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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.
mesch
mesch OP
3,481 posts
 
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.  
atmasphere's avatar
atmasphere
11,807 posts
 
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.
mesch OP
3,481 posts
 
Thank you Ralph. That was the answer I was looking for with this question. As we discussed when you refurbished this unit last April, I intend to keep it as original. Enjoyed your interview in TAS. 
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