Dynaco ST-70 Series 3 Tube Amplifier Review

[Blumlein 88]

@amirm, thank you for the review and the enlightenment.
We certainly have a come a long way, in the intervening 6+ decades.
I had difficulty trying to rate this resto-mod ST-70, but not because of the price: By the standards of those foregone days; pickins' were slim!

Although that perforated cover adds a nice look, listening to cage-free tubes may allow reduction in its perceived distortion products.

Yes, I prefer my tubes to be free range and not caged.
Even had a free range SS amp at one time.

 

[Blumlein 88]

Thats one way to look at it but here is another perspective. I grew up riding motorcycles and over the years about 80% of the people I ride with ended up with Harley's, many after owning a dozen or more other motorcycles. The pleasure of riding a motorcycle has little to do with ultimate performance and technology. You could make an argument that the ultimate motorcycle design happened in the 1940's and can't be improved upon.

I hope not.

If I loved the HD thing, I think a good modern update with the pluses and not the minuses were the Honda V65 Magna bikes. Some styled something like Harley's. Lots of torque, much more power, smoother being a V4 design and a low CG. The two things it does not have would be the shake from the V-twin which I detest myself and the sound of the V-twin which I also detest especially when it is loud. Of course even these are now more than 35 years old.

 

[Platypus20]

@amirm, thank you for the review and the enlightenment.
We certainly have a come a long way, in the intervening 6+ decades.
I had difficulty trying to rate this resto-mod ST-70, but not because of the price: By the standards of those foregone days; pickins' were slim!

Although that perforated cover adds a nice look, listening to cage-free tubes may allow reduction in its perceived distortion products.

I thought the purpose of the cage, was to keep unwanted (rogue) distortion under control

 

[ElJaimito]

why is there no "audiophile" saturation hardware? there should be a market for this. you just need to inform the user that that is actually what they are looking for

There is. Try a ReVox 736/G36, heavily cooked sound, couldn't bear them. Gyraf Audio used the 736/G36 preamp stage as a basis for a studio device, forget which one...

 

[ElJaimito]

I thought the purpose of the cage, was to keep unwanted (rogue) distortion under control

I reckon it's for whisker protection (moggies and otherwise)...

 

[dfuller]

My guess is the unstable channel was a not-quite-right performing tube.

Also, I'll be honest... I can't fathom the point of pentodes over beam tetrodes for any tube power amp making an attempt at high fidelity. KT77s bias like EL34s (and are generally electrically compatible) but they are somewhat cleaner especially in a UL config due to the aligned grid and screen - lower grid current is good, it makes them more linear even open-loop.

 

[Brian Hall]

Harley Davidsons have terrible frame geometry, are unbelievably heavy, terrible outdated shaky engines with very low output for their size, brakes are terrible. Awful outdated suspensions. One of the worst mechanical/maintenance issues of any motorcycle. All this without the stigma of a low price. Like riding on a water buffalo. A stand out product in the 1940's, an embarrassing American dinosaur today.

All true in the old days. Things have changed. None of those apply now except for saying their large touring bikes are heavy and they do lack the "stigma of a low price".

Harleys have a low center of gravity and don't feel heavy when you are riding them. No more shaky engines. If you like motorcycles, go do a test ride.

 

[DMill]

I don’t know about motorcycles but have had a few tube amps. This one looks decent enough assuming that bad channel was a bad tube? The design of these is simple enough I’d like to believe it’s not a flaw in the circuit.

Also, just a giant thank you to Amir for taking the time on tubes. We all know they aren’t going to test well against modern gear. Car analogies are annoying but when you see that dude in his 57 Chevy and he likes it for whatever reason… good for him. For me tube amps are the same. I keep one in my office and it reminds me of a better time when we didn’t look down on people for having fun.

 

[Cogito]

I will - for me at least it is the aesthetic. But I'm not willing to pay the price of entry.

Right! There's a shit TON of great amps in the $3k price range.

All true in the old days. Things have changed. None of those apply now except for saying their large touring bikes are heavy and they do lack the "stigma of a low price".

Harleys have a low center of gravity and don't feel heavy when you are riding them. No more shaky engines. If you like motorcycles, go do a test ride.

"Shaky" at idle. Quite smooth at speed. Unfortunately, reliability issues persist, just like tube gear... <See how I cleverly slipped back onto topic? LOL>

 

[AndreaT]

See what you made me do…after my post on value of class D, modern DAC…I reconnected my ST-35/PAS 3x (about 1968, recapped, one new transformer in the ST-35, new faceplate and LED)…Hidizs music player as source…ProAc Tablette Anniversary, enjoying Selling England by the Pound and Berta Rojas…

 

[pseudoid]

Ooohh, La, La!
What a nice looking combo.
Don't get mad: You may need to tame that bright blue LED.

 

[AndreaT]

Ooohh, La, La!
What a nice looking combo.
Don't get mad: You may need to tame that bright blue LED.

It looks much worse than it is: interference with the lens of the camera?

 

[leebergtally]

For a guitar amp, the reduced regulation and consequent sag under drive is a virtue for tube rectifiers. For hifi, there is zero advantage.

I'd be interested to see those. Assuming (safely) that this is one more variant of a Mullard circuit, there may be some simple circuit changes that I can suggest which will improve performance. Unfortunately, nothing will improve LF power delivery shortfalls- that's on the output transformers.

Here’s a Dropbox Link to all my ST70 docs https://www.dropbox.com/scl/fo/pvh5...S4e-0eL8?rlkey=71w7essdsuminkl4q327udfbr&dl=0

 

[MaxBuck]

I wanted to rate this thing, but I didn't see a choice labeled "Probably as good as you're going to get with tubes."

 

[ElJaimito]

I wanted to rate this thing, but I didn't see a choice labeled "Probably as good as you're going to get with tubes."

In fact it is possible to get much better performance than that out of a tube/valve mixed semi output stage with hybrid architecture. See for example (took me ages to find it!) this article:

https://www.worldradiohistory.com/UK/Wireless-World/00s/Electronics-World-2004-03-S-OCR.pdf - "Hybrid audioamplifier" on Page 48. There was also a hybrid design I think by JLH (EWW August 1991), but I can't find a copy anywhere online.

I still wouldn't bother with one: valve amp performance is contingent mostly on the quality of the transformer, the specific design referenced avoids that, but many would argue it's not really a valve amp! Also, valves wear out often rather quickly and fail in interesting and expensive ways (I had used inter alia to be the service engineer at Orange Music in Soho - London). I sold my valve tester many years ago!

 

[Theta]

Take a 2017 or later Low Rider with the 107 engine for a ride and then tell me it doesn't make enough power.

It only weighs ~650 pounds with the 1750cc engine and sounds good completely stock. My Honda NC700 weighs ~470 pounds with a 670cc engine and sounds like a sewing machine.

Sorry for the offtopic.

I don't understand why anyone would want a tube amp in 2024.

Harley low rider is more like agricultural equipment or the scooter I once made with my family lawn mower. Dynaco while not up to date is not as bad as all that. Just a sympatic reply to the offtopic.

 

[SIY]

Here’s a Dropbox Link to all my ST70 docs https://www.dropbox.com/scl/fo/pvh5...S4e-0eL8?rlkey=71w7essdsuminkl4q327udfbr&dl=0

Thanks much! It looks like what they did was mash together an old Heathkit circuit (triode voltage amp direct coupled to a split load inverter) with a short tail diff amp. More tubes than needed because of the low mu of the chosen (for reasons unknown) 12AU7 input tubes. 12AU7 have notoriously poor linearity. So this can be improved but it will take some skill- get the open loop gain boosted in the first voltage amplifier stage (change out to a 12AT7, adjust the cathode bias resistor and bypass it, or better yet use an LED there), then fool around a bit with compensation to get things stable.

More advanced mod: the short tail would be better if it were converted to a long tail using a CCS in the cathode tied back to the B- bias supply; this would also give you an opportunity to get rid of the split load and the poles created by the capacitive coupling to the diff amp by direct coupling from the voltage amp to the diff amp. If that sounds like a pure Mullard 5-20 circuit, it's because it is.

 

[leebergtally]

Thanks much! It looks like what they did was mash together an old Heathkit circuit (triode voltage amp direct coupled to a split load inverter) with a short tail diff amp. More tubes than needed because of the low mu of the chosen (for reasons unknown) 12AU7 input tubes. 12AU7 have notoriously poor linearity. So this can be improved but it will take some skill- get the open loop gain boosted in the first voltage amplifier stage (change out to a 12AT7, adjust the cathode bias resistor and bypass it, or better yet use an LED there), then fool around a bit with compensation to get things stable.

More advanced mod: the short tail would be better if it were converted to a long tail using a CCS in the cathode tied back to the B- bias supply; this would also give you an opportunity to get rid of the split load and the poles created by the capacitive coupling to the diff amp by direct coupling from the voltage amp to the diff amp. If that sounds like a pure Mullard 5-20 circuit, it's because it is.

This might shed some light on why they chose 12AU7. From their design team via a review on the Absolute Sound
"The first challenge involved the obsolete 7199 pentode-triode. There have literally been dozens of DIY-inspired upgrades attempted over the years aimed at replacing the original front end. Radial Engineering selected three input/phase-splitter circuits for extensive testing using various tube configurations. The first recreated the original design using a pair of EF86 small-signal pentodes and a 12AU7 dual triode as the phase-splitter, even though there was concern over the limited availability of the EF86. The two other circuits evaluated were all triode, based on circuits designed by Hafler over his long career. The first of these used a 12AX7 voltage amplifier and a pair of 12AU7 as long-tailed-pair phase-splitters. The practical issue with the long-tail-pair phase-splitter was the need for a trimmer adjustment for proper balance. The third circuit was derived from a Williamson design by Hafler that was not sold commercially for cost reasons. It uses a 12AU7 dual triode as a voltage amplifier and cathodyne phase-splitter, and another 12AU7 as an additional voltage amplifier and output tube driver. That’s a total of four 12AU7 triodes for both channels, so not surprisingly this version was dubbed the Quad Triode by the design team.
Six months were spent evaluating prototypes of all three designs using the same output transformers and power supplies. During the listening phase of the testing process, the pentode-triode design ranked lowest. And although both triode designs were comparable in midrange and treble performance, the Quad Triode nudged ahead by virtue of its punchier low end. It’s worth noting that the lower gain 12AU7 was selected instead of the common 12AX7 as the driver tube to reduce dynamic distortion levels. It also offers a lower source impedance and thus less interaction with the capacitance of the EL34’s control grid. Many years ago, David Manley opined that he could tell the IQ of a tube-amplifier designer by his choice of driver tube; a 12AX7 would score poorly in this context. Another advantage of the Quad Triode circuit is that a second feedback loop is no longer required as the cathodyne phase-splitter is buffered from the power tubes by the driver stage."
https://www.theabsolutesound.com/articles/dynaco-st-70-series-3-tube-power-amplifier/

 

[leebergtally]

Here's the test report for my ST-70 Series 3. Interesting how the right channel distortion goes up immediately at 20k and the left closer to 45k.

 

[Art of sound]

This is a review and detailed measurements of the Dynaco ST-70 Series 3 tube stereo amplifier. It is on kind loan and cost $3000 (seems to be discontinued now).
View attachment 379392
The amplifier is gorgeous looking and better than just about any tube amplifier I have seen. You can see the style and modernization when looking at the back panel:

View attachment 379393
The view is even nicer when looking from the top and seeing stylish labeling on transformers and such. As the name indicates this is another attempt at bringing the famous ST-70 to market. The owner tells me that they seem to have stopped producing it even though it was brought to market in 2018.

A pair of LEDs aid in the bias adjustment for the two stages in each channel. You are supposed to adjust the trim resistors until the two have similar intensity. Out of box, the left channel was not so and I attempted to adjust the bias. I got it close but it is hard to make fine adjustment both on the trim resistor and ability to judge the intensity of the yellow LEDs.

I have read that measurements were not only used to optimize the design but that each amplifier produced would get measured at the end of assembly line to achieve THD of 0.03% (SINAD of 70).

Dynaco ST-70 Series 3 Measurements
Let's start with our warm up measurements:
View attachment 379397
I don't know what is going on the left channel. Clearly there are some variabilities. Looking at the spectrum of it (not shown) distortion would rise up and then go back down in that channel. Maybe a bad tube? Or one that is not firmly seated? (Visual inspection didn't show such through the grill.)

Here is our usual dashboard measurement with 4 ohm load with same setting used on the back panel:
View attachment 379396

This is better performance than some of the other variations of Dynaco ST-70 I have reviewed. Gain though is on the low side and is less than its older incarnations. For our modern use though, it is fine as amplifier clips before reaching 2 volt nominal we get these days:
View attachment 379398
As noted, these are good numbers for a tube amplifier.

Frequency response was impressively flat at 4 ohm and almost so at 8 ohm:
View attachment 379399

There is a high-pass filter with two settings:
View attachment 379400
Good thinking as this amp is not capable of producing much clean power at the lowest range (see measurement below).

Multitone shows large amount of intermodulation distortion:
View attachment 379401
So likely low level detail is masked. Same is true with dual tone test:

View attachment 379402

Crosstalk was surprisingly good:
View attachment 379403

Here is how much power we have:
View attachment 379404
View attachment 379405

View attachment 379406

Certainly not much by today's norms. Best to get a high sensitivity speaker as to live in the lower portion of that curve.

Finally, here is the power sweep at different frequencies:
View attachment 379407
Generally good other than the 20 Hz response.

Conclusions
It seems that a nice clean up pass has been performed to optimize the performance of this famous design. Alas seeing how the original cost just $100, the inflation adjusted cost is $1000 which is a lot less than retail of ST-70 S3. Granted, the original was a mass product while this one appeals to small segment of the market. I see no role for such products in a system you build today but I know there are fans of tube amplifiers so here you are with another option.

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How can one tell if there is random distortion vs intentional harmonics being added. If the SNR is 65db shouldn't everyone who uses it hear distortion within the 12w power to sensitive speakers none the less.