From: apogosso@tpg.com.au   
      
   "John Byrns"  wrote in message   
   news:byrnsj-7C2B68.15594724062012@news.giganews.com...   
      
   > The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation   
   > between a cathode-grid "self excited" oscillator circuit and modulation   
   > applied   
   > to G3 as the total cathode current appears to be largely independent of   
   > the   
   > voltage on G3, minimizing "FMing".  If this weren't the case, the local   
   > oscillator frequency, in an AM receiver using one of these tubes, would   
   > vary   
   > with changes in the AGC voltage due to fading.   
      
   Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on   
   G3.   
   With 6BE6 it is almost impossible to cut it off completely, so 100%   
   modulation os impossible, and deep modulatio will be distorted, even with   
   feedback.   
      
   By the way, in this case NFB shall be applied from the plate of a heptode,   
   not from its cathode.   
      
   It is better to use a separate oscillator and geef it to G3 of a heptode,   
   while feeding audio to G1. In this case any heptode can be used. NFB can be   
   taken from the cathode since plate current is *sort of* proportional to   
   cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND   
   and use a large electrolytic for passing AF as well, not RF only. But still   
   it is better to take NFB from the plate (I mean AF component, not RF).   
      
   Even in a best heptode, even with a separate oscillator you will have   
   residual FM -- due to space charge and stray capacitances. A small FM of say   
   100...1000Hz is acceptable for listening on an AM radio, but will not be   
   listenable on a synchrodyne or an SSB receiver.   
      
   >   
   > Both of these schemes differ from the common phono oscillator circuit in   
   > that   
   > they require two tuned circuits, one for the oscillator and a second for   
   > the   
   > plate circuit feeding the antenna, the plate current can be completely cut   
   > off,   
   > neglecting leakage and so forth, without affecting the oscillator circuit   
   > to any   
   > great extent.  The single tuned circuit approach of the common phono   
   > oscillator   
   > circuit would have a greater sensitivity to "FMing", and the oscillator   
   > dies   
   > completely when the plate current is cut off.   
   >   
   > --   
   > Regards,   
   >   
   > John Byrns   
   >   
   > Surf my web pages at,  http://fmamradios.com/   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)