This circuit fails in one truly important aspect: in spite of the symmetry of driver parts, it does not offer an equal drive signal to both output tubes. As configured, the only case wherein the top and bottom tubes see identical drive signals is when the load being driven is 0 ohms. At any higher impedance, the top triode sees less drive voltage than the bottom triode. Probably the easier path to seeing this asymmetry is to imagine a pulse being reflected back into the output transformer's primary. The bottom triode's grid is oblivious to the negative going pulse at its plate, but the top triode's grid sees a small negative going pulse because the driver tube's rp defines a voltage divider with its plate resistor. So the positive going pulse at the top triode's cathode is not relayed in its entirety to the top triode's grid, but rather diminished, making it appear as a negative pulse to the grid. The only way to ensure a symmetrical drive signal is to use triodes with infinite rp's (FETs, MOSFETs, transistors, and pentodes come close). Or another alternative is to effectively increase the rp of the triode used.

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TCJ Filter Designer lets you design a filter or crossover (passive, solid-state or tube) without having to check out thick textbooks from the library and without having to breakout the scientific calculator. This program's goal is to provide a quick and easy display not only of the frequency response, but also of the resistor and capacitor values for a passive and active filters and crossovers. Tube crossovers are a major part of this program; both buffered and unbuffered tube based filters along with mono-polar and bipolar power supply topologies are covered.   

For more information, read the article "Tube-Based Crossovers" in the Tube CAD Journal.
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