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Long-tail phase splitter input
The circuit below establishes a balanced drive signal by using dissimilar plate resistors to correct the unbalanced output of the long tail phase splitter. Equal plate resistors do not give equal gain per phase leg, as the second triode of the pair does not realize as much gain as the first triode. Reducing the first plate resistor equalizes.
In fact, what we have here is an example of how a circuit's liability can be turned into an asset. The subtraction of the 2.2k resistance from the 10k plate resistor allows us to add a balancing capacitor easily. This capacitor is needed to balance the drive to both output tubes. Without this capacitor, the output stage would not be balanced in drive or output impedance, as the top triode's cathode moves with the output signal, whereas the bottom triode's cathode is fixed. By adding this capacitor, the output stage functions as two cathode followers in parallel, as both output tubes equally buck a perturbation at the output by halving the top triode's cathode movement against a fixed grid and the bottom triode's grid movement against a fixed cathode. The feedback is DC coupled and it keeps both the input and the output stage biased at the right voltages.
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All in all, not a bad little amplifier. The input coupling capacitor allows the two resistor voltage divider to set the DC input voltage to the input grid to 20 volts DC, which also allows a large-valued common cathode resistor, thus eliminating the need for a negative power supply. And the feedback loop is DC throughout. This is circuit is, in fact, a small power amplifier that would be perfect for driving low-impedance headphones.
The one remaining problem that burdens this circuit is that it must be used with a high quality power supply, as power supply noise is not equally presented to the output tubes' grids. The added feedback capacitor shields the bottom triode's grid from the power supply noise, but not the top grid. Unfortunately, the work around is to add more components. One extra resistor and one extra capacitor are needed. With these added parts, both phase legs are shielded from the power supply's noise. And while we are at it, let's change the output stage to a grounded-cathode mode just to show what it would look like. (This remake relies on the feedback to bring down the output stage's distortion and output impedance.)
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