Pros and cons of vintage Dynaco ST-70

The Dynaco ST-70 is quite competent, and definitely lower priced than the amps it challenged in its day. As long as you don't need more power, this amp is excellent if properly refurbished. Older amps need all the filter capacitors replaced and the bias supply rectifier replaced as well.

The big weakness of the amp is the 5AR4 rectifier tube, which can't handle the current of the power tubes at full power. As a result, if you are pushing the amp hard, the rectifier tube will fail. There's no good fix for it; you can replace it with solid state, but then all the voltages in the amp (including the filaments) will be a bit too high.

The 7199 driver tubes are not as easy to find any more either. So it might be a good idea to obtain a few spares.

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

Don’t know if anyone mentioned Bob Latino’s version of the Dynaco 70 but worth a look at his site as it details some of the improvements he has made to the original model to address the older design concerns:

http://www.tubes4hifi.com/bob.htm

Full disclosure: I recently purchased one of Bob’s ST120 amps, and while not perfect, it is a nice addition/change from my SS amp for certain types of music (plus I love sitting in the dark staring at the glow of tubes).

I owned a mint McIntosh C22, MR 67 and MC240 years ago. They sounded great, To my amazement though, my Dynaco Stereo 70 produced a deeper soundstage and had a bit more midrange clarity. Both amps were in great condition. 

Don't get me wrong, the McIntosh 240 is a wonderful amp with a superior build quality and measures better than the Dynaco at the frequency extremes. Still the Stereo 70 is an example of superb, cost effective engineering.

As for the original matching preamp there is a Dynaco Pas 3 for sale here on AG. If one owns a ST-70 one can have Ralph of Atma-Sphere refurbish it. I did with mine and am happy.