ratbagp wrote:
With so many pp amp designs, what is special about the Williamson design? Is there some special magic or is it a rite of passage?
Just asking, no axe to grind or anything.
ray
You might get a lot of different answers on this (or not!) but to my ears, the beauty of a true Williamson amp is the sheer naturalness of tone. There is nothing "hi-fi" about it--there's no artificial brightness or edge to the sound, no weird phasiness, no incoherency. Instruments and voices sound utterly realistic and honest. The original was an "all triode" design, using 6J5 triodes for the voltage amp, phase splitter and driver stages, then KT66s in triode mode. The triode-wired KT66 was considered a significant upgrade from 2A3s or 300Bs in that you got more power, more transconductance, along with the convenience of indirectly heated power tubes, which meant *much* less noise and a vastly simplified power supply--one filament supply could serve for *all* the tubes.
The other huge leap was *bandwidth*. The Williamson was the first amplifier to employ global negative feedback. In the late 1940's a typical 15 watt push-pull triode power amp using 2A3s (300Bs were rarely used outside industrial theater amps) could reasonably produce a frequency range on the order of 100Hz to 12kHz, or 15kHz if it was a quality piece. The Williamson boasted "flat" response of 20Hz to 200kHz!! This meant that transients were reproduced with astonishing faithfulness. It also employed a very high load on the output tubes--10K plate-to-plate--to achieve incredibly low distortion.
But there was a catch. The Williamson has a daunting number of frequency poles, all threatening to send the amp into oscillation, and the output transformer had to be capable of enormous bandwidth to accomodate the amount of feedback employed without introducing deadly phase shifts at the output. You *cannot* build a Williamson with any old output transformer. In England, the Partridge was built to Dr. Williamson's specifications. In America, the Peerless S-265-Q, the Stancor A-8054 and a handful of others were found to be adequate to produce a reasonable stability. The Peerless is a killer transformer, and that's what I used on both mine and my friend Jim's amps, after an incredible run of luck on eBay at sourcing *six* of these rara avis within the space of three months.
But then there was *another* catch. When all is said and done, and everything is in order, the stock Williamson can produce low and high frequency oscillations if you look at it the wrong way. *Any* vintage Williamson amp needs a fair amount of work to be stabilized. Once stabilized, what was a great amplifier becomes a *brilliant* amplifier, because all the beauty is there without the distortions that supersonic oscillation can produce.
There's a great read at Audiokarma about cleaning up the Heathkit W2, which was Heathkit's answer to the Musician's Amplifier. It's an interesting and enjoyable read about the Williamson design, and anyone interested in the history of hi-fi would find it interesting:
https://audiokarma.org/forums/index.php ... 2m.767851/