Posted by Bob01605 on January 26, 2007 at 12:49:05

Last Saturday afternoon Jan. 20th my audiophile buddy Paul and myself conducted some informal listening tests on three Dynaco ST-70 amplifiers that I own.

Amp # 1 - A bone stock factory built Dynaco ST-70 in nice condition with the stock driver board. This amp has all it's original tubes (Mullard output, Mullard GZ34 and Sylvania 7199) and all tubes tested strong. Came from an estate ..

Amp # 2 - A complete rebuild with the three original cloth lead transformers and a VTA (Vacuum Tube Audio) driver board. This amp had a Mullard GZ34, JAN Phillips 12AT7 driver tubes and Svetlana Flying "C" output tubes. The photo above shows this amp with Electro-Harmonix output tubes but I pulled them and installed a lightly used second set Svetlana Flying "C"'s that I had on hand.

Amp # 3 - A complete rebuild with three new Dynaclone transformers and the VTA driver board. This amp also had a Mullard GZ34, JAN Phillips 12AT7 driver tubes and Svetlana Flying "C" output tubes.

Other equipment used ..
Classe CDT-1 CD transport
Bel Canto DAC-2 upsampling DAC
BAT VK-30SE preamp - (has a stepped volume control with 140 steps)
The three Dynaco ST-70's
Signal Cable interconnects and bi-wired speaker cables
B&W 703 speakers

We played the same six tracks from the Stereophile Test CD 3 (tracks 4-9). Paul never knew what amp he was listening to as he left the room when I made all the cable changes and then I threw a sheet over my stereo rack so he couldn't see what amp was in there. After the 6 tracks were over I let Paul write down what he heard and made no comments although I made a mental note of what I heard.

Synopsis of what we heard.

Amp # 1 - Very smooth and mellow sound but noticeably rolled off at the top end. Bass not as tight as the other two amps. At higher volumes this amp would seem to run out of gas sooner. You couldn't really go beyond "94" on the BAT's volume control without the sound breaking up on peaks. I don't think that the stock quad cap stores enough energy to play continuously at high volume levels.

Amp # 2 - Noticeably better top end - not rolled off like amp # 1 - bass tighter with very slightly more bottom end extension. Amp will play louder without breakup. You could get up to about 97 or 98 on the BAT's volume control before breakup would start to occur on peaks. This amp has a 40, 80, 30, 20 quad cap which is almost double the capcitance of the stock amp. More accurate sound. Larger soundstage than Amp # 1 especially noticeable on track # 4 (Dick Hyman's "Topsy")

Amp # 3 - The sound was virtually the same as Amp # 2 - tighter bass with a little more extension and a better top end than Amp # 1 with noticeably more extension. This amp also has the same 40, 80, 30, 20 quad cap upgrade for more energy storage. Paul thought he heard even more top end extension than Amp # 2. I honestly didn't hear it but Paul is younger than me and maybe hears higher frequencies a little better. Same excellent soundstaging as Amp # 2.

On a short and subsequent test we pulled the original Mullard's from amp # 1 and installed the Svetlana output tubes from Amp # 2 into Amp # 1 - after rebiassing there was no real noticeable change in sound quality from Amp # 1 - same mellow sound but slight shortcomings as noted above. The Svetlana tubes acquitted themseves very well against the legendary Mullards.

Conclusions - Both Amps 2 and 3 gave better sound reproduction than the stock amp and would play louder with less strain. The VTA driver board on both Amps 2 and 3 made a noticeable difference in extension at the top and bottom end and provided a somewhat larger soundstage in both width and depth. The newer A-470 Dynaclone transformers are apparently every bit as good as the original A-470's with the cloth leads and sounded IMHO identical.

Bob (and Paul)