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VTA ST-70 Review (Stereo Tube Amplifier)

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 125 62.5%
  • 2. Not terrible (postman panther)

    Votes: 47 23.5%
  • 3. Fine (happy panther)

    Votes: 17 8.5%
  • 4. Great (golfing panther)

    Votes: 11 5.5%

  • Total voters
    200

GXAlan

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Why would I ship it to Amir, putting aside the issue of building a non-breadboard version?

My 6DJ8-oid phono preamp has terrific performance. Best tubes for distortion were CCa. Best for low microphonics were a rare 12V version made for Tektronix.

Oh -- just to showcase the measurements more publicly? I vaguely remember seeing a blog post where your APx555 was a loaner and not something you have? I suppose with tubes any modern AP instrument will be sufficient to showcase the performance?
 

SIY

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Oh -- just to showcase the measurements more publicly? I vaguely remember seeing a blog post where your APx555 was a loaner and not something you have? I suppose with tubes any modern AP instrument will be sufficient to showcase the performance?
I have a 525 with every bell and whistle. More than good enough for any tube device, even the high performance stuff I’ve designed.

If I need better than 0.0004%, I am set up with some specialized gear, but honestly, the 525 is so ridiculously better than needed for audio that I rarely break it out.
 

GXAlan

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I have a 525 with every bell and whistle. More than good enough for any tube device, even the high performance stuff I’ve designed.

If I need better than 0.0004%, I am set up with some specialized gear, but honestly, the 525 is so ridiculously better than needed for audio that I rarely break it out.
That's awesome! Do you have any measurements from "your best" tube-powered gear to share?

If you have the interest, I'm happy to gift you a few JAN Sylvannia 6DJ8 tubes to see how they perform in a well designed circuit ;)
 

SIY

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That's awesome! Do you have any measurements from "your best" tube-powered gear to share?

If you have the interest, I'm happy to gift you a few JAN Sylvannia 6DJ8 tubes to see how they perform in a well designed circuit ;)
Better yet, I'll throw them on a jig I use for testing noise and linearity.

Here's a couple examples from my MM phono stage using the 12V version of ECC88/6DJ8. I'd have to do some serious digging to find my MC stage's measurements.
1660948026991.jpeg
1660948092771.jpeg


And a high voltage adaption using 12AT7 was discussed here. At 65VRMS out, 3rd was the only significant harmonic at -62dB. This will be the input stage for my next tube power amp.
 

JP

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JP

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Never saw those.
 

JP

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Yep assumed as much (x2). Lost to the labyrinth of ‘I’ll remember what I named that, and where I put it’? Went through that just yesterday. Again.
 
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I have recently built a VTA sound valves st70 kit and I was happy with the results. I had a gsi st70 back in the early 80s and I wanted to remember that sound. I have multiple amps and I can’t say it is the best amp but it is no where near the worst sound either. I have to say that it gives me pleasure to listen and I think it is a winner for this reason.
 

CrowDaddy

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I'm surprised no one has mentioned Kevin Carter's superb ST-70 upgrade, with Lundahl transformers as phase splitters, a modern power supply, and (mostly) class A operation. My impression is that he is retired and no longer offers the kits, but his webpage about them is an interesting read:
http://www.kandkaudio.com/power-amplifier/ (scroll down to the ST-70 section)

Here is a European version of the VTA circuit with auto-bias:

As SIY suggested, an EF86 as input is a much better choice than a 12AX7, and here is a board using EF86 inputs:
or a complete amplifier kit: http://www.triodeelectronics.com/st70buildkit.html

My ST-70 is original, except for upgraded coupling caps, a few resistors, and a new(er) electrolytic can. It has never failed in the decades I have used it.

However, a couple of years ago I was fortunate to find a pair of Atma-Sphere M-60s, used, in good condition. They are more neutral and transparent than most amps, whether SS or tube, and they have more glowing tubes than anything else.
 

Radtraveller

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I'm surprised no one has mentioned Kevin Carter's superb ST-70 upgrade, with Lundahl transformers as phase splitters, a modern power supply, and (mostly) class A operation. My impression is that he is retired and no longer offers the kits, but his webpage about them is an interesting read:
http://www.kandkaudio.com/power-amplifier/ (scroll down to the ST-70 section)

Here is a European version of the VTA circuit with auto-bias:

As SIY suggested, an EF86 as input is a much better choice than a 12AX7, and here is a board using EF86 inputs:
or a complete amplifier kit: http://www.triodeelectronics.com/st70buildkit.html

My ST-70 is original, except for upgraded coupling caps, a few resistors, and a new(er) electrolytic can. It has never failed in the decades I have used it.

However, a couple of years ago I was fortunate to find a pair of Atma-Sphere M-60s, used, in good condition. They are more neutral and transparent than most amps, whether SS or tube, and they have more glowing tubes than anything else.
More tubes than anything else?
I’ll bet my tube amp has more (and my daddy can be up yours!)

:)
 

CrowDaddy

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How much do you want to bet? The M-60 has 12 tubes per channel, the MA-1 has 19, and the MA-3 has 54.
 

Radtraveller

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My Carver Silver Seven 900 has 40..(50 if you want to count non output tubes)

so the MA-3 has a few more total..
but only makes 500w vs the Silver Seven 900(tested output > 1000) watts per channel..
never heard of MA-3 though, nor seen one for sale, though I read the price was close to $150k/pair(see below)

not only that, but each “mono block” is actually 3 mono amps in parallel.. with the ability to pull 1 or two out of the circuit at any given time and still have a ”functioning” mono at 1/3rd to 2/3rd of the 500w available when all 3 are in circuit.

which means each of the three amps that make up an MA-3 mono only has 14 tubes per channel.
It was a marketing trick.. 3 amps in one “mono block” .. it wasn’t a complete, self contained single mono block with 42 power tubes, just a few working together in a single chassis.

very little info exists on any actual MA-3 amp produced, (nor has one ever been “measured” to ASR “standards”, same goes for the Silver Seven 900 though I am well aware of Amir’s stance on tube amplifiers… along with many lf the readers here) but:



“Very few exist, and I doubt that anyone has heard them. They go for roughly $147k/pair.

A pair were on static display at THE Show at the St. Tropez in Las Vegas in I believe January 2004. A sight to behold.

They are unique in that they have built in power conditioning and are capable of full output power even when the voltage out of the wall sags to as little as 100 volts. They also have built in tube testing. The 42 output tubes (6AS7G) in each are configured in 3 banks of 14 tubes each, which can be switched on or off depending upon listening conditions. Like all Atma-Sphere OTL amplifiers, there is a single stage of gain, no global feedback, and full Class A operation, all triode. The MA-3s put out 500 watts into virtually any load, yet they are pretty darn efficient, so heat output would be less than what you might guess.”
 

PeteLeoni

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Having been married to a McIntosh MC275 for 25 years must make me unqualified for both being an audiophile or a golden-ear.
bum!

Having been married to a McIntosh MC275 for 25 years must make me unqualified for both being an audiophile or a golden-ear.
bum!:oops:
If you hid your amps from yourself and others you would know the answer to both of those Qs. (-:
 

milosz

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I had a relatively unmodified ST-70 for a while. Got it pretty cheap, The only modification it had was upgraded film caps - "Multicaps" - as coupling caps.

I bought the thing and then wondered what I would do with it. I then saw a pair of Quad ESL-57's for sale nearby, so the ST-70 inspired me to buy those. The Quads needed a little restoration- the diode voltage multipliers were bad, the tweeter panel had been arced a few times- so I had the panels rebuilt by Quads Unlimited and also bought their voltage multiplier boards and their Zener protection boards. Once I got the Quads working, I tried them on the ST-70. I was underwhelmed. It sounded OK but although their didn't seem to be any high treble rolloff, the highs still sounded like any transient was being mushed, rounded off, smeared- however you want to put it. A real noticeable lack of detail. I tried a decent solid state amp and THAT sounded great. I also had a Stax SRD-7 transformer/bias supply type adapter for my Stax Lambda headphones, I tried the ST-70 on that and heard the same sad thing. I sold the ST-70 after that.
 
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Jud-the-prof

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March 28, 2024: I've just bought an all-new, Dynaco ST-70-inspired, basic amp (for $1,195) that an enthusiast has created for sale (with a Bob Latino input section that he has lightly modded). I bought it on a lark just to see what this much-loved simple tube amp circuit would sound like. I'm fascinated by this antique. I'm 82 years old. When I was a kid I longed to get my hands on the Dynaco ST-70 (in the late 1950s). I couldn't afford $100 then. And it would have needed a pre-amp too, which I could not afford either. But I did buy a 12-watt-per side Heathkit integrated amp featuring four EL84 tubes in push-pull circuits (for $59). I soldered it up, plugged it in, and I relied on it for the next that 13 years---on down to 1973 when I got my first real job as a college teacher. It finally blew a power tube, so I set it aside.
The AudioScience review of an original Dynaco ST-70 that started off this thread a couple years ago, which I found just recently online, was quite off-putting. That amp measured terribly. It was a dog! Could that have been due to the age and wear on the old amp? Installing the Bob Latino input section in that tired old example was not much of a test for the mod in question. Resistors and caps whose values were no longer nominal, solder joints that were questionable, wire insulation that had rotted, corroded tube sockets, transformers past their prime----all these aspects (and more) could account for the reviewer's disappointment.
When I receive the new David Hafler inspired amp from my enthusiast builder, I'll come back to this thread and report. My builder has beefed up the power transformer and obtained bigger, better-wound output transformers. He's put bigger caps into the amp where that might make a difference. He's used high quality resistors with guaranteed, precise values. He's also supplied a connector for a fancy power cord. But he has retained tube rectification in the power supply. Maybe I'll attempt to replace the tube with a solid-state rectifier just to see what such a mod might do for the amp's performance. Report to come.
 
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SIY

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Maybe I'll attempt to replace the tube with a solid-state rectifier just to see what such a mod might do for the amp's performance.
Be VERY cautious about that! Because the tubes won't start drawing current for 30 seconds or so (whatever warm-up time is), the power supply voltage will be much higher than under normal load. If the "enthusiast builder" didn't use parts with a high enough voltage rating, you could be cleaning capacitor juice and carbon out of the insides.

Most "enthusiast builders" are not particularly competent. Don't know about yours, obviously, but there's not many of us around who understand this old technology thoroughly (we tend to be old!).
 

milosz

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I had a Stereo 70 which was the original circuit except for new capacitors and the film caps in the front end stages were replaced by fancier caps ("Multicap" brand)

I did some A/B tests with it against a Forte 4a amp and also against a Harman-Kardon Citation II using Quad ESL-57 speakers, small conventional sealed 2-way system using SEAS 5 inch magnesium midwoofer and SEAS Millennium soft dome tweeter as speakers.

The Stereo-70 sounded noticeably "clouded" in the treble range on both speakers. Not rolled off, but with the treble sounding "smeared" or in some way lacking transient detail. The flaw was NOT subtle. Various listeners heard the same thing in A/B listening. This was most interesting in the A/B between the H-K Citation II and the Stereo 70- both being tube amps. The conventional expectation is that solid-state will have a certain kind of treble sound which some would call detailed and others would called etched or grainy. And indeed there was a difference in sound between the Forte 4a and the Stereo 70, though I would call the Forte 4a's sound transparent and not "grainy." But, OK, this confirmed the audiophile mythos to some degree regarding highs from solid state amps. So, let's compare the two vintage tube amps - the H-K and the Dynaco. This revealed that there was something flawed in the Stereo 70's treble. The Citation II sounded clean and clear, with good detail. The Stereo 70 did not lack treble range but the treble was... smoothed out somehow. I ran ran frequency response curves of each amp into each speaker and amplitude response matched within 0.5 dB from 100 Hz up to 20,000 Hz for each amp. So, like I said the Stereo 70 had no treble roll-off but still the highs were flawed. I sold the Stereo 70 and still have the Citation.
 
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