And if eight 6AS7's are used, the output impedance would be 280/[8(2 + 2)] or 8.75 ohms.  Not nearly as low as we had hoped.
    "But wait, there's more!" the assumption being made here is that the rp is constant; alas, it is not. Nor is the mu. Both vary with plate voltage and current. (Of the two the mu is closer to being an actual constant.) The RCA Receiving Tube Manual lists the mu as 2 and the rp as 280 at 135 volts of plate voltage and 125 mA of plate current. If we reexamine the tube at 100 volts and 340 mA, with a grid  voltage of -10 volts, we find that the mu has climbed to 2.5 and the rp fell to 148 ohms.
   Now, if we redo our calculation, the output impedance is 148/[8(2.5 + 2)] or 4.1 ohms, which, when placed in parallel with the 8 ohm load, becomes 2.7 ohms. (Yes, using the load to lower the advertised Zo is cheating, but common.) Not bad, but then not great. Once again, these formulas assume true Class A operation. This makes sense, for if a triode stops conducting, its Gm falls to zero and it offers no resistance to applied voltage.

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   Here is where we can see that amplifier classification is more than just pedantic taxonomy. If we desire a flat, i.e. a consistent output impedance from 1 watt to full output, then this amplifier output stage or the rearranged version, the standard Totem Pole, must either be run in strict Class A, strict Class A2, or optimized Class AB. Optimized Class AB means that the bias point has been carefully set so that the overlap of the two output devices occurs where each device has half its normal transconductance so that when paralleled, a constant transconductance is realized. This means that the optimized Class AB amplifier will have twice the output impedance of the same amplifier run in pure Class A.       
                                //JRB

  Audio Gadgets is software for the technically minded audiophile. The quickest way to understanding what Audio Gadgets is all about is to imagine a programmable calculator designed for the audio enthusiast. Audio Gadgets does far too much to fit in even a 21" monitor; consequently, the notebook metaphor is used to hold ten pages of audio topics. Stepped attenuators to tube circuits.             
           
              Windows 3.1/ 95/98/NT

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