Contact Joe Curcio. There are a plethora of upgrades, that can be done to the ST-70's power supply(simple and complex). He's an expert on the piece. (http://www.curcioaudio.com/)
Output threshold using a Dynaco ST-70 amplifier with passive pre.
I am concerned with overdriving my recently refurbisher Dynaco St-70 amp. I is my understanding that the GZ34/5AR4 tube based rectifier is the 'weak link' regarding this amplifiers output power capabilities. Am I right on that (Question 1)?
I am using a HRT Music streamer Ii which has a maximum output of 2.25 volts. This is connected via short interconnects (0.5m or less) to a Schiit Audio SYS passive pre which is in turn connected to the amp via short interconnects. Source is Macbook pro via USB.
Question 2) At what input voltage might I be overdriving this amp? Sensitivity rating states "1.3 volts rms input for 35 watts out". I am thinking that the 1.3v may be at the upper limit given this amp is rated 35wpc.
Question 3) Can I measure the output voltage on the SYS pre at at various output levels as driven by the DAC using digital meter without harm to pre or DAC? I am thinking of using RCA connecters on pre outputs having striped wire ends for attachment of meter probes.
I am using a HRT Music streamer Ii which has a maximum output of 2.25 volts. This is connected via short interconnects (0.5m or less) to a Schiit Audio SYS passive pre which is in turn connected to the amp via short interconnects. Source is Macbook pro via USB.
Question 2) At what input voltage might I be overdriving this amp? Sensitivity rating states "1.3 volts rms input for 35 watts out". I am thinking that the 1.3v may be at the upper limit given this amp is rated 35wpc.
Question 3) Can I measure the output voltage on the SYS pre at at various output levels as driven by the DAC using digital meter without harm to pre or DAC? I am thinking of using RCA connecters on pre outputs having striped wire ends for attachment of meter probes.
- ...
- 7 posts total
If you are concerned about overtaxing the 5ar4 take a look @ the following discussion/mod. http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod |
Thanks rodman and dekay. My ST-70 remains as original with the exception that I had the caps and resistors replaced. I then purchased new tubes. At this time I am inclined to keep it as original. That said, I will follow up on the resources you provided to gain a better understanding of this amp. It was factory made, I believe around 1968, thus I believe 8-10 years into the ST-70's production. |
Regarding Q1: the lack of dual rectifiers in this amp results in the rectifier tube being the number 1 failure mode in the amp as far as tubes go, if the amplifier is routinely driven hard. There are not any good solutions- if you go solid state, use HEXFRED rectifiers or similar for best results. As long as the amp is not being pushed hard the rectifier will hold up fine. |
- 7 posts total