The Tube CAD Journal: 300B Power Amplifier Design Example

Push-Pull Amplifiers

There is no reason why push-pull amplifiers cannot be used with the partial feedback technique. The Williamson amplifier topology makes an easy conversion to partial feedback. The driver stage in the stock amplifier is a differential amplifier of sorts. Breaking this compound circuit into two grounded cathode amplifiers assures a consistently high output impedance from this stage.

Other Uses for Partial Feedback

Feedback is feedback; it can be used in preamps, crossovers, equalizers, line stages, and power amplifiers. The power amplifier readily benefits from the greatly lowered plate resistance, as the output transformer has limited amount of inductance, which the lower rp aids. But another benefit of the partial feedback topology is that it can be switched in or out of circuit. For example, problem we face in designing a phono preamp is the dissimilar output voltages of the MM and the MC cartridge. Do you design for high gain and MC use or lower gain and MM use? Or do you try to compromise and aim for some center point that serves neither cartridge type well?

One solution is a preamp that has adjustable gain. Here is where switching the partial feedback in and

out of circuit shines. If resistor Rfb is removed, we are left with a grounded cathode amplifier; with it in place, we have a plate follower of sorts with greatly reduced gain.

Conclusion

This topology deserves a first and second look from tube fanciers. Quick and short, it bestows many of the benefits of a cathode follower without needing huge input voltage swings.

If you have any questions or experience with this topology, please drop us a line. For my own research, I plan on building a push-pull amplifier based on this circuit. The output stage will consist of 8 6BX7s; the input and driver stage, 12SN7s; the output transformers will be Dynaco MK3 outputs; the mode