The upcoming "new" Dynaco ST-70x - opinions?

Think I'd probably get the VTA ST-70 if inclined to explore the Dynaco mystique, either kit or assembled ...

 

$3k ? So much for an amp that anyone can afford. The original ST70 was the Model T of stereo tube amps. Cheap, worked well (enough), sounded good (enough) and mostly reliable. Discounting for inflation, this $3k ST70 would have been about $500 in the mid 60's....or about 2x more expensive than a Mac MC240.....

There are many amplifiers both new and used available for $3k (or less) that will sound much better than this new ST70 could ever possibly hope to. HK Citation II anyone ? Easily found for about $1000-$1500, and another $1k for restoration..... Several ARC and CJ tube amps..... Line Magnetic, Rogue, Prima Luna....

 

Exactly. The original appeal of the stereo 70 was that it was a cheap, and easy to assemble amp kit which contained better than average output transformers. It was made as simple s design as could be made to build a push-pull 35 wpc amp in its day.

The later appeal was to modders who figured-out that stereo 70s original circuit could be improved upon quite a bit by various mods, and that this amp could sound great with the right mods. This has fuled cottage industries from the late 70s on who love to rebuild these amps into some really nice sounding little amps. I purchased and built an audio-research kit for my stereo 70 back in the early 80s (which I still have to this day), and it transformed the sound of this little amp into something very special.

But there has always been one problem with all of these mods: limited space inside of the chassis to fit a the kind of beefy power supply which is required to take any of these new designs to their limits.

Frankly this new amp is only a stereo 70 in terms of its form factor, and possibly in terms of its transformer designs. Other than that, this is really a completely new design. The big mistake with doing this is that the stereo 70s original form factor was the biggest limiting factor to any of its mods. Plus most people who are interesting in the stereo 70 know that there are already a couple of upgraded clones available for a lot less than this new unit is selling for. So I have a difficult time envisioning much of a market for a yet another new stereo 70 which sells for much more than does its very similar competition.

 

It looks like a winner to me. Addresses the original stellar output transformers with supplying same quality as original. Tube rectifier is not needed as newer SS diodes operate as quiet as the 5AT4 tube and likely supplies a hefty current surge for deep bass notes. Another plus is not using the so-so sounding 7199 tube. Only issue I have is a 12AU7 driver/phase splitter. Phase splitters are not known for being particular with regard to tube quality, but simply use a 6SN7 to 12AU7 socket adapter and a Sylvania 6SN7GTA. If you must go with 12AU7, use CBS Hytron 5814 or Brimar 13D5.

Note- when the 12Au7 was newly released, a year later Collins Radio complained about the 12AU7 high distortion. RCA revised the 12AU7 and released the new 5814 tube as a lower distortion 12AU7.

 

I have had one for many years. It worked magic with my Infinity IM 4.1s. (Well, his U70, which is basically the same thing)

 

I sold mine like a fool before I even hooked it up (moving) because I had a one-year-old and was worried about him burning himself. In hindsight, sometimes, one has to take a few risks....

 

Got one off eBaY. Going to do a head to head Van Alstine Ultravalve vs Van Alstine Ultimate 70 (Van Alstine Modded Dynaco ST70 PCB kit).
I'm also going to pit it against my Conrad Johnson MV60 (EL34/6CA7) tube power amp.

 

Mullard EL-34s and GZ-34 rectifier, David Hafler mentioned in an interview in Vacuum Tube Valley that the Sylvania 7199 was a lower noise choice than the RCA 7199 (I’ve seen both brands with Dynaco labeling so those may have been whoever had adequate on hand stock for a bulk order or had the best price when Dynaco called for pricing).

 

Ah! Two of my favorite tubes! The Mullard EL34 (XF2) and the Mullard GZ34 (built to last a lifetime, or two!).

 

They are dandy tubes that last a long time in the ST-70.

 

I’m demoing the new Dynaco ST70 and it sounds really good. Output seems higher than the rated wattage, drives my Vienna Mozarts nicely. I’m using a Mytek Brooklyn DAC+ as the preamp. Not sure about $3000, but I think dealers will be able to discount reasonably.

 

Price drop! Now HALF OFF! Normally $2,999 now just $1,499 with FREE shipping to Canada or USA ! Good deal for a made in Canada, EL34 amp! Not sure when the sale ends...

Dynaco ST70

 

I wonder if the Latino amp can be purchased built with the octal board?

 

I went ahead and ordered one. I felt it had several things going for it including:

* Front Panel L/R Trim Controls
* Bias Controls for Each Output Tube
* 3 Year Transferable Warranty
* Made In North America (Canada)
* Winner of the 2018 The Absolute Sound Product of the Year

Here’s what TAS said about the new Dynaco ST-70 Series S:

“The new ST-70 was obviously not intended to be an exact copy of the original, but rather aims to considerably raise the sonic bar compared to what was expected in the early days of sterphonic sound. Development was driven by the notion of what David Hafler would do today, given modern components and available tubes. The Series 3 captures what is good about vintage tube sound while dodging its weak spots. Harmonic textures are consistently sweet and refined without sounding overly liquid. Tonal balance is reminiscent of the Quad 57 ESL, offering a middle-of-the-hall perspective. Transient decay is accommodated by an impressively low noise floor. The Series 3 maintains the sonic soul and virtues of the original ST-70 while injecting a dose of steroids into the mix. The ability to resolve individual threads in a complex passage is highly evolved. The bass range is much improved. And detail resolution and microdynamic intensity are significantly sharpened. This is an amp that David Hafler would most definitely have been proud of.”

But the thing that sold me was the price drop from $2,999 to $1,499! Looking forward to it!

Dynaco

 

Sigh. Still waiting for someone to make a decent copy of the original kit and sell it for a price the average audiophile Joe can afford.

 

I would rather have an updated better sounding ST-70 myself.

 

I haven't read article yet. But over a 1000 hell yes it'll be.

 

It would be nice to see kits offered again, but, I don't think they sell very well which would mean that cost-savings from mass production would not be there as well. A long time ago, I built a Van Alstine kit that updated the ST-70. I started with a very old ST-70 (with the most highly regarded output transformer); the kit involved populating and installing a new input/driver board (did away with using 7199 tube) and installing a new power transformer and a solid state bridge rectifier. I used this ST-70 has a headphone amp. It sounded decent (slightly rolled off on top, which actually helps with most headphones of the day).

At the last Capitol Audiofest, I heard a very decent sounding 300b amp that is offered by a Japanese outfit that sells kits. The representative at the show was also a very nice gentleman that answered all questions honestly and frankly. I think there kits are worth looking into. The company is called Elekit.

Elekit TU-8600 300B SE Tube Amplifier Kit - Limited Run | TubeDepot.com

 

Odd that the Elekit TU-8600 does not mentioned the type of preamp tubes. If the preamp tubes are the notorious non-linear 12AU7, I would have to pass.

 

Interesting. Hafler (current owners of Dynaco) did listening tests of 12AX7, 12AU7 and EF86 and decided the 12AU7 sounded best in the Williamson design dual triode voltage amplifier and cathodyne phase splitter driving the EL34's. Here is an exert of the article:

"The original 7199 driver tube with pentode and triode sections that was serving as both voltage amplifier and phase splitter has long been out-of-production and unavailable as are all triode-pentode types except for expensive or hard to come by new old stock (NOS) tubes. David Hafler’s approach would be to find a solution that would enable the ST-70 to be produced without part shortages while delivering the very best sound possible. This drove us to searching for and testing various options until we could uncover the best overall solution.

We began by recreating the design using a pair of the EF86 Pentodes and a 12AU7 dual triode as the phase splitter. This worked reasonably well. However, we discovered that the EF86 is only available in limited supplies and did not feel secure with this reality. This would not only inhibit production requirements, but also make it difficult to impossible for Dynaco owners to find replacement tubes down the way. Yet even though quality and price would be called into question, we built one anyway and called it the 2T.

As we delved further into the original ST-70 design, we came to the conclusion that this driver circuit was likely developed by David Hafler to meet a target price point based on the available parts that were obtainable over 60 years ago and would not reflect what he would have done today. Over his illustrious design history David Hafler had developed a number of more advanced designs – many of which were never commercially produced. In fact we discovered a design that used three dual triode tubes with a 12AX7 as a voltage amplifier and a pair of 12AU7 tubes as long-tailed pairs to act as a phase splitter. This configuration cost a little more to produce than the original design and had very good sound quality. However, the long tail pair phase splitter has unequal gain on the two phases which require a trimmer to balance the phase for lowest distortion. To make sure it has been set correctly, it requires carefully adjusting the trimmer in the field and this is extremely difficult. We set the course and built a prototype and called this version the Triple Triode (3T) design. We then proceeded to investigate the advanced Williamson circuit that Hafler had drawn up.

This Williamson design used a dual triode as a voltage amplifier and cathodyne phase splitter. Another dual triode is used as an additional voltage amplifier and output tube driver. The cathodyne phase splitter used here and in the original ST-70 had the advantage of being as accurate as the resistors used in the circuit. With today’s proliferation of high tolerance 1% resistors, this made investigating this option completely viable. We called this version the Quad Triode. (4T)

It is important to note that our goal was to deliver the best sound possible, regardless of the tube configuration. The fact is, whether we employ 2, 3 or 4 driver tubes in the design makes very little difference to the overall cost when building an ST-70. Well over half the bill of materials costs (BOM) are in the transformers, case and labor. So we took a six month detour and built prototypes of all three designs (2T, 3T and 4T) for comparative listening. To ensure fair testing, all three designs used the same output transformers and power supplies. We set them up for blind testing and conducted multiple tests. To ensure there would be no listening bias or sonic memory to contend with, we built a switcher that instantly switched between amplifiers and carefully made sure each had the exact same output level. In measuring, we found after warm-up and bias adjustment the triple triode (3T) unit with the long tailed pair phase splitter had the lowest distortion by a very tiny margin but it had to be laboriously set up with an Audio Precision in order to bring the distortion lower than the quad triode. The benefit of the quad triode (4T) and pentode-triode (2T) designs is that they do not require balance/trimmer adjustments. The quad triode (4T) design measured a slightly higher distortion through the critical 500mW to 5W range as compared to the triple triode (3T) while the pentode-triode (2T) design measured to have about twice the distortion of the quad triode (4T) design.

During the listening phase of testing process, we found the sound quality of the pentode-triode (2T) design to have the lowest sound quality. Still good - but not as good as the other two. When comparing between the triple triode (3T) and the quad triode (4T) the mid and high end seemed indistinguishable. Both were excellent. However the quad triode (4T) with the Cathodyne phase splitter was felt to have a punchier low end. Likely because the dedicated drivers were able to feed the EL34 grids from identical lower impedances giving it more punch.

After considerable deliberation, we decided that while the quad triode (4T) design cost a little more to produce, the improved sound quality was worth the extra.

All of these variations followed David Hafler’s original design philosophy and are still valid today. The low open-loop gain principles are maintained, thereby minimizing Transient Intermodulation Distortion (TIM). TIM distortion is regarded in many circles as the type of distortion to which the ear is most sensitive. This is also why we chose to use lower gain 12AU7 tubes for all four driver circuit tubes versus the more common 12AX7 as the excessive gain and subsequent dynamic distortion levels appeared higher."

 

BIGGER Dave,

Thank you for the interesting excerpt on how the new Dynaco ST-70 design choices were made. Do you know if they make their own output transformers or use transformers based on Hafler designs? The Hafler/Acrosound transformers from many years ago are still very much valued and sound great. I have a pushpull 2a3 amp that uses Acrosound TO-330 transformers and that amp is quite nice.

 

At age 19 the Dynaco 70, a vintage cloth wired one, was my first tube amp ever - bought in 1989 from one of my friends, who was big into audio gear. He was 21 at the time and had Quad ESL-63 speakers driven by a "Frankenstein" modified Mac240 - run on regulators (including the output stage) and modified by one of the local tube gurus to handle KT88s. It was - and still is - one of the best stereos I've ever heard.

He asked me if I would be interested in buying a tube amp for $125. I said - heck ya! - with visions of McIntosh chrome dancing in my head. Instead he brought out the little homely Dynaco 70 - complete with Siemens EL34s. I wasn't exactly enthralled by the amp - at first. Well after listening to it, I began to appreciate what it did with my less-than-stellar front end and cheap speakers. It certainly was musical. I had that amp for a long time - did a bunch of mods to it, sold it, bought it back, rebuilt it, and finally sold it again.

It was a great little amp but not one I would rush out to buy again considering the better stuff I've heard since then. To be fair, this latest iteration may take care of some of the worst aspects of the original, namely the undersized power transformer. With my Dynaco 70, even with a rebuilt front end and SDS power supply board, I could hear compression on the musical peaks. One could hear the soundstage depth shrink if it tried to deliver too much power; it was sort of a "spring effect" where the images would be pulled back into the speaker. This was, of course, at fairly loud levels. I imagine a new power transformer and solid-state rectification would take care of this sag effect, with some loss of the golden hue that the originals make.

 

This is sounding like a very well thought out and nicely priced amp. I'm waiting to read more reviews and hopefully some actual user reviews, but I am leaning heavily toward this one.

 

Here is information regarding the OT's. Please refer to Dynaco for this article along with the associated charts and graphs:

"The output transformer inside the Dynaco ST-70 is often considered to be one of the primary contributors to its legendary sound. Before we set out to build a ‘new version’, we invested heavily by purchasing various ST-70s ranging from original to updated. This laid the foundation and set the goal in developing the right transformer for the job.

We began by taking these apart and then scouring the planet to find possible solutions. After testing various ‘off the shelf’ output transformers we found that we could not find any that had the high-end response of the original David Hafler designed A470. The square wave response of the original transformer at 20kHz is telling. In looking at the old part we felt that this was part of what made the original amplifier so well loved by audio fans. The picture at the right (refer to Dynaco ) shows the square wave response at 1 watt output as are all the rest of the tests herein. They are representative of the other tests we did at various power levels to clipping.

The bottom end is like this at 40 Hz square wave (refer to Dynaco ). The angle of the tilt indicates the low-end response. The less the tilt; the better the bottom end. Tilt is caused by both the frequency response and the ability of the power supply to deliver current at low frequencies.

Following is the frequency response of a professionally upgraded ST-70 with the original ST-70 transformers in it (refer to Dynaco ). The amplifier used to obtain these results already had upgraded coupling capacitors and a somewhat upgraded power supply.

Our first attempt at a custom transformer looked like what you see to the left (refer to Dynaco ). This is as good if not better than every off-the-shelf transformer we tried. Please note the vertical scales are different. The original is set to 2V per division while this one is at 5V per division. 20 kHz at the left. 40 Hz below.

Though the top end was disappointing, the bottom end was stunning in comparison. The greatly improved power supply is a large part of it but still the transformer contributed a lot as well.

In our second try after exhaustive work by our transformer vendor over many months including testing of the original A470 we got what you see here at 20kHz (refer to Dynaco ). While the rise time is not quite as fast, that was the price to pay to get the excellent bottom end. The issue is that there is no overshoot and no ringing or distortion of the wave shape that cause issues. Our listening tests confirmed that the rise time was not an issue. The lack of ringing is what makes the high end magical. Here is what we got with the new transformer.

Besides preserving the high frequency and square wave response of the original transformer, we were able to extend the low frequency response to do justice to the dynamics and range of today’s music, recorded with the modern recording technology that is now prevalent. As far as we can tell, we feel these may well be the finest output transformers made for any tube amplifier, anywhere near this price point.

Two speaker output impedances are now offered for 4-ohm and 8-ohm operation via a recessed selector switch and feedback is now taken from whichever impedance tap is selected by the switch."

To read the entire article on the creation of the new Dynaco ST-70 Series 3, click here - www.dynaco.com/inside-story

 

Current owners of Dynaco have no interest in offering a kit. Here’s what they said regarding offering a kit:”

“Then there was the question of offering it as a kit. We debated this as it would save the cost or getting safety certification and they reduce the costs in production. But in today’s onerous legal environment, sending out products with high voltages that could potentially be played with by amateurs, the legal risk was simply too great. Instead, we chose to sell the ST-70 as a completed product and undertook the costs to have it properly safety certified.”