I'll be glad to snap a pic of mine when I get a chance. They are much like yours were -- a gold chassis and a gray chassis. In addition, the gray chassis has a Chicago OPT, while the gold chassis has a Stancor OPT and a replaced power transformer (by a previous owner) which is massive and could likely run both amplifiers by itself. Strangely, its 6.3 volt winding is only good for about 1.5A, but does have a high current 12.6 volt CT filament winding. For balanced operation of the heater circuit then, I had to effectively wire the output tube heaters in series and the 6SN7 heaters in series, and connect these series strings across the 12.6 volt winding. They still have their original (good) can caps, and a hodge-podge of tubes installed.
I mention all of this because many of my pieces do not have the refined, finished look that the product of a "build" has. The closest I've ever come to that was the Fisher SA-100 Clone I did some years ago. Rather, many of my pieces are truly developmental pieces, and have remained as such. Not a chassis used to house some completely different kind of electronic design mind you, but an existing piece modified to a demonstrably worthwhile level of improvement over its stock existence. With my Eico pieces, I've pushed that envelope in some cases with HF-60s sporting a Williamson front end, triode connected 6550 tubes, and HF-20/22/35 OPTs, or HF-35s also converted to a Williamson front end, but using 5881 output tubes and higher drop 5V4GA rectifier tube. Both of these efforts make for an outstanding use of the Eico HF-20/22/35 OPT, and produce superb results. All were the product of an evolving effort in trying to achieve the best possible performance from a given OPT offered in Eico products. By comparison, the stock HF-20, HF-22, and HF-35 -- certainly nice amplifiers -- all have significant design compromises too, that are avoided with the mix and match efforts described. The stock HF-50/60 went through a similar modification development effort as well.
With the Heath W4-AM amplifiers, the original design - like other Heath amplifiers I've chronicled -- went through its own evolution process with Heath, starting with a very unstable version of the amplifier, and ending up with one that was stable enough alright, but (imo) sucked all the life out of the music, and had notably deteriorated specifications (confirmed by measurement). My first modification was aimed at resolving those issues, by creating an amplifier that was very stable, but didn't achieve that end by clamping the response so much that it affected performance. This is the modification that includes the more complex dual 4/8Ω feedback network. The modified amplifier both sounds audibly better, and measures extremely well. But through that process, it became apparent that the output transformers -- while excellent -- were not really up to the full power capability of the amplifier, being taxed on the high end of the audio spectrum due to the high primary impedance (and therefore higher shunt capacitance) of the transformers (10K). The result was that the power curve rolled off somewhat on the high end. But within the flat power bandwidth the transformers could handle, they were superb. That fact morphed into the amps that Derek built here.
I always felt that the original Williamson amplifier was never really developed to the full potential of which it was capable. Early versions that were actually stable were few and far between, and even those that were/are stable, are/were only marginally so at best. Ultra Linear came along which instantly made the old triode output stage obsolete. By the time that OPTs caught up to the wide frequency response needed for stable NFB to be applied, designers had moved on from the Williamson design, with the new darling of design being Laurent of Dynaco fame. With better OPTs, fewer stages, and high performance miniature tubes, by the end of the 50s, the Williamson was a relic of the past, with even Heath moving on from its absolute dedication to the basic concept. The original Williamson amplifier then never got fully developed because it never got a chance before technology replaced it.
But, with the OPTs of the W-4 amplifiers being ideally suited for the original Williamson design, and the triode connection of the output stage able to make the most of those transformers, the modified W4-AM amplifiers then evolved into their final configuration of being a really well executed version of the original design. There are differences of course, but those differences are what make this version so superb. They include:
1. The use of EL34 output tubes: These tubes allow the amplifier to develop the full power output of the original KT66 version from Williamson (definitely more than the 807 Americanized version), but because of their high Gm, produce an input sensitivity that is over twice that of the original design -- now requiring just 1.0 vac to develop full power output. Williamson's use of KT66 tubes produced an input sensitivity requiring 2.25 vac to produce full power output. The EL34s operate with a standing current of 60 mA, which produces extremely low distortion levels, while keeping plate dissipation levels safely below the rated dissipation of the tube. True to the original Williamson concept, the output stage operates in pure Class A throughout the entire power output range of the amplifier. Distortion at full power output (about 12 watts rms) anywhere from 20 Hz to 20 kHz is well down in guaranteed McIntosh performance territory (no greater than 0.5%).
2. Proper bias applied to the 6SN7 driver stage: Williamson's original version used a tube that was only similar to the 6SN7, but not a direct copy. When Sarser and Sprinkle introduced the Williamson amplifier to America, they made the good choice to use a 6SN7 in the Americanized version, but failed to adjust the cathode resistor appropriately for the characteristics of the 6SN7. By using the value Williamson used, it left the driver stage underbiased, and also needlessly cooked the tube. The proper cathode resistor allows the driver stage to develop even more undistorted drive capability, with less distortion in the process.
3. Altered coupling cap values: Even Williamson's version was famous for the woofer element "breathing" in and out slowly due to poor LF stability caused in large part by the poor choice of coupling cap values used. The values used in this version allow for full power output at 20 Hz with very low distortion, but eliminate all traces of LF instability.
4. Inclusion of a step network at the plate of the First AF Amplifier stage: This was something Williamson only reluctantly added to his design, admitting in his updated version that it does in fact provide improved HF stability. And it does. But he didn't not take it near far enough, because at the time, extended HF response well up into the supersonic range was championed far more than stability was. History has shown that this was a major mistake. As long as response is flat out to 20 kHz, a tapered response above that point is very important to achieve -- not only to achieving high levels of HF stability, but also in eliminating RFI and other nuisances that and extended response causes.
5. Inclusion of a phase advance cap across the NFB resistor: Williamson stated that such a cap would not help stabilize his design. While this is generally true, such a cap does aid in achieving a very flat response out to 20 kHz, and so it is included. True to Williamson's specifications, the loop operates with 20 db of NFB.
6. Inclusion of an input grid stopper resistor: Williamson did not include this, but such a resistor aids in achieving a smooth reduction in HF response in the supersonic region, and contributes to the excellent HF stability achieved.
So there are differences from Williamson's published schematic. But the point is, all the modification efforts of this amplifier were aimed at achieving all of the intended goals of his design, using the true topology of his design, while using more modern tubes, an improved OPT, and all the wisdom that history affords. IMO, the results produce an amplifier that is extremely hard to turn off.
Dave