The ST70 is an iconic amplifier for very good reason. Radial Engineering now owns the Dynaco name (Hafler as well) and is reviving some of the classic pieces of gear from these brands. The new ST70 ditches the old pentode front-end in favor of a grounded cathode feeding a cathodyne, feeding a grounded cathode driver. Classic Williamson:
According to Dan Fraser (lead engineer, Dynaco)
During the listening phase of testing process, we found the sound quality of the pentode-triode design to have the lowest sound quality. Still good – but not as good as the other two. When comparing between the triple triode [12AX7 feeding 12AU7 LTP] and the quad triode [12AU7 Williamson] the mid and high end seemed indistinguishable. Both were excellent. However the quad triode with the Cathodyne phase splitter was felt to have a punchier low end.
With the high level of negative feedback, more stages proved better sound. It was also better behaved with regards to balance, according to Fraser. The long-tail-pair version had trouble maintaining balance (though it isn’t clear whether constant current sinks or negative voltage rails were used to maximize the tail).
Fascinating stuff, doubly so because I’ve been working on a large push-pull project recently. By “working on” I really mean collecting parts, obsessing over minor details, and waiting for free time to start cutting wood and punching holes in aluminum plate. I’m planning an Allen Wright-inspired designed with only two stages. Rather than the all-tube cascode Wright employed (successfully), it will use a shunt cascode hybrid for maximum nerd. Hoping for material updates to this project soon!