But moving the plate resistor to a new position in between the two power supplies will force many a scratched head. Yet the circuit functions identically as before: the same gain, same output impedance, same inverted phase at the output, and the same Miller effect capacitance. 

      What happens when the two tubes share an identical idle current? In this case, then the load current must equal zero, as is given by
    ILoad = ITube1 - ITube2. 
A further proof is a quick thought experiment: if the tubes see the same current flow, then these tubes must see the same voltage from cathode-to-plate, as they share the same electrical characteristics. The tubes can either conduct or not conduct. If they do not conduct, then the voltage across them must equal zero. If they do conduct, this voltage must equal the B+ voltage, as the two tubes are in series they must split the total voltage potential within the circuit (any other voltage would lead to electrical absurdities, such as the B+ voltage not equaling the B+ voltage). Now, since this voltage equals the B+ voltage, the voltage at the cathodes must equal. Equal cathode voltages means zero voltage across the load, and thus zero current through the load. In this situation, it little matters whether the load impedance equals zero of infinity, as no current flows through the load.

Single-Ended Circlotron!
   Push-pull circuits confuse many beginners. So let's level the playing field and confuse everyone. The grounded cathode circuit consists of as little as one triode, one cathode resistor, one plate resistor and one power supply. Simplicity itself. Replacing the power supply with two power supplies (in series) of half the voltage should not confuse anyone, as the total B+ voltage remains the same.

    Yes, we have shifted voltage relationships, for example the output is now at ground level, but we have not altered the total potential voltage, current, or dissipation in the circuit. The triode still sees the same cathode-to-plate voltage and the same idle current. The plate resistor still sees the same voltage across its leads; and thus the same current flow and dissipation; thus, the same functioning.
    Increasing the grid voltage causes the tube to increase its current conduction, which when relayed through the current path, creates a greater voltage across the plate resistor, which in turn forces the plate negatively, as the other end of the resistor is fixed by the first power supply. Conversely, decreasing the grid voltage causes the tube to decrease its current conduction, which lessens the voltage across the plate resistor, which in turn forces the plate positively, as once again, the other end of the resistor is fixed by the first power supply.
   If the triode ceases to conduct any current, then the resistor will see zero voltage across its leads and the tube will see the total voltage of the two power supply placed in series. And if the tube conducts fully, then the resistor will see almost all the  voltage from the power supplies in series and the tube will see a cathode-to-plate


pg. 4


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