AC Balance Adjustment


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Earlier (all pre-1961 designs), H.H. Scott basic and integrated amplifiers use a "floating-paraphase" phase splitter (inverter) circuit which featured an adjustable AC balance potentiometer for each audio channel. Such types include: the 99, 209, 210, 222A/B, 299A/B, 272, 240, 250, 280, 290, & 399.

Later (post-1961) Scott amps (299C/D, 222C/D, 233, 296 and all Scottkit Amps) eliminated this AC balance control by the introduction of a "split-load" phase inverter circuit. So, if you own one of these types, no need for you to read further.

Typically, this adjustment permits the floating-paraphase phase splitter to send perfectly adjusted symmetrical signals to each side of the push-pull output tubes, even under very high signal levels. To perform this factory adjustment procedure properly, you will need an oscilloscope, low-distortion audio oscillator, a (non-inductive), 16-ohm resistive dummy load, capable of handling the amplifier's full-rated output, and a high-quality distortion analyzer. 

If you don't have the test equipment -- don't worry too much about performing this AC balance adjustment. If your output tubes are anywhere close to being a matched duet (and the rest of your amps is healthy), then this adjustment is just not that critical.

To quote a factory H.H. Scott Service Bulletin:

"All H.H. Scott Amplifiers contain a dynamic self-balancing phase inverter stage which automatically balances the output of the amplifier beyond normal limits of audible detection. This accomplished by the feedback developed in the plate-to-plate voltage divider from which the second grid of the phase inverter obtains its signal. For this reason, the adjustment of the (AC) balance control is not absolutely necessary for good performance, although such adjustment may improve performance at high signal levels.

If the amplifier is used for fixed-frequency or other laboratory applications, however, readjustment of the (AC) balance control may be desirable for optimum results. The control is factory set for the original output tubes, but the characteristics of these tubes may change with time."


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