The IC is the LF441, which has a .5 mV offset and draws almost no current. The zeners establish the artificial floating negative PS for the IC. The resistor in series with these zeners will have to be adjusted to yield the break voltages.

     In the circuit above, the two Op-Amps compare the DC voltage developed across the DCR of the output transformer's primary to the reference voltage of 8 volts. The output tube's conduction is then adjusted to equalize the input's of the Op-Amps.

     I recommend bridging the IC's inputs with two low-leakage diodes to protect the IC, although the IC may come pre-protected.
     Before applying this circuit to a 60W power amp, I would test bench the design with a 12AU7 in place of the many 6AS7s and see how it behaves with some huge tone bursts (40 VPP). My guess is that the music will sufficiently fall outside of the time constants to prevent any problems.
    Understand that this circuit is not an auto-bias circuit; it only auto corrects DC offsets. The idle current is still set by the potentiometer.
     On the other hand, if the amplifier were to be run in TRUE Class-A mode, then DC offset correction and auto-bias could be implemented. It is unlikely that anyone will be willing to forgo the practicality of Class-AB operation in the direct coupled version of this amplifier. However, if an output transformer is used, then Class-A operation makes more sense.

Circular, single power supply, ultra-linear KT8 based Class-A amplifier with auto-bias

     In the above amplifier, all the stops have pulled. The center-tapped choke allow the use of a single power supply, which can be used to feed the screens either the opposing plate voltage or a constant voltage or 50% of the cathode-to-plate voltage swing. The choke is meant to decouple the two power supply capacitors not to provide any further filtration. Of course, in a true Class-A amplifier, much of the power supply will drop out of the output.
    Why brother with such an elaborate scheme? In an earlier letter, we covered the advantages of always keeping the whole of the primary under tension. This scheme allows us to run the output stage in a lean Class-AB and yet always keep the primary under load.
    The ultra-linear aspect will no doubt confuse many readers. The key to understanding it is to reference every voltage relationship to the output tube's cathode, not ground. Here the screen sees 50% of the cathode-to-plate voltage swing.

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