From: info@turneraudio.com.au   
      
   On Friday, 9 November 2012 19:07:56 UTC+11, Alex Pogossov wrote:   
   > "John Byrns"  wrote in message news:byrn   
   j-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   
      
   I've tried caps between OP anodes and g1 or to driver cathodes, but usually it   
   just causes more open loop phase shift of 90 degrees at a lower F than do the   
   existing amounts of C shunting Ra in the input-driver amp stages. So with the   
   ultimate OLG phase    
   shift of 180d occuring at a lower F the amp becomes harder to stabilise with   
   other networks with a pure C load. The C you put there adds a high Miller   
   effect where it was smaller to begin with. But perhaps its possible to use an   
   R&C zobel to shelve the    
   HF that way to avoid the 180d shift at such a low F. The R&C zobel is best as   
   a passive network across V1 anode to 0V, ie, it shunts Ra, and this works   
   better IMHO to reduce ringing on C loads and keep away from FB C having to   
   load down any driver stages.   
      
   I'll do it your way after I have seen your many schematics which show/prove   
   your method to be superior. When I test many amps it is revealing to set them   
   up with a 0.22uF cap load and with 5kHz square wave at the -12dB level. One   
   will often see some    
   ringing at Vo which is to be expected. But if you examine the anodes of OP   
   tubes, the ringing is often a lot worse than at Vo. Just what is done to   
   reduce the ringing is debateable, but I recall having tried every possible way   
   without reducing the widest    
   full power BW with pure R load but while making the amp unconditionally stable   
   with an possible load or none at all. Whay appears in schematics at my site is   
   a distilation of ideas that worked, and priciples that can be applied to   
   nearly all amps without    
   wasting too much of my precious time.   
      
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
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