I had a customer Email me and mention that at one time he had a version of the Dynaco ST-70 made by Audio Research between 1970 and 1974). He said it ran really hot, broke down a few times but sounded good when it was working right.
Audio Research had 3 versions of the Dynaco ST-70 that they put out during the early '70's - the C1, C2 and C3 (photos of each and a schematic of the C3 below).
C1 below > The C1 used a tube rectifier with two OA2 voltage regulator tubes. The driver circuit used SIX driver tubes (three for each channel). This amp had THIRTEEN tubes on a relatively small 9 1/2 inch by 13 inch chassis. No wonder the amp ran hot! It had 4 X EL34 or 6L6 output tubes, 2 X 12AX7, 4 X 6FQ7, 2 X OA2 and a 5AR4/GZ34 tube rectifier.
C2 below > Pretty much the same tube line up but now the front tube sockets were capped and the amp has better output terminals on the rear.
C3 below > Gone is tube rectification and the circuit has been altered now with 3 X 12AX7's and no OA2 voltage regulators. (only 11 tubes now) This last version was available now as an UPGRADE to existing ST-70's. You sent your ST-70 to Audio Research with a check for $385 and they did the upgrade for you and shipped the amp back to you. I do not think that many were sent in. The $385 was about triple the cost of the original ST-70 in kit form. The C3 version also used a much larger board that was suspended below the original board opening.
ARC C3 schematic below ..
Bob
Audio Research had 3 versions of the Dynaco ST-70 that they put out during the early '70's - the C1, C2 and C3 (photos of each and a schematic of the C3 below).
C1 below > The C1 used a tube rectifier with two OA2 voltage regulator tubes. The driver circuit used SIX driver tubes (three for each channel). This amp had THIRTEEN tubes on a relatively small 9 1/2 inch by 13 inch chassis. No wonder the amp ran hot! It had 4 X EL34 or 6L6 output tubes, 2 X 12AX7, 4 X 6FQ7, 2 X OA2 and a 5AR4/GZ34 tube rectifier.
C2 below > Pretty much the same tube line up but now the front tube sockets were capped and the amp has better output terminals on the rear.
C3 below > Gone is tube rectification and the circuit has been altered now with 3 X 12AX7's and no OA2 voltage regulators. (only 11 tubes now) This last version was available now as an UPGRADE to existing ST-70's. You sent your ST-70 to Audio Research with a check for $385 and they did the upgrade for you and shipped the amp back to you. I do not think that many were sent in. The $385 was about triple the cost of the original ST-70 in kit form. The C3 version also used a much larger board that was suspended below the original board opening.
ARC C3 schematic below ..
Bob
