I climbed the great mountain of audio each step bringing me closer to the top?? When my BAT VK-200 fried a transistor my tube buddy loaned me a Dynaco ST-70 to produce some sound while I shipped my prize amp off for repairs. Driving my Von Schweikert VR-4s for once music emerged not sound..When the amp arrived back I hurried to reconnect. Oh no where is my music?? Then my buddy said wait I have something else-a pair of Dynaco Mark IV 40 watts each monoblocks. More of the same only better bass (LOTS) midrange sweet top end Equipped with Mullard EL-34s and only changes made WBT binding post and Cardas inputs. No Synergistic Research power cords just these little lamp cords how can that be?? The BAT amp is gone and now I have decided to sell a 4200.00 amp which is somewhat better but vs. cost not. The Dynaco ST-70-4 stars the Mark IV monos 5 stars..MUSIC

I built one of these amps in 1968 together with the PAS-3X preamp that was its companion. I used it to drive a pair of Acoustic Research AR2ax 3-way speakers, which were considered quite good in their time. In about 1970, I replaced it with Dynaco's new SCA-80 40wpch solid state amp. Here's what I remember about the stereo 70. First, it was quite sensitive to the condition of the EL-34 output tubes. After a while, the amp would just not sound as good. Four new EL-34s would bring it back. There were premium tubes under the "Mullard" brand that made the amp a little "sweeter." I do not think the Stereo 70 really delivered 70 watts; more like 40 - 50. In comparison to the SCA-80 that replaced it, the Stereo 70 had markedly inferior bass response (definition, articulation) and somewhat veiled top-end. Remember, the speakers I was using (like almost all of that era) had a substantial roll-off in the top octave. As to the mid-range; quite honestly the old ARs rolled off the 10 inch woofer at 1.5KHz so the speaker just didn't deliver the kind of midrange that we now take for granted. So I can't really compare. I think for those who like the tube sound, the ST70 might be good for running a pair of small speakers at modest volume levels if you had a subwoofer to take care of everything under 100 Hz.

Soundstage is wonderful. Good bass, sweet midrange, and good highs are heard.

Less than reliable if all original as these are now well over 35 years old. Can be easily upgraded to near bullet proof with simple modifications.

These are without a doubt one of the most popular vacuum tube amplifiers of all time due to the number of units sold over the years. A true bargain in the $200-450.00 price range. My ST-70 was purchased for $200.00 to my door. There was no rust on the nickel plated chassis. The amp was in need of a complete gutting, and rewiring however. I went with the highly regarded KTA (Kennedy Tube HiFi) audio circuit board designed by Kevin Kennedy. We replaced the selenium rectifier, all capacitors under the chassis, and a new large can cap on top. For the new driver board I used Dale, Vishay, and Roederstein Resista resistors. The coupling caps are Sequa paper in oil. I added a Mullard 5AR4 rectifier recently as mine went South. I now use Electro Harmonex EL-34 tubes. My amplifier is wired in triode and produces and honest 19 watts a channel. We (Kevin Kennedy and I) just measured the THD on my ST-70 last week for fun. The results were more than good. At normal listening levels the distortion is around .35%. At full power before clipping at 19 wpc the THD was only 1% before clipping occured at 20 wpc. The sound is sweet, yet natural and yet very detailed with the modifications done to my amplifier. For a "taste" of the tube sound you can not go wrong with a Dyna ST-70. I use my ST-70 with a Pas 3x that has also been highly modified with the addition of the Curcio upgrade boards. I use 4 ohm 91db efficient Celestion Impact 40 speakers. I would heartily recommend this amplifier to anyone interested in tube sound. Just remember if you buy a "stock, or non-modified unit" do change at the least the selenium rectifier with a new diode. Also replace the large can cap (the big silver one on top) wiht a new replacement one. Also consider replacing the original capacitors on the board on top with new replacements for added reliability. Then sit down and smile at the glorious sound you will be rewarded with!

Similar Products Used:

Dynaco Pas 3x modified with Curcio Engineering upgrade boards, Scott 350 tubev FM stereo tuner.

Great Tube Sound, inexpensive

some high-end roll off, finding one in near mint condition

When I first got this amp, I plugged it in and was very, very disapointed. My first tube amp and it made music sound like it was coming from an old trashcan.
AHHHH! But wait!
These things are very to modify and rebuild. That's why I wanted it.

So I went to Triode Electronics (www.trioeel.com)and purchased their ST70 conversion board and new tubes. The conversion uses two EF86 tubes and one 12AU7. I soldered it and biased it. This is my first tube project but I hope for many more.

WOW!!!!!!!!
Even through my BOSE speakers (very midrangey and flakey sounding) it sounds great. I play a lot of Jazz vinyl through it on a Technics 1200 markII deck.

I have no experience with other tube amps but have been told the ST70 with modifications can sound better than McIntosh and Audio Research amps.I hope to compare it at a later date with a MAC 275 or 240.

I hope to buy some NHT's or PSB speakers in the near future. I am sure the ST70 will sound even better then.

My only recommendation is this: If you buy the ST70, don't give up on it until you have tried a modified board and experimented with tubes. There are a lot of web-sites that address mod-ing the ST70. You won't regret it.