Pros and cons of vintage Dynaco ST-70

Think of it this way, they were designed when speakers had much more limited frequency response and digital music didn't exist.  I've played with a lot of vintage Dynaco gear and am not thrilled.  They need modded.  

When I was playing around and modding Quad II, Radfords, Croft, Leak, TVA, EICo tube amps, the Dynaco was always considered low rent.

Frank Van Alstine had (and has?) an extensive ST70 mod which greatly improved it, as did Bill Johnson in the early days of Audio Research. Frank also modded the Dynaco PAS 2/3 pre, which was a fitting companion.

@elevick @noromance @bdp24 

Many thanks, very helpful. I will proceed with caution. 

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.