In Reply to: RE: Here is the whole build manual and schematic for the heathkit AA-100 posted by burwoodc on January 24, 2015 at 13:48:54:
"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.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- OPTs and EFB - Triode_Kingdom 20:46:44 01/24/15 (8)
- RE: OPTs and EFB - burwoodc 21:00:44 01/24/15 (0)
- RE: OPTs and EFB - burwoodc 20:57:27 01/24/15 (6)
- RE: OPTs and EFB - Triode_Kingdom 21:33:39 01/24/15 (1)
- RE: OPTs and EFB - burwoodc 23:21:00 01/24/15 (0)
- AA100 OPT imp... - Steve O 21:23:59 01/24/15 (3)
- RE: AA100 OPT imp... - burwoodc 23:33:57 01/24/15 (2)
- RE: AA100 OPT imp... - Steve O 07:55:28 01/25/15 (0)
- RE: AA100 OPT imp... - sony6060 05:52:33 01/25/15 (0)