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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
 
Thank you AL!  I was hoping you would provide guidance. 

I will forget the meter approach for the reasons you stated. I have been playing with the Schiit SYS pre and will continue this way to find what I believe to be a reasonable volume limit.

Regarding the 'weak link' question, I this was something I was told and I know that many approaches to upgrading the ST-70 include replacing the rectifier stage with a different design.  

In any case I am still looking for a speaker to use with this amp that could be bought used for $300-$400. I believe such a speaker should have a TRUE 93dB sensitivity and a reasonably linear 8 ohm (or better) frequency plot. This has been difficult.
rodman99999
6,132 posts
 
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/)   
dekay's avatar
dekay
5,565 posts
 
If you are concerned about overtaxing the 5ar4 take a look @ the following discussion/mod.

http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod
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.
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