From: apogosso@tpg.com.au   
      
   "Alejandro Lieber"  wrote in message   
   news:k54s5k$2r6g$1@adenine.netfront.net...   
   > Very interesting. Do you know if it has been done in practice ?   
   > Will the final valve still work in class C all the modulation cycle ?   
   >   
   > I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the   
   > cathode of the RF output valve.   
      
   Cathode modulation has more advantages.   
      
   1. It is a sum of control grid modulation and partly screen modulation. As   
   the cathode voltage rises at the troughs of modulation, voltage difference   
   between the screen and cathode reduces. Thus the "pull" of the screen grid   
   reduces which cooperates with the effect of the cutting of by the control   
   grid. Thus the cathode voltage excursions are smaller than control grid   
   excursions if it were purely grid modulation.   
      
   2. Modulation is not affected by variation of excitation voltage. In a   
   common grid modulation, if the excitation voltage say is modulated by 50Hz   
   hum, output will also be modulated. With the proposed cathode current   
   modulation output will not be affected. Cathode voltage might be modulated   
   by hum, tracking it like in a cathode detector, but since the current is   
   stable, the anode RF component will be almost unchanged (ignoring cut-off   
   angle effects).   
      
   But the greatest advantage is no need for any adjustments of bias, etc. The   
   only thing to be set is the cathode current. It is done by adjusting bias on   
   the gate (base) of the modulator series transistor.   
      
   Regards,   
   Alex   
      
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