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
Hi Mesch,

Q1: I don’t know the answer.

Q2: Yes, based on the specs I would expect that the amp would be driven to the approximate vicinity of its 35 watt maximum continuous power capability by an input of 1.3 volts. Although the dynamic (short-term) power capability of the ST-70 is apparently significantly higher.

Q3: Yes, you can do that, but there’s probably no point in doing so. It would probably be more meaningful to gradually increase the volume control settings you use while listening for early indications of the onset of clipping (i.e., perceivable increases in distortion).

For a voltage reading to be meaningful, the meter would have to have bandwidth extending over all or nearly all of the audible frequency range; it would have to be fast enough to fully capture the amplitude of a brief transient; and if it is fast enough to accomplish that capture it would probably have to provide a peak-holding function that would allow the value to be displayed for enough time for you to see it. Also, the 35 watt and 1.3 volt specs are probably based on the amplitude of a sine wave as defined on an "RMS" (root mean square) basis, while music is of course not a pure sine wave, and depending on the particular meter what it may indicate could be a peak value, an RMS value calculated from a measured peak based on a sine wave assumption, or a "true RMS" value. So there are lots of potential issues that would be involved in obtaining a meaningful measurement.

In any event, enjoy! Best regards,
-- Al
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.
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/)   
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.
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.