From: apogosso@tpg.com.au   
      
   The recent thread about the SET 300B based amplifier got me to question: Why   
   triode?   
      
   1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output   
   and reasonable damping WITHOUT GNFB, but the penalties are: low plate   
   efficiency, complexity of filament supply, cost, possible self-destruction   
   if shorted load, full volume and fixed bias.   
      
   2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in   
   parallel) design needs GNFB to achieve the same benchmarks, but more   
   efficient, cheap and will not blow the tubes. A penalty here is probably the   
   GNFB loop which encompasses the OPT and would require shelving or other   
   elaborate compensation, which only Mr Turner can properly master to achieve   
   stability over the wide range of the load impedance.   
      
   3) Yet there is another possibility -- use pentodes (or beam tetrodes) with   
   local feedback. In effect the goal here is to reduce the gain (from G1 to   
   plate) of the stage to about the typical mu of a rival triode. In other   
   words, replace the internal electric field local NFB in a triode by an   
   external local feedback around a pentode.   
      
   One way is to use cathode feedback with the cathode winding having about 1/4   
   of number of turns of the plate winding. A drawback here is a special   
   transformer to be made.   
      
   Another simpler method is to arrange a resistive local feedback. Throw say   
   R1=470K from the output tube plate to grid, through a DC blocking cap of   
   course. Feed the input signal to the grid via a series R2=150K resistor.   
   Thus the output stage will resemble an inverting op-amp, the gain of which   
   is defined by the ratio of  R2 / R1. This is because an inherent gain of a   
   pentode from grid to plate is high. Say for two EL34s in parallel it can be   
   up to 50. (More practical to throw R1 from the output tube plate to the   
   driver tube plate without a DC blocking cap.)   
      
   Thus the local feedback can reach 15...20dB. As a result:   
   a) Drive voltage will be comparable to 300B;   
   b) Distortion will be same if not lower than with 300B;   
   c) The local NFB will be unconditionally stable, since it does not include   
   OPT;   
   d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)   
   + 1). With a paig of hi-gm tubes as EL34s it will be even lower than with   
   300B;   
   e) Will yield higher efficiency with the same DC input power;   
   f) No real need for resonance damping circuits across OPT primary;   
   g) No GNFB will be needed;   
   h) Not prone to self destruction with shorted load.   
      
   A drawback though is a low input impedance (about R2). However, with a   
   cathode follower in the driver stage it will not be an issue.   
      
   I am wondering if such "op-amped" pentode stages are common. What am I   
   missing?   
      
   Regards,   
   Alex   
      
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