flow through the top triode, as its grid swings positive with the collapsing of the voltage across the 1K resistor. 
  The Bridge amplifier confuses most tube audio buffs. (I wish I had a share of Microsoft stock for every time I have had to explain how this circuit is not anything new, radical, or mysterious and how it is just a push-pull output stage that can be run in Class A, or AB or
even B or C, all advertising copy and high-end magazine reviews to the contrary. The quickest path to understanding this amplifier topology is to replace mentally all the tubes with switches and replace the power supplies with 6 volt batteries. Work the switches in anti-phase and evaluate the voltage relationships. 

   In fact, the amplifier can be inverted by using the plates as the output (quick rush to the patent office or trademark this as Ultra-Supra Linear or Supra-Ultra Linear or Mega-Linear). Next month we will continue exploring the bridge, AKA balanced, AKA circular amplifier.
   For now, the best explication of this circuit is to be found in Zimmermann and Mason's
Electronic Circuit Theory: Devices, Models, and Circuits. John Wiley & Sons, Inc., 1959. Chapter 11, section 5, "A Balanced Power Amplifier with Direct-Coupled Load."
  After examining the foregoing circuits the simple trans-former coupled push-pull amplifier may now seem trivially easy to understand. But if we could objectively measure mental complexity of a circuit, it would actually rate as being much more difficult to understand as it would require an understanding of magnetic fields and winding ratios, which the other circuits mercifully do not entail. (For example, in a transformer coupled push-pull amplifier, how does the plate swing to a voltage greater than that of the power supply?)

Totem OTL Amplifiers
  The push-pull output stage not yet covered is the type that is used in the Futterman OTL amplifier. Here one tube stands on top of the other. Each tube receives an anti-phase input signal relative to the other. As one conducts more, the other conducts less. Both can conduct continually over the entire waveform or for only portions of it. Push-pull in every sense. All that remains is understanding how the input signals are established between the tubes.
   Two modes of operation present themselves: push-pull Grounded Cathodes or push-pull Cathode Followers. In other words, we are presented with two choices: gain, but high output impedance or no gain, but low output impedance.
    Almost all transformer coupled push-pull tube amplifiers accept the first choice. As tubes can easily swing lots of voltage, but only meager current, the choice is easy to make. The output transformer reduces the voltage swing and increases the current and reduces the output impedance. Even with an output transformer, the output impedance is

Bridge amplifier with right switch open

Ground Paradox
  Notice that the output seemingly only swings positively, never negatively. This paradox only occurs because where we choose to define as ground changed from one example to the next. Had we placed a voltage divider across the outputs and fixed ground as its midpoint potential, then the output terminals would have read, -3v and +3v in the first example and +3v and -3v in the second. Of course the loudspeaker is indifferent to what we choose to specify as ground, it only responds to changes in the amount and direction of current flow across its leads.

Bridge amplifier with ground referenced to output midpoint


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