The Williamson is not unconditionally stable, even with a WWFB. We can measure that well now, and some subtle circuit changes can make it unconditionally stable at low and high ends. Sadly, many ‘Willys’ had issues back in the late 40’s and beyond as few had measurement equipment, and many commercial output transformers did not meet spec.

What is the name of your book which you wrote on the the true history of the Williamson amplifier? Is it in English?
Unfortunately I can not read Italian.

Chris please send me your circuit diagram my friend.
harmonsalmon583@gmail.com
Thanks

“But the biggest flaw of the Williamson was its lack of unconditional stability. Because feedback is put across so many stages, including the completely unnecessary differential driver stage, it is not stable into certain reactive loads.”

Rubbish, Bafflegab. There is only one type of feedback applied in the Williamson amplifier (as can clearly be seen from the schematic) and that is overall feedback from the output transformer’s secondary back to the input stage.
Remember: the Williamson is in effect a simple three-stage design because V1 is directly coupled to V2, making V1 plus V2 a single stage.
Furthermore, Partridge established themselves as THE prime supplier of output transformers for those seeking to build the Williamson (Williamson himself never sold OPT’s to the consumer, he merely laid down the transformer specs his amp called for). And what an OPT the Partridge was! It was never equalled by any competitor, let alone surpassed.

“It runs a pair of 6L6 class power tubes into 10 K ohms, and gets perhaps fifteen to twenty watts. A more modern design would get a minimum of 35 and perhaps as much as 50 watts from the same tubes.”

“More modern”? Well, that would have been counter-productive to the whole idea of the Williamson, wouldn’t it? The “Willie”, after all, is a pure class-a design, that goes for maximum sound quality, not for max output power (and thus distortion). By running the KT66’s well below their max. plate dissipation one gains (beside superior sound) the added benefit of dramatically prolonged tube-life.

The distortion that tubes make is not the same type as transistor distortion … just look at the se amp figures. I have heard amps that measure perfect but sounds like cardboard . When you can hear the ring and harmonics of piano strings ,clear vocals , and acurate reproducion , these are remarkable amps . I heard an original years before I built these …. clear and acurate . Pilot used this design in their amps with 6sn 7’s . I use the original kt66 outputs and glass 6j5’s are better than the metal ones . The way Ihave it set up is 1%@1000 HZ . .. 20 WATTS ….Max is 10 % @50 watts .. . . Sounds great driving my AR-9’s with my SAE amp driving the 4-12″ woofers -bi amped. Cymbals sound like brass . I have heard a lot of amps over the years . I have had these in use since 1990 . Super reliable . It works for me . Nothing like 14 tubes heating the room on a cold winter night!

If you look at specs , you are not hearing the music . I have heard amps that test great on the bench and sound like cardboard. Specs give no indication of how an amp reproduces music and sounds accurate. When you hear all the harmonics of a piano , and cymbals sound like cymbals , vocalists sound like they are in the room with you -its awesome . You listen to your specs -I will listen to the music . I have the feed back loop connected and switched … better with it off …