From: apogosso@tpg.com.au   
      
   "flipper"  wrote in message   
   news:jvkhu7t0q8atdfn2n6rm6qjjfutpbpkfur@4ax.com...   
   > On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov"   
   >  wrote:   
      
   > Yes, audio to G3 is the most common implementation with people looking   
   > to the G3 curve for linearity, which is why dual control pentodes seem   
   > to be the preferred choice. For one, as you mentioned, the 6CS6 is one   
   > of the few sharp heptodes, the ECH84 being the only other one I can   
   > think of off hand, and there's more DC pentode choices. Second, the   
   > dual control pentode G3 'linear' region remains relatively constant   
   > over bias while the 6CS6 cutoff region shifts with screen volts and   
   > bias.. That makes the DC pentode easier to bias.   
      
   Likewise in a dual control pentode the "linear G3 region" shifts with the   
   plate voltage.   
   Dual control pentodes have one undesirable disadvantage. Their plate   
   impedance is not high and more importantly, it is not constant. It is high   
   at G3 cutoff (obviously!) and high at G3=0...+5V (where it works as a   
   conventional pentode without much current returning to G2). At the very   
   middle of the "linear" G3 zone the plate impedance is low, as in a triode   
   where the cathode is the space charge rather than a red hot physical   
   cathode. If such modulator runs into a LC tank, its Q factor will be   
   modulated. Peaks and troughs will be emphasized, carrier level -- reduced.   
   Hence -- the second harmonic of the envelope.   
      
   >   
   > There's a ton of Dual Control Pentode AM broadcaster schematics online   
   > and I've breadboarded a couple of them. In fact, my LO was originally   
   > developed for a 6GY6 version using a 1 MHz brick osc.   
   >   
   >>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).   
   >   
   > This, using a dual control pentode, is what John was suggesting as an   
   > alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to   
   > work similarly.   
   >   
   > I managed to get a dual control pentode (6HZ6) model working with   
   > Circuitmaker, although I'm not sure how 'good' it is. At any rate, it   
   > seems to not like going over 80%, or so, mod. Can't drive it to near   
   > cutoff. Another thing, input impedance to G3 seems incredibly low, at   
   > least at RF, and, if the simulation is even remotely accurate, there's   
   > no way my LO can drive it.   
   >   
   > Both those 'problem's may be the model, so I'll probably try building   
   > it anyway, but if the simulation is valid then the 6ME8 works quite a   
   > bit better.   
      
   Yes, 6ME8 seems a good choice, though this tube is "exotic" -- not found in   
   evereyone's junk box, unlike say 6BE6.   
      
   >   
   >>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.   
   >   
   > I'm curious how space charge makes it across to a separate LO.   
   > Modulating grid capacitance, which is coupled to the LO tank?   
      
   Exactly. Likewise, in 6ME8 density of the beam (cathode current) affects the   
   space charge between the deflection plates which modulated capacutance   
   between the deflection plates. I do not know the magnitude of this effect   
   compared to a heptode. In a heptode typically space chage "equals" 0.3pF.   
      
   As far as the parasitic FM is concerned, do two simple experiments.   
      
   1. On a quality comms receiver in SSB mode listen to the carrier (set beat   
   frequency to a few hundred hertz) of your transmitter.   
   Without applying any audio, halve the cathode resistor of 6ME8 (touch it   
   with another resistor in parallel), effectively simulating +100% modulation   
   excursion. Ideally, pitch of the beat tone shall not change. If it   
   changes... you have FM.   
      
   2. On a quality comms receiver in SSB mode listen to the carrier (set beat   
   frequency to a few hundred hertz) of your transmitter.   
   Without applying any audio, play with the antenna tuning capacitor of your   
   transmitter. Ideally, pitch shall not change, only the volume as you tune to   
   and detune from the resonance peak. If it changes...   
      
   If you do these tests, could you "report" the results here. Then we can   
   discuss what causes FM in each of these cases.   
      
   Regards,   
   Alex   
      
      
   >   
   >   
   >>> 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/   
   >>   
      
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