Mad -- I surely didn't mean to muddy up the waters with my comments, although now that I've seen the pics of your unit, it adds a whole new element (of mystery?) to the series beginning with the original Custom Sixty.
The Custom Sixty I am aware of is that which has been described in this thread -- triple rectifier tubes on a separate chassis, and separate amplifier chassis with push-pull-parallel 6B4 or 6A3 triodes (same tubes, different base) in the output stage. In my comments, I stated that the Custom Sixty was the forerunner of the 50A -- and that is true, but more specifically, I meant that by topology. By that analysis, the Custom Sixty is very much the fore funner of the 50A, with a split load phase inverter, pre-driver stage, and cathode follower driver stage driving a Class AB2 output stage all coming over from the Custom Sixty, into the 50A, long about 1952. To the original design however, certain compromises had to be made for it to morph into the 50A. In was transformer/capacitive coupling into the output stage, and gone was the third rectifier tube. In was a new bias regulator and preamp input stage to create the 50A.
But your unit adds even more fuel to that fire, as it is a unit I've never seen before, and by everything mentioned/shown here, appears to be the final iteration of the 50A design. Still a dual chassis build like the Custom Sixty, but now with the transformer coupled driver stage and push-pull output stage of the 50A.
So your amplifier is a blend of both the original Custom Sixty, and the 50A -- it would be fascinating to see the schematic of your amplifier to see how that blend was accomplished. It was probably a pretty good move on Fisher's part to help reduce aging inventory so it could be a cleanup piece, but still very much a member of Fisher's Class AB2 effort, and as I say, one I wasn't aware of, as I thought the 50A line ended when the 55A line began -- but apparently the 50A line continued on in the background as the 360A/P you have for console duty.
Nearly a couple years ago, I did a deep dive study into the original Custom Sixty for my own purposes when I became aware of that unit. I was already well published on the 50A and 55A amplifiers, and my intent was to design a "modern" stereo 55A amplifier, with fully regulated power supplies and mosfet drivers to eliminate the driver transformer of the 50A, without adding more tubes to what was already a well establish ultimate Williamson amplifier chassis, while still achieving Class AB2 operation. I wrote up a paper on it, but never published it, as it became rather involved to say the least. But in it, I discuss the necessary changes Fisher made to morph the Custom Sixty into the 50A, which at the time were rather bold for the high fidelity world. That said however, that project has had a very successful outcome, and has become one of my most favorite pieces of all time -- so again, your unit is worth everything you can put into it!
Will definitely be following along!
Dave
