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Amplifier Bias

If these amplifiers are biased AB1, none should be rebuilt with self bias. As I wrote recently in another thread, DC voltage across the self-bias network increases at higher amplifier outputs and anode currents. That causes bias voltage to increase, moving the output stage closer to Class B. Unless the bias is set very high at idle, this movement creates crossover distortion that is easily observed on a scope. This distortion will persist into quiet musical passages due to the time constant of the bias circuit, muddying nuance and degrading fidelity.

The only reason manufacturers ever got away with this is that printed specifications minimize the impact. At higher amplitudes, the crossover distortion represents a numerically small percentage that simply adds to the naturally increasing distortion levels. In addition, the loudness of the system at higher levels tends to hide what's happening from the listener.

These older designs can be greatly improved with either the "Enhanced Fixed Bias" cathode bias technique described at the tronola.com site, or with the addition of fixed grid bias. This should not be neglected if you want your restoration work to be fully worthwhile.









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