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OPTs and EFB

"However I want to use my heath aa-100 outputs and el84 so reduced voltage in the circuit."

What is the primary Z of the AA-100 OPTs? Nearly all 7591 amplifiers use OPTs with a 6.6K plate-to-plate primary. Unless the AA-100 in unusual in this respect, those transformers may not be a good match for EL84s in pentode mode.

"EFB" is an acronym for "Enhanced Fixed Bias." The name is trademarked, and I think the designer sells PCBs. The basic idea is that amplifiers with cathode bias, which are at the mercy of varying tube current to create bias voltage, can be easily changed to a more stable bias system. The author uses the LM337 voltage regulator for this. In addition to stabilizing the bias voltage with respect to tube current, the regulator works from resistively-divided B+. This means cathode bias will track changes in B+ voltage. Pretty clever, actually, and the net effect in terms of bias stability is similar to amplifiers using fixed negative bias, but at less cost. The only downside I see (from an outside perspective) is that the circuit elevates the cathodes above ground. In comparison with fixed negative bias, less B+ is available to the output tubes, and that reduces output power. Of course, there's no such penalty when the circuit is compared to amplifiers using cathode bias, and it's as an improvement to those amplifiers that the author intended EFB to be used.



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  Kimber Kable  


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