2.06.2004
Suggestions for upgrading the ST-35
Some ST-35 upgrade suggestions found on http://www.audioasylum.com/forums/dynaco/bbs.html:
1. Remove C1 and replace with plain wire jumper (modern preamps have DC blocking caps in their outputs.)
2. Replace C4 and C5 with a high quality .1mfd 400v. coupling capacitor. The caps in the amp are old, cheap Radio Shack Parts. Recommend (econmical) Sprague orange drop or Dayton film and foil cap or (better) Audiocap or Auricap. See Parts express or other online supplier.
3. C6 and C7 are disk ceramics which have a notoriously bad sound. While not in the direct signal path, they are in the global feedback circuit and replacing them with silver mica units (available at Antique Audio Supply on the net) should have a noticeable effect. AAS has a $25 minimum order so if you buy other parts there, it might be more practical.
4. Replace C3 with an economical .1 mfd cap. It is a blocking cap in the positive feedback loop, so it doesn’t need a premium cap, but it should be a better one than Dynaco put in there.
5. Cut or punch a hole about the size of a quarter in the chassis next to the power transformer opposite the can electrolytic. Mount a Ruby 50/50 mfd, 500v electrolytic with its mounting bracket (Parts Express 020-607 and 020-615) Use both sides in parallel as C8C. Tie the triangle and square coded sections of the can electrolytic together and use as C8B.
6. Mount a 2200 mfd, 25v. electrolytic with silicone adhesive under the chassis next to the terminals of the exisiting can electrolytic. Use it to parallel C8D.
This little amp has a superb circuit design and great output transformers and choice of tubes, but suffered from the use of cheap circuit components in the signal path due to cost considerations in manufacturing and marketing. Putting in high quality capacitors and decreasing the P/S impedance will really make this unit sound the equal of many high-end systems.
p.s. please resist the temptation to increase the value of C4&5. They are looking into a 470k grid resistance and I’m afraid increasing the time constant here will cause low-frequency instablity.
1. Remove C1 and replace with plain wire jumper (modern preamps have DC blocking caps in their outputs.)
2. Replace C4 and C5 with a high quality .1mfd 400v. coupling capacitor. The caps in the amp are old, cheap Radio Shack Parts. Recommend (econmical) Sprague orange drop or Dayton film and foil cap or (better) Audiocap or Auricap. See Parts express or other online supplier.
3. C6 and C7 are disk ceramics which have a notoriously bad sound. While not in the direct signal path, they are in the global feedback circuit and replacing them with silver mica units (available at Antique Audio Supply on the net) should have a noticeable effect. AAS has a $25 minimum order so if you buy other parts there, it might be more practical.
4. Replace C3 with an economical .1 mfd cap. It is a blocking cap in the positive feedback loop, so it doesn’t need a premium cap, but it should be a better one than Dynaco put in there.
5. Cut or punch a hole about the size of a quarter in the chassis next to the power transformer opposite the can electrolytic. Mount a Ruby 50/50 mfd, 500v electrolytic with its mounting bracket (Parts Express 020-607 and 020-615) Use both sides in parallel as C8C. Tie the triangle and square coded sections of the can electrolytic together and use as C8B.
6. Mount a 2200 mfd, 25v. electrolytic with silicone adhesive under the chassis next to the terminals of the exisiting can electrolytic. Use it to parallel C8D.
This little amp has a superb circuit design and great output transformers and choice of tubes, but suffered from the use of cheap circuit components in the signal path due to cost considerations in manufacturing and marketing. Putting in high quality capacitors and decreasing the P/S impedance will really make this unit sound the equal of many high-end systems.
p.s. please resist the temptation to increase the value of C4&5. They are looking into a 470k grid resistance and I’m afraid increasing the time constant here will cause low-frequency instablity.
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