in favour of automatic bias. Automatic   
   bias will be more robust and forgiving to tubes mismatch. It will not care   
   if this selenium rectifier dies altogether. I would throw resistors from   
   each cathode to ground, plus another between the cathodes to be able to   
   check unbalance.   
      
   Now my question is, from your experience, what should these resistor values   
   be, to get a benign class AB? I would guess 300R from each cathode to ground   
   and 100R between the cathodes?   
      
   Regards,   
   Alex   
      
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