Hi Bob -- I apologize for the confusion you've experienced. I introduced EFB™ to the late version units because (1) that's what my unit was, and (2) the as produced design can be very hard on the output tubes. Fisher was certainly counting on the unit being used in the classic way it was likely to be used in the mid 60s -- for nice background music at a party, or dedicated listening sessions requiring little more than 2 or 3 watts average per channel -- and in that scenario, everything is fine, and nonbody's the wiser to any lurking issues. For the target individual Fisher was selling to, it was a pretty safe bet to take: FM only reception meant classical music during that day, and rather conservative listening levels. Now on the other hand however, if mom and dad leave for the weekend and junior has a party with the tunes cranked, now the tubes could get into real trouble. And finally, for me, part of my listening enjoyment is knowing that the equipment is all working properly within acceptable design standards for the components used, so that the piece can be operated without concern at any level within it's published capabilities, rather than only within some lower safe range within those capabilities. With today's use environment being very different from that of the mid 60s, I set out not only looking to improve performance, but primarily to fix the conditions of concern in the later version receivers. So that is how EFB came to be applied to the later version receivers.
The early version receivers to not suffer the potential damaging effects of the later versions, regardless of what power level the unit is asked to produce. Therefore, without the need to "fix" any circuit condition, and the fact that honestly, I had no idea how many folks (literally world wide) would actually take on the task of applying such a unique modification to their receiver, an EFB workup for the early receivers was never presented. And to be fair, the vast majority of those who have installed it (that I know of) have in fact been folks with later version units. So, the request for an EFB schematic for the early version units has really never materialized. Complicating that is my presentation in this thead, where I present a client's early 400 receiver with most of the bells and whistles installed, including EFB. Since a schematic for those versions was never posted (I don't recall anyway), it leads to folks like you shaking your head. Sorry about that.
When applied to the early version receivers, there are benefits to be had, although (obviously) not as great as that when applied to the later version units. In later version units, besides eliminating the possibility of damaging operating conditions, output tube quiescent current was (able to be) significantly lowered, as was (as a result) output tube and power transformer operating temps, which resulted in cooler operating temps for the unit as a whole, and an extension in output tube life. Power output was largely unchanged, but distortion for the power produced was significantly reduced.
For early version units, the application of EFB can provide for somewhat lower quiescent current (from the otherwise ideal operating point), having less impact therefore on operating temps and tube life (but more on that in a moment). However, power output under conditions of both channels being driven is definitely improved, as is distortion under that condition as well.
The ideal quiescent output tube cathode current in an early version receiver (in a perfect world), is 38 mA per tube. From the early version receiver's I've serviced, it seems clear that the Fisher operating conditions produce a quiescent current of about 43 mA per tube when B+ is 425 vdc. This gives a little leeway to account for sagging power supply voltages as power output is increased. It won't account for all of it, as quiescent current will certainly fall below optimum under continuous high power conditions -- but it (obviously) won't fall as far as if it were initially set to 35 or 38 mA. Against this however, I've read many accounts of folks who set the quiescent current in their receivers to 30 or 32 mA per tube, and often enough, as low as 25-27 mA -- erroring on the side of safety and tube life, given all the stories of earlier foreign produced tubes reaching critical mass if operated too hard -- except that these lower numbers (particularly the latter ones) will really impact the performance in early version units -- easily enough to be audible. Foreign produced tubes have gotten much better, but still the practice of setting the bias too low continues.
Against this backdrop, EFB applied to an early version receiver can help performance significantly. Quiescent cathode current can be set for the optimum setting of 38 mA (Pd = 15 watts), and the EFB circuits will hold that optimum B+/current relationship regardless of power output level produced (making power supply sag irrelevant). This lowers distortion, but equally importantly, allows the unit to produce an honest 30 watts RMS per channel with both channels driven, which represents an increase of 10 more available watts (5 per channel) over the stock design -- and at lower distortion than the stock design produced at lower power output capability.
So the benefits are there for early version units, but somewhat different from those provided for later version units. If this is something you are interested in pursuing, I'll try to get at least a hand drawn schematic sketch draw up and posted soon.
I hope this helps!
Dave
