Despite owning some very exotic and expensive amplifiers over the years, I’m a huge fan of the Dynaco ST70. Although it employed circuitry already in existence in the guitar world, it became the torch bearer for the high-end audio implementation.
Such wonderful sounding and highly esteemed amplifiers as the Quicksilver 8417 and Jadis Orchestra use the same design, which testifies to the relevance and efficacy of its architecture. Like those amplifiers, the ST70 brings much joy to those who treasure the midrange. Sonically, with modern component (resistors, capacitors, etc.) parts and the right tubes, while not the last word in speed, power, tautness of bass, extension in the upper or lower registers, or absolute transparency, there’s an inherent rightness and beauty to the sound that’s lacking in most amplifiers, vintage or modern. Tube rectification explains much of the low end complaints. But I believe its benefits far outweigh its sins, though that’s obviously a personal thing. And for all the criticisms of the ST70 from stem to stern, in all honestly, though I’ve listened to many "improvements" to the original (power supply, input and output stages), both from Dynaco itself and third parties, I’ve never heard any better the original, holistically. In my opinion, that’s a reflection of the purity that comes out of its simplicity, and aligns with why SETs have emerged as some of the most beloved and emotionally involving amplification in current times.
On my journey, I’ve found two particular subtle and very low cost changes address its deficiency in the low-end. One is quite obvious, the other not at all. But in no way do these affect the sonics of the amplifier outside of the bass, and that’s why they appeal to me.
Regarding reliability and overall design, the fact that the majority of them got up and running by people who had no previous experience in building such creatures, and continued to perform with little trouble for many, many years running their original tubes says it all
