From: apogosso@tpg.com.au   
      
   "Alejandro Lieber"  wrote in message   
   news:k52ck9$1765$1@adenine.netfront.net...   
   >I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a   
   >EL34 in class C as final.   
   > The idea is to modulate it by varying the negative polarity of the   
   > suppressor grid (g3).   
   > Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma   
   > with 0 volts suppressor, does anyone have any idea what negative voltage   
   > in g3 is needed to reduce the anode current by half, needed for screen AM   
   > modulation ?.   
   >   
   > Alejandro Lieber  LU1FCR   
   > Rosario Argentina   
      
   Does not seem like a good choice. EL34 is not for RF. It has high Cag   
   capacitance = 1pF. You are looking at interdependance of grid and antenna LC   
   tanks or parasitic self-excitation at worst. And 75W DC input is too much --   
   unreliable.   
      
   Suppressor modulation is not a good choice -- requires about -150V dc and   
   the same AC component. Nonlinear. Negative g3 voltage repulses the electron   
   flow to the screen grid. It is easy to burn, unless a series resistor is   
   used to limit Pg2. (Ask Patrick what happens if you run a push-pull audio   
   amp without negative feedback at full power and then disconnect its load!)   
      
      
   If you (for whatever strange reason) decide to build AM transmitter (in the   
   21-st century when even SSB starts to look old fashioned), then use 807 tube   
   or any other specially designed for RF.   
      
   The best modulation is the cathode modulation. For example with 807:   
   1) get about 35Vm of excitation, connect g1 to ground via a 100K resistor,   
   apply AC excitation through a cap to g1.   
   2) Use Vg2= 250...300V   
   3) Between cathode and GND connect a darlington of a MOSFET with a 100R   
   resistor from emitter (source) to GND. Bias base (gate) accordingly to get   
   about 40...50mA DC on idle. Apply AF to base (gate) via a 1uF cap or so. You   
   will need about 5VmAC. Such transistor, as you can see, works as a   
   controlled current source. Thus cathode current of your 807 will be   
   perfectly following the AF input. Cathode of the 807 shall be decoupled to   
   GND via say 4700pF. Since it is the current that is controlled, this scheme   
   does not depend on the non-linearity of the curves of the tube. Cathode   
   potential sets by itself.   
      
   Regards,   
   Alex   
      
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