This is a review and detailed measurements of the Bob Latino's take on Dyna ST-70 stereo tube amplifier. I believe (the kit?) costs US $1,430. This unit was bought used for around US $900.
The polished stainless steal case looks very nice. The three transformers give it substantial weight. As you see, the front-end/bias is a new design:
The indicated bias voltages are from original Dyna design and don't hold for this configuration. Per Bob, the right DC bias voltage for the EL34 tubes is 0.4 volt which is what I adjusted them to. There is fair amount of interaction between the four pots so I had to go back and forth half a dozen times to get it adjusted so.
There is a switch for operation in triode in addition to pentode. Most of my testing was done in Pentode mode but I also ran a dashboard in triode mode.
Tubes that are included are: NOS Mullard EL-34's and a NOS Brimar CV4003/12AU7 and new production Prima Luna 12AU7s.
The RCA connector was loose as was the ground terminal in one channel. Neither seemed to impact operation unless you really tugged on the RCA connector the wrong way.
Back panel is modified as well with inclusion of modern binding posts:
The taps are 4 and 8 ohm (labels are old and don't apply). New power cord is included.
VTA ST-70 Measurements
I let the unit warm up good deal before setting the bias. As such, output was quite stable with time:
Here is our dashboard in pentode mode:
I could not impact the power supply spikes with any grounding scheme. They are inherent to the design of the unit. Worst offender though is the third harmonic which peaks to nearly -50 dB and sets SINAD accordingly. With median SINAD score of all amps tested at 78 dB, this is copious amount of distortion of course. But it betters the Carver Crimson 275 amplifier by a few dBs (46 dB SINAD).
Then again, distortion is up by some 10 dB relative to the true vintage Dynaco ST-70 amplifier. That unit achieved a SINAD of 63 dB in good channel.
There has been some discussion in their forum about the correct bias voltage so I also tested the unit by setting it to 0.45 volt. That helped the weak channel gain about 2 dB in SINAD and also brought its gain closer to the other channel. Not sure what the impact on longevity of the unit is to operate it at that level.
Triode mode costs you more performance in the form of increased distortion:
As noted, watch out for difference in volume as you AB because the gain goes down in Triode mode. Continuing with 0.4 volt bias and pentode mode we get reasonable SNR values for a tube amp:
Again, this betters the Carver to the tune of 7 dB.
Frequency response was fine in one channel but had an odd notch in the other:
Fortunately it is outside of the audible band so just a curiosity (although lack of gain/level matching remains a small issue).
Crosstalk was surprisingly good:
Multitone shows the effect of all the distortion/intermodulation products (and power supply noise):
Let's sweep for power:
So we get 10 more watts than Carver 275 before clipping. The distortion rise is more steep though, resulting in much less power if we limit distortion to just 1%:
Using the 8 ohm tap we get the same power as 4 ohm:
I was hoping to listen to the amp but forgot and took it all apart for the review. When I get a chance, I will give it a listen.
Conclusions
Other than the unique look of the unit, I am the wrong guy to ask about the appeal of these tube amps.
There is little to hang your hat on as far as fidelity merit. That aside, comparing to the Carver Crimson 275, performance is better in most areas. Sadly distortion is much worse than the original ST-70. Not sure if this is planned or artifact of not measuring the performance of all these mods.
Anyway, this is not for me but some of you have different priorities than me.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
The polished stainless steal case looks very nice. The three transformers give it substantial weight. As you see, the front-end/bias is a new design:
The indicated bias voltages are from original Dyna design and don't hold for this configuration. Per Bob, the right DC bias voltage for the EL34 tubes is 0.4 volt which is what I adjusted them to. There is fair amount of interaction between the four pots so I had to go back and forth half a dozen times to get it adjusted so.
There is a switch for operation in triode in addition to pentode. Most of my testing was done in Pentode mode but I also ran a dashboard in triode mode.
Tubes that are included are: NOS Mullard EL-34's and a NOS Brimar CV4003/12AU7 and new production Prima Luna 12AU7s.
The RCA connector was loose as was the ground terminal in one channel. Neither seemed to impact operation unless you really tugged on the RCA connector the wrong way.
Back panel is modified as well with inclusion of modern binding posts:
The taps are 4 and 8 ohm (labels are old and don't apply). New power cord is included.
VTA ST-70 Measurements
I let the unit warm up good deal before setting the bias. As such, output was quite stable with time:
Here is our dashboard in pentode mode:
I could not impact the power supply spikes with any grounding scheme. They are inherent to the design of the unit. Worst offender though is the third harmonic which peaks to nearly -50 dB and sets SINAD accordingly. With median SINAD score of all amps tested at 78 dB, this is copious amount of distortion of course. But it betters the Carver Crimson 275 amplifier by a few dBs (46 dB SINAD).
Then again, distortion is up by some 10 dB relative to the true vintage Dynaco ST-70 amplifier. That unit achieved a SINAD of 63 dB in good channel.
There has been some discussion in their forum about the correct bias voltage so I also tested the unit by setting it to 0.45 volt. That helped the weak channel gain about 2 dB in SINAD and also brought its gain closer to the other channel. Not sure what the impact on longevity of the unit is to operate it at that level.
Triode mode costs you more performance in the form of increased distortion:
As noted, watch out for difference in volume as you AB because the gain goes down in Triode mode. Continuing with 0.4 volt bias and pentode mode we get reasonable SNR values for a tube amp:
Again, this betters the Carver to the tune of 7 dB.
Frequency response was fine in one channel but had an odd notch in the other:
Fortunately it is outside of the audible band so just a curiosity (although lack of gain/level matching remains a small issue).
Crosstalk was surprisingly good:
Multitone shows the effect of all the distortion/intermodulation products (and power supply noise):
Let's sweep for power:
So we get 10 more watts than Carver 275 before clipping. The distortion rise is more steep though, resulting in much less power if we limit distortion to just 1%:
Using the 8 ohm tap we get the same power as 4 ohm:
I was hoping to listen to the amp but forgot and took it all apart for the review. When I get a chance, I will give it a listen.
Conclusions
Other than the unique look of the unit, I am the wrong guy to ask about the appeal of these tube amps.
There is little to hang your hat on as far as fidelity merit. That aside, comparing to the Carver Crimson 275, performance is better in most areas. Sadly distortion is much worse than the original ST-70. Not sure if this is planned or artifact of not measuring the performance of all these mods.Anyway, this is not for me but some of you have different priorities than me.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
