Another walk down Memory Lane. This time, we’ll go into the late Forties, when the Williamson burst on the scene. This English design wiped out all other designs in the USA until about 1955 or so, with the exception of the McIntosh and a few others.
How does it work? There’s an input tube, typically a triode like the 6SN7, direct-coupled to a split-load inverter, also called a "concertina" stage. This always has identical plate and cathode resistors, and gain a bit lower than unity. The plate output drives the upper half of the push-pull amplifier, while the cathode drives the lower half. Despite appearances, the voltages on top and bottom are equal and opposite ... provided the total loads match, as well.
The inverter is then cap-coupled to a separate push-pull driver stage, which is sometimes also set up as a differential stage, depending on the resistance presented to the common cathodes. High impedances move it towards a differential stage, with the limit being modern constant-current sources. 6SN7’s were typically used here, with later designs replacing them with 12AU7’s (which typically have more distortion).
The drivers are then RC cap-coupled to the output tubes in the usual way. The drawback of a classical Williamson are the two stages of cap coupling, which can introduce low-frequency instability unless the output transformer has extremely wide bandwidth. The Partridge transformer specified for the original design had one of the widest bandwidths of any output transformer ever made ... but lesser transformers introduced stability problems, sometimes "motorboating" at low frequencies, but more commonly long recovery times from overload.
The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz, the 20 dB of feedback nicely corrected it, since the input section used a high-gain pentode and there was plenty of "excess gain" to drive the feedback network.
The Dynaco had the advantage of being the cheapest of all to build; a combined pentode/triode, the 7199, took care of the entire front end, and all that was left were a pair of EL34 output tubes and an output transformer. In addition to Dynaco, many receivers used this approach as well. It was simple, saved money, and saved space, which was at a real premium in a low-profile AM/FM stereo receiver.
Receivers in the early Sixties (Fisher, Scott, Sherwood, Harman-Kardon, etc.) all had Bass and Treble tone controls, an AM and FM tuner with two different IF strips, an FM multiplex stereo decoder, a stereo power amp with at least 20 to 35 watts/channel, and last but not least, a stereo phono preamp. All with vacuum tubes, in a very crowded chassis, with marginal ventilation and caps of much lower quality than we have today.
We don’t see many Williamson amplifiers today. The dominant PP-pentode designs are Mullards and Dynacos, depending how price-sensitive the amplifier is. The monster tube amps with 4, 6, or 8 output tubes per channel typically throw in a dedicated cathode-follower section to drive all those grids ... sometimes one cathode follower to drive them all at once, or preferably, each output tube gets its own cathode follower. The RC coupling is then moved to the input side of the cathode follower, and the CF directly drives the grids of the output tube(s). This easily provides independent biasing of each of the output tubes, which is important when that many tubes are used.
