3a6440a5   
   From: apogosso@tpg.com.au   
      
   "John Byrns"  wrote in message   
   news:byrnsj-5CF687.12071508112012@c-131-121-196-216.gonavy.usna.edu...   
   > RCA published a paper describing the 6L6 beam power tube and its   
   > development in   
   > the Proceedings of the IRE and also the RCA Review back in the mid 1930s.   
   > Besides describing the development of the 6L6, they showed the op amp like   
   > connection you are speaking of and what the characteristic curves of the   
   > 6L6   
   > look like when this connection is used, basically triode like IIRC.   
   >   
   > I don't think the connection was very common although I believe I have   
   > seen a   
   > couple of amps that used it.  Much more common is a variant on the   
   > connection   
   > where the resistor from the output tube plate is feedback to the driver   
   > tube   
   > cathode, I have seen this connection quite a bit over the years.   
      
   Interesting to calculate will such feedback be deeper tham the op-amp   
   connection? Possibly, if Rfb is relatively small (and here you start loosing   
   AF power) and driver plate load resistor is large. As J.L.Stewart mentioned,   
   a pentode in the driver will be an advantage.   
      
      
   > The problem with either of these schemes is that it destroys the pentodes   
   > inherently good power supply rejection.  That means that we either need to   
   > provide greater power supply filtering to reduce the noise on the power   
   > supply   
   > line, or go to parallel feed to improve the power supply noise rejection,   
   > either   
   > solution adds to the cost.   
      
   Yes, but all 300B / 2A3 lovers are bound to face the same issue.   
      
   >   
   > If I were doing this and was free to spec the output transformer I wanted   
   > I   
   > would go with the cathode feedback scheme in the output stage.  Taking the   
   > feedback from the plate doesn't improve the low frequency stability   
   > issues, it   
   > only helps with high frequency stability, so what I would do if I couldn't   
   > spec   
   > the output transformer I wanted, would be to take the negative feedback   
   > from the   
   > secondary at low frequencies and from the plate/primary at high   
   > frequencies.   
      
   I did the two branch feedback exactly in the same manner. One solution is   
   just to throw a small capacitor (10pF) from the plate to grid. It will work   
   as a compensation cap in the op-amp, creating a dominant HF pole. In other   
   words it makes 6V6 work as integrator at HF.   
      
   Later I realised that far better is to throw a larger cap from 6V6 plate to   
   unbypassed cathode of the previous driver stage. (To the same cathode a   
   second resistive (regular) NFB is fed from the OPT secondary.)   
      
   Regards,   
   Alex   
      
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