From: info@turneraudio.com.au   
      
   More from me about wien bridge oscilator.   
      
   The idea of passing idc through fet does not make any difference to the nature   
   of the effective Rd - it remains a very non linear resistance.   
      
   WB oscilators with lamps or thermistors are probably all capable of producing   
   far less THD. But perhaps using current source from a bjt collector with PNP   
   MJE350 to pass some dc through 3 x 12V/50mA lamps to warm them up so R = 100r,   
   so 3 give R = 300r,    
   and then NFB network total R = 900r, and that may work well. The main FB R =   
   600r approx, and would be cap coupled to lamps, and Idc controlled with   
   rectified voltage to MJE350 base and current limited with say 220r emitter R   
   between +20V and emitter.    
      
   "Grain of wheat" 12V lamps have about 27r at 20C and when hot are 240r.    
      
   I could get a type 327 lamp, 28Vx40mA = 700r at max on current, and maybe 240r   
   at situation where you wan to use them in WB oscilator, sometimes with cathode   
   Idc flowing through one at V1 cathode of a 2 stage tubes oscilator. I recall   
   HP's first WB    
   oscilator had this.   
      
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)