to the end means the grid or grids will be driven beyond a negative voltage into a positive voltage relative to the cathode voltage. What Class A Isn't     The logic used at advertising departments goes something like this:     Class A is a feature of some desirable        amplifiers; our amplifiers are desirable;     therefore, our amplifiers must be Class A     amplifiers. Another twist recognizes that current and voltage is part of the issue:    Since the total idle power dissipated by the    entire amplifier, which includes the heaters    and panel light, is greater than the power   output, our amplifier must be operating    in pure Class A. Or they may be more subtle:    Since the total idle power dissipated by just    the output stage of our amplifier is greater    than the power output, our amplifier must     operate in Class A. This last piece of misinformation almost seems right, but isn't. The classic Stereo 70 from Dynaco put almost 35 watts per channel and its output stage dissipated over 40 watts of heat at idle per channel and yet no one has ever claimed that it was a Class A amplifier. The inefficiency of an output device does not make it ipso facto run in Class A operation.      Once again, Class A means that at no time while reproducing the output signal does any of the output devices turn off and stop conducting current. This is the test that must be met.      If a small resistor is placed in series with an output device and an oscilloscope's probes are attached across that resistor, we will be able to see how much of the waveform is handled by this device. If the entire sine wave is present, then the device has never turned off and the amplifier has remained in Class A operation. If any portion of the sine wave ever goes flat, then the device has momentarily turned off and the amplifier has moved into Class AB operation.
 At Last the Circular / Bridge Amplifier      As we have mentioned in previous issues, the use of two floating power supplies makes this amplifier inscrutable to far too many tube fanciers. In the murky understanding of how this push-pull amplifier topology works hides the deception that this topology must be Class A in nature, that the output stage could only work if both output devices provide a current path to the other.      Contrary to the dictums (dicta for those I the know) from the advertising department, ignorance of the actual functioning of a circuit not an excuse to bestow magical attributes on the circuit. The Circular / Bridge Amplifier is just as much a push-pull amplifier as the Stereo 70. It too obeys the laws of physics. It too can only deliver twice its idle current into a load before leaving Class A operation. It too can be biased down to Class B or even Class C operation.    Examine the absurdity of claiming that an amplifier puts 60 watts of Class A power independent of the idle current. If we assume that Class A operation can be independent of idle current, why not bias the output stage down to 1 mA? And then we will have a wonderful paradox: a cold Class A amplifier. For that is the assumption being made when we are told that this push-pull amplifier must be Class A by its very nature and it is no less insane than claiming since the bottle the wine is in is very attractive, the wine must be delectable.    One last time, Class A means that at no time while reproducing the output signal do any of the output devices turn off and stop conducting current. Imagine a tug of war: if one side lets go, the war is over. If one leg of a push-pull amplifier stops conducting, the amplifier has left Class A operation. Now, just what would be the minimum requirements for a true Class A, OTL amplifier that put 60 watts RMS? The first step, how much peak current is needed to put out 60 watts RMS into an 8 ohm load? The math:
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