I had built many ST-70s in the Sixties and I decided this time that I would experiment a little as I already had a fine working system and a lots of time now that I was a retiree.
The main decision (apart from buying the VTA board) I took concerning this project was to built the amp following an integral hierarchical star grounding system. To this effect:
- I changed completely the return paths on the VTA board by cutting and severing all return lines that created a mixed ground system;
- I grouped every stage return path to a common point;
- each one of those return paths were connected to the return path of their own B+;
- I ran 4 separate B+ lines (on each channel one for the driver and one for the phase splitter) and each one twisted with its return path from the fourth section of the quad cap;
- the common return for the push-pull output tubes cathodes of each channel was connected to an insulated lug between the tubes and a separate ground return connected those lugs to the common star point;
- finally, the feedback return wire was twisted with the OPT secondary ground return and this ground return was connected to the common signal return of THE FIRST stage.
Top of the modified VTA board
Underneath the chassis
Notice: the VTA board is not mounted under but on top of the chassis. Notice also the feedback returns twisted wires.
Underside of the VTA board
Output tubes common insulated signal return ground point
Another change I made was to fit the modern five-way binding post while keeping a choice of output impedance. See the picture:
Speaker binding posts with impedance selector
Overall, this is the final result:
ST-70 45° view
ST-70 front view
St-70 front view slightly above
ST-70 rear view 45°
The sound is fine and has all the delicacy of a good tube amplifier. A final point: this amp was tested with the JJ ECC82 tubes sent by tubes4hifi and with NOS Canadian Marconi Radiotron 12AU7s. To my surprise the sound, with the Marconi tubes is much better! Timbres are more accurate and the fine harmonics in violin ensembles come through with a delicacy absent from the JJ tubes. I'm amazed because I didn't believe in the intrinsic superiority of those vintage tubes. I'm tempted to disconnect the feedback returns and measure the frequency response above 20 KHz to see if the Marconi and the JJ behave differently.
Thanks for looking at the pictures and any comment would be appreciated.