Try to understand
 

I have read about the Eico HF-81 amp in the new Stereophile.
The author write that the Eico a very good amp, and it use the
Williamson amplifier circuits .
Can you explain what is the "Williamson amplifier circuits "?
And is this circuit realy so good ?

Many thanks

The Williamson style of amp was designed in the '40's, I think. It is a gain stage direct coupled to a split load phase splitter, then to separate common cathode drivers that have a shared cathode resistor. To this point it used 6SN7's throughout. The output stage was originally push pull triode, I think, KT66's with a special custom wound output transformer.

Though this design did use gNFB, it was relatively simplistic for it's time. It had a cult following due to its good sound. Many clones were built, with many different valves and variations but there is supposed to be something magic about the specific original operating conditions.

I have built a reasonable clone of the Williamson. It was my first all valve amplifier. It convinced me that I'd taken a road from which there was no return. Though I go single-ended these days, I have no hesitation in recommending a well built Williamson.