@jetter Hello, and thank you for asking.
Actually, I really love my Dynaco ST70. I’ve owned and own a lot of amps over the years, and most of them have another zero in their price tags, including my Jadis amplifiers. Over the years, I learned that my beloved Jadis Orchestra Reference and Quicksilver's initial and what many call their best sounding amplifier the 8417 are just Dynaco amplifiers. In my opinion, there’s no more fun audio component than an ST70. Of course, one could have the same feelings for the very similar MKII, MKIII, and MKIV.
In terms of sonics, mine is not a classic ST70. Those are as you mentioned, very old school sounding in that vintage lush, warm, caramel colored, slow, tubby sort of sound. I have had that.
I’ve built a few ST70s, and have experimented, pondered, and learned from each section of this very simple amplifier. It’s taught me more than anything else in audio. Personally, I find it beautiful in its simplicity on many levels, and from many directions. Where I’ve ended up is discovering what differentiates vintage and modern tube amplifiers is the metal film resistors used in today’s products compared with the vintage carbon composition resistors. People, especially those who build and sell amplifiers love to tell everyone about their ingenuity and innovation regarding their circuits. That’s all fiction, these circuits all originated in the 1930s - 1950s. In fact, apart from SETs, 99% of today’s tube amplifiers use the Mullard long-tail pair driver circuit given freely to generate sales of their tubes. And most of the ST70 "upgrade" boards use this circuit as well, with the intent of fixing so much of what Dynaco poorly implemented. However, working through all of those things brought me full circle. It’s the concertina driver circuit Dynaco took from the guitar amplifier industry that’s the better implementation, in my opinion. But that ended up being so obvious that I felt stupid upon coming to that conclusion. I don’t want to bore folks, and write too much except to say simple usually is better, and that’s a big reason so many have come to treasure what an SET provides.
So my ST70 is a mostly stock implementation of the original, but with metal film resistors that give it the clarity, openness, and neutrality of a modern amplifier, with the speed and lithe nature their more complex circuitry never will. It can possess the sweetness you mentioned, but that’s a reflection of the tubes employed. I happen to like that quality, and I pursue it. One ST70 weakness was the low-end, and there are some logical reasons for that. One should jump out as completely obvious to anyone familiar with tube amplifiers, and the other one is not at all obvious. Taking care of those two things brings a heft the original longs for, though the obvious mod also must be done with a sort of a judicious hand, as it also causes some of the magic of the original to dissipate. Another mod I’ve discovered of late is one I’m infinitely thankful for, as it protects the rectifier tube from blowing upon initial turn on. And for anyone who loves the sound of the vintage Mullard 5AR4, the peace of mind of not blowing a $120 tube when powering up the amp is a true gift. What weaknesses remain? Mostly that it never was the sort of monster power amplifier, and it remains that way. That said, like most any decent push-pull tube amplifier, despite what most folks worry over, it will drive any reasonable loudspeaker to lease breaking volume without a second thought.
