With these modifications -- as with all designs to begin with -- it is all about compromises, and principally, achieving the best mix of them.
The basic topology of the W4 circuit is wonderful, and in its original form, is capable of providing about 18 watts of superb performance. Its only drawback is the poor stability produced by the original design. The modifications offered for that design correct those issues.
For the modification using EL34s, the decision to use that tube should be kept in context, as the goal was not driven by the sound of that tube, but by the sound that a different topology would produce.
The original W4 employs a class AB1 UL 5881 output stage, where as the goals that ended up becoming the EL34 modification were to convert the unit to a class A triode output stage, in keeping with the original Williamson concept.
The original version of Williamson's design employed KT66 output tubes, which produce 10 watts RMS power output in his design. However, at the time of America's introduction to the Williamson, the KT66 was not available. The closest tube by characteristics that could handle the voltage was the 807, so all of the earliest American versions of the amplifier used that tube.
When the 5881 come along, the ability to have the same characteristics as the 807 bottled up in a convenient octal base format with no top cap, and voltage ratings that were acceptable for the Williamson design, meant that all such copies of the amplifier quickly converted over to the 5881. However, compared to the KT66, both of these tubes resulted in a power loss, as they could only realistically provide 6-7 watts RMS of power output in Williamson's design. By the time the KT66 did become available in America, UL had already made its big splash, and the triode output stage was quickly abandoned, whether the KT66 was available, or not.
But returning to Williamson's design, the only practical tube he had available for his design at the time was the KT66. Therefore, even though it is a wonderful tube in its own right, it became his tube of choice principally by default. The design was published, and the rest is history.
One of the drawbacks of his design -- as history shows -- is that it requires a notably greater input signal to achieve full power output. This is typically over 2 volts, compared to the 1+ volt figure that was quickly becoming the norm of the day with faithful conversions of his design to UL operation, and other designs as well. In any event, the 2+ volt driving requirement coupled with only 6-7 watts of usable power here in America, meant that the stage was set (no pun intended) for UL to quickly take off, and take off it did.
In my efforts then to produce a really good version of Williamson's efforts -- using his topology and design goals -- then the EL34 -- unavailable at the time of Williamson's design -- makes for an ideal tube. Consider that:
1. It can easily handle the voltages of the Williamson design.
2. All else being equal, it can produce more power output than the KT66 can in the Williamson design and still run well within rated dissipation levels.
3. Due to its higher Gm rating, it can be used in the design and produce a very normal 1.0 vac drive requirement, while still employing the same amount of NFB specified by Williamson.
Therefore, use of the EL34 became an obvious choice, as it overcame the original American problem of low power output, and even improved on Williamson's original design specification for sensitivity.
The only other matter requiring attention was the famous high and low frequency instability problems associated with the design, which I have also addressed with the EL34 modification.
So the effort of converting the W4 to use EL34s was not so much an effort to convert the W4 simply to use another tube, but because of the excellent and perfectly suited transformer set offered, convert the unit to represent the finest example of the heritage it grew out of, with appropriate fixes for its blemishes attended to along the way. Using the EL34 then was just one of the considerations made to achieve that goal.
I hope that helps!
Dave
