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VTA ST-70 Review (Stereo Tube Amplifier)

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 126 64.0%
  • 2. Not terrible (postman panther)

    Votes: 46 23.4%
  • 3. Fine (happy panther)

    Votes: 16 8.1%
  • 4. Great (golfing panther)

    Votes: 9 4.6%

  • Total voters
    197

amirm

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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.
VTA ST-70 Review Bob Latino Kit Dynaco Tube Amplifier Stereo.jpg


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:

VTA ST-70 Review Bob Latino Kit Dyna Bias Board Tube Amplifier Stereo.jpg

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:
VTA ST-70 Review Bob Latino Kit Dyna Back Binding Posts Tube Amplifier Stereo.jpg


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:
VTA ST-70 Measurements Warm-up Dynaco Tube Amplifier Stereo.png


Here is our dashboard in pentode mode:
VTA ST-70 Measurements Dynaco Tube Amplifier Stereo.png


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:
VTA ST-70 Measurements Triode vs Pentode Dynaco Tube Amplifier Stereo.png


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:

VTA ST-70 Measurements SNR Dynaco Tube Amplifier Stereo.png


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:

VTA ST-70 Measurements Frequency Response Dynaco Tube Amplifier Stereo.png


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:
VTA ST-70 Measurements Crosstalk Dynaco Tube Amplifier Stereo.png


Multitone shows the effect of all the distortion/intermodulation products (and power supply noise):

VTA ST-70 Measurements Multitone Dynaco Tube Amplifier Stereo.png


Let's sweep for power:
VTA ST-70 Measurements Power into 4 ohm Dynaco Tube Amplifier Stereo.png


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%:
VTA ST-70 Measurements Peak and Max Power into 4 ohm Dynaco Tube Amplifier Stereo.png


Using the 8 ohm tap we get the same power as 4 ohm:
VTA ST-70 Measurements Power into 8 ohm Dynaco Tube Amplifier Stereo.png


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. :)

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temps

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I'm often tempted to get a tube amp just for the excellent, classic cosmetics... then I'd tell listeners we're running it real hard and this is how music is supposed to sound ("you can hear the tube warmth!" etc) when really a tiny silver box housing NC252MPs hidden in the corner is what's actually running everything
 

EchoChamber

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I think within the DIY community, many mods are based on assumptions and beliefs. It is a lot of trial and error and “measured” by ear, really hard to tell if one is improving or not the resolution of a component… I’ve been guilty of it. And still am, tweaking USB bridges… ;-)

Also, the NOS tubes could be the culprit, modern tubes might provide less distortion.
 
Last edited:

Doodski

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I'm often tempted to get a tube amp just for the excellent, classic cosmetics... then I'd tell listeners we're running it real hard and this is how music is supposed to sound ("you can hear the tube warmth!" etc) when really a tiny silver box housing NC252MPs hidden in the corner is what's actually running everything
I like this Dyna ST-70 and over the years I have met a bunch of peeps that have one and swear by them. I was curious about tube amps too and I had the opportunity to buy a trade-in so I jumped on it. So I ran this vintage Sherwood integrated amp for a couple of months on a very bright sounding pair of speakers and it sounded OK. It needed to warm up a lot but once warm it became not bad. I had extra company over every week because people wanted to hear it and see it. It was a great conversation piece and looked lovely. I miss it.
 

SIY

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Distortion is higher because the open loop gain is low. They’re probably using a variant of the Mullard circuit, but without the high gain pentode as the input voltage amp. 12AU7 is not a great choice for that, and the results show it.
 

EchoChamber

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I like this Dyna ST-70 and over the years I have met a bunch of peeps that have one and swear by them. I was curious about tube amps too and I had the opportunity to buy a trade-in so I jumped on it. So I ran this vintage Sherwood integrated amp for a couple of months on a very bright sounding pair of speakers and it sounded OK. It needed to warm up a lot but once warm it became not bad. I had extra company over every week because people wanted to hear it and see it. It was a great conversation piece and looked lovely. I miss it.
I love vintage gear from that era… Eons ago I found a very nice Scott 299A amp and 350B tuner to restore. Scott gear was popular on Audio Asylum at the time (where I used to hang out 20+ years ago). Unfortunately I never got to it and ended up reselling it unrestored… :-(

3AC226B7-30F5-4E26-9FF7-C04087C9D429.jpeg

50AD6B65-B66E-4F84-A3A0-08CFFDC9015B.jpeg
 

Doodski

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I love vintage gear from that era… Eons ago I found a very nice Scott 299A amp and 350B tuner to restore. Scott gear was popular on Audio Asylum at the time (where I used to hang out 20+ years ago). Unfortunately I never got to it and ended up reselling it unrestored… :-(

View attachment 196361
View attachment 196362
That's some nice looking gear. What year do you think it is from?
 

EchoChamber

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That's some nice looking gear. What year do you think it is from?
Early 60’s. They were designed to be mounted on audio furniture or wood cabinets.
 

phoenixdogfan

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Any test on the output impedance? While the FR curve looks flat into a resistive load, it won't employed to drive a purely resistive load, and we know that most tube amps can cause FR variance because of their interaction with the speaker's impedance curve.
 

dfuller

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I have to agree with @SIY on this one, 12AU7s seem an awfully strange choice as an input stage considering the original design uses a much higher gain pentode (the pentode half of a 7199, as it turns out). EF86s probably would have been a better choice, with something along the lines of a JJ ECC99 or 6SN7 as the driver stage.
 

SIY

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I have to agree with @SIY on this one, 12AU7s seem an awfully strange choice as an input stage considering the original design uses a much higher gain pentode (the pentode half of a 7199, as it turns out). EF86s probably would have been a better choice, with something along the lines of a JJ ECC99 or 6SN7 as the driver stage.
The original design used an input pentode coupled to a less-than-unity-gain cathodyne phase splitter. If this is indeed a variant of Mullard, the phase splitter will be a long tail pair with gain, but not enough to equal the original circuit's. The original Mullard circuit used a pentode up front with the long tail pair, so had oodles of open loop gain for distortion and source impedance reduction.
 

Dan Clark

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I love vintage gear from that era… Eons ago I found a very nice Scott 299A amp and 350B tuner to restore. Scott gear was popular on Audio Asylum at the time (where I used to hang out 20+ years ago). Unfortunately I never got to it and ended up reselling it unrestored… :-(

View attachment 196361
View attachment 196362
My grand-dad had this setup driving some Altec horns. I remember being endlessly fascinated by the rotary capacitor plates behind the tuner.
 
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