ST 70 best sounding version?

 "....just not a 5AR4."  Noted :)

I have two Stereo 70s as well as Adcom 5802s and Sonic Frontier amplifiers. Both Stereo 70s have been updated. One has the original stock circuit with all caps and resistors replaced with modern polypropylene caps and metal film resistors. The updated stock amp sounds very nice. Sonically it has a lush midrange with a deep soundstage. The midrange is sweet sounding, bass is OK but not tight and the highs are pleasant.

The Curcio designed mod has both boards (4 6DJ8s). Sonically the amp is excellent, if the 6DJ8s are in good shape. The amp has a very open, clean sound, a deep and wide sound stage and tighter bass than the stock unit. Sonically it's not sweet sounding but seems accurate. 

The Adcom 5802s by comparison (bi-amped) are Nelson Pass designs (3 gain stages). The amps produce very tight bass, a wide and deep soundstage, very low noise, and a smooth and pleasant high end with excellent reproduction of spacial cues (when they exist). 

The Sonic Frontiers sound similar to the Curcio design with a bit more punch likely due to the larger power transformer and output transformers. 

Triode Electronics of Chicago offers a new power transformer for the ST-70 that has increased current for the rectifier circuit. This allow you to run dual 5AR4s in parallel rather than one- thus fixing the biggest single weakness of the original amp (which had distortion low enough that it rivals the classic Marantz 8B), which is that it eats rectifier tubes.


You can reduce distortion in the amp by replacing the 7199 driver tube with a 6GH8A, by using an inexpensive adapter for the conversion available on eBay. 6GH8s are a lot cheaper and easier to find than 7199s!


Finally, a set of ODAM V-Caps work out very nicely as the main coupling caps in the ST-70- better sounding and better performance. Of course the stock filter caps should be replaced, along with the selenium rectifier used in the bias circuit. The filter cap to use is a 4-section part made by JJ, available from CE Distribution. Its small enough it can be mounted beneath the chassis, allowing the 2nd 5AR4 to be located where the old filter can was.
Out of spec resistors should be replaced. I don't like the speaker terminals but without chopping up the chassis there's not much for it.

I needed some music in the room I have placed my treadmill.

I hooked up my all original Dynaco ST70 to an even older Heathkit AA141 tube preamplifier that a friend gave me years ago. Tubes used in the Dynaco are old stock Mullard el34s and rectifier along with RCA or Amperex 7199’s (I can’t remember). I threw some inexpensive Chinese 12ax7s that I had laying around in the Heathkit..

Hooked up to a Yamaha T85 tuner and B&W DM110 speakers, this antique system is surprisingly pleasant.

So bottom line, the Dynaco ST70 in its original configuration can sound OK.