Tomlang -- Thanks for your interest and quires. They are in fact quite timely.
First -- as to your questions:
1. If you are looking to install EFB in an amplifier that already operates with conventional fixed bias, then in fact you would use an EFB supply regulator, versus an EFB cathode regulator. I mentioned this only briefly in my original article introducing EFB.
Since an amplifier with fixed bias already operates (obviously) with fixed bias, then only the unique "Enhanced" element needs to be added to achieve EFB operation. It is this element that reduces distortion so significantly in spite of significant changes in B+ level, be it either due to changing AC line conditions, or changing dynamic (power output) conditions. This too is an equally simple circuit to install, but in releasing it, I got somewhat distracted along the way, which I will explain further in a moment.
To finish the possible existing scenarios then, the cathode regulator version already released not only provides true fixed bias operation, but also of course produces the Enhance element as well. It is appropriate for traditionally cathode biased amplifiers where conversion to fixed bias is desired, or for amplifiers designed with fixed bias, but where chewing up some of today's higher B+ voltages might be of benefit.
In getting ready to release the supply regulator version of EFB however, I thought it best to back up and also address the numerous requests I've had for installing EFB in pentode output stages as well. To be done properly, this of course has added yet another version of EFB to the EFB stable, so that you now have:
1. The original EFB employing a cathode regulator, which is appropriate for cathode biased UL designs to be converted to (or requiring) fixed bias,
2. The original EFB employing a supply regulator, which is appropriate for fixed bias UL designs,
3. EFB II employing a cathode regulator, which is appropriate for cathode biased pentode designs to be converted to (or requiring) fixed bias, and
4. EFB II employing a supply regulator, which is appropriate for fixed biased pentode designs.
To document the validity of EFB II, I first built a clone of the Fisher SA-100 amplifier which I documented here:
http://www.audiokarma.org/forums/showth ... p?t=466229
I chose this amplifier because it is one of the highest regarded EL84 fixed bias pentode amplifiers, and frankly, because I also wanted one. Its initial intention was to be a development mule for EFB II, but ultimately became much more than that. For the record, my clone uses the version of EFB II described in #3 above, because paleenty of B+ was available, and the cathode regulator version chews some of that up as previously mentioned.
With that project complete now, a kind AK member has let me borrow a stock SA-100 not only for performance comparisons against my clone with EFB II, but also to actually install EFB II in after I'm finished documenting its stock performance. Since the SA-100 is already a fixed bias amplifier, the version described in #4 above will be used, using the supply regulator element you are asking about.
Work on installing EFB II into the SA-100 is underway now, and should (if all the planets align) wind up in another article by the end of the year, not only releasing the simple supply regulator version described in #2 above (which you are asking about), but also both versions relating to EFB II.
I mention all of this not only to provide a long needed update on EFB, but also because most particularly, the version described in #4 above (and being installed in the SA-100) should be of great interest to the gazillions of Fisher, Scott, and Eico integrated amplifier owners (to name a few) using 7591 output tubes. For those units, EFB II allows for lower dissipation levels, increased power output with both channels operating, and of course lower distortion as well. With how expensive the NOS examples of 7591 tubes are, it should not only improve performance, but increase life as well, which is a valuable commodity for those tubes.
And finally,
2. Yes. Because any form of cathode bias chews up available B+ voltage, then converting to the use of a negative voltage supply for bias turns that "lost" B+ into available power output and increased headroom.
Thanks so much for the interest. I hope this answers your questions!
Dave