Power switch added to front of another reworked ST-35
The Dynaco ST-35 was one of the last tube power amps designed by Dynaco. Many feel, and I concur, that this little amp, within its power range, is probably the best sounding ALL STOCK power amp that Dynaco ever put out. Only 17.5 watts per channel but, even unmodified, a very clean sounding 17.5 watts. EL84 output tubes, a 12DW7/7247 all triode driver system and a solid state rectifier gave this little amp excellent frequency extension and very low distortion figures. Dynaco did cut a few corners to save a buck. There was no on/off switch, no choke and no adjustable bias system like on the ST-70. With a little time and a few modifications you can still improve the sound of this little gem.
1. Put in an ST-70 C-354 choke in place of the 50 ohm resistor on the quad cap. There is a nice place for it on top of the chassis right in front of the left output transformer. Just drill two 9/32" holes that are 2 3/8" apart. The choke will cut down on AC ripple current and lower the noise floor of the amp somewhat.
2. Remove C1 on both channels and replace it with a jumper wire. C1 is a DC blocking cap but isn't really needed since all modern preamps have a DC blocking output cap.
3. Pull C6 (18 pF) and C7 (27 pF), two disc ceramics, from the feedback circuit and replace them with better silver mica units. These two capacitors have usually drifted pretty far off their original values in 40+ years. If you still have the original ones in there now both channels probably have different amounts of feedback now.
4. Replace the main coupling caps C4 and C5. Dynaco used relatively cheap caps here. Auricaps will fit in here nicely. Be careful which replacement caps you buy because there is limited space on this smallish sized amp.
5. Replace C3. You don't really need a premium cap here but you might as well replace it since the cap in there is 40+ years old.
6. Replace the input jacks with gold plated input jacks. The original ones were aluminum and over the years they develop a hazy coating and sometimes cause "non contact" issues. Do you sometimes have to jiggle your interconnects to get sound out of the amp ?
7. Replace the output terminals with modern 5 way binding posts. Dynakitparts sells a replacement set.
8. Check the resistance of all those carbon composition resistors on the old board. (The resistor values may be found in the ST-35 manual - Link below) They usually drift high and sometimes start to make noise. (Do you hear a little "hisssss" that maybe shouldn't be there ?) Replace them with metal film resistors. You could also replace both boards and all parts with the Dynakitparts ST-35 replacement driver boards and parts set.
9. Add a power switch. Any smaller 1/4 inch AC switch will work just fine. Just drill a 1/4 inch hole in the side of the chassis somewhere on the front of the amp.
Dynaco ST-35 assembly manual