From: apogosso@tpg.com.au   
      
   "flipper"  wrote in message   
   news:qhbsu7dtce6cv7uj55oenpjn3426lsohgb@4ax.com...   
   > On Thu, 28 Jun 2012 20:31:40 +1000, "Alex Pogossov"   
      
   >>> With carrier at 640 KHz frequency deviation from idle to twice idle is   
   >>> roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to   
   >>> .64022 MHz.   
   >>   
   >>It is not bad. I expected worse.   
   >   
   > Were you thinking maybe a CF would be needed?   
      
   In a quality equipment -- yes, but not in your case when you knowingly allow   
   for some FM.   
      
   >   
   >> Probably 6ME8 has less space charge   
   >>coupling than a regular heptode.   
   >   
   > I suspect so and it's one of the things I think are 'different'   
   > because of the beam deflection.   
      
   In the deflection tube the electron beam is far away (relatively) from the   
   deflection plates, not brushing them even when the deflection plates sit at   
   positive bias. In a heptode the space charge is right in the vicinity of the   
   G3, enveloping it and nearly touching it. Therefore the deflection tube   
   space charge effects are smaller.   
      
   But... the deflection tube requires +/-30V for full beam cutoff/switchover,   
   while a heptode probably would cutoff at -5V. Six times less. Now if you   
   arrange a tap on the oscillator running at 30V at 1/6-th, and feed it to a   
   heptode G3, the effect will be 6*6=36 times smaller. So you end up with the   
   same FM normalised to an available oscillator voltage. A hetode will give   
   low FM if coupled to a tap of the oscillator tank.   
      
   Because of huge deflection drive requirement, such tubes never gained   
   popularity as generic mixers.   
      
   >   
   >>Still 70Hz is not acceptable for listening on a synchronous or SSB   
   >>detector.   
   >>In these cases parasitic FM index shall not exceed about 0.25, so with the   
   >>lowest audio frequency of 50Hz, FM deviation shall not exceed 15Hz.   
   >>   
   >>But who would deliberately listening to your transmitter on an SSB radio?   
   >>No   
   >>one.   
   >   
   > Right. What I'd rather know is how much deviation is 'acceptable' for   
   > the BCB.   
      
   Up to 1kHz, I would guess.   
      
   >   
   >>So, well done anyway!   
   >   
   > Thanks. I'm getting the impression it might not be worth even trying a   
   > heptode or DC pentode.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)