Review and Measurements of Dynaco ST 70

[rebbiputzmaker]

Yes, they were good transformers. No, modern units from Lundahl, James, Tamura, Sowter, and the like will outperform them.

Transformers from the Golden Age of tube electronics were very special. Magnequest has kept many of these classics alive.

http://magnequest.blogspot.com/p/about-magnequest.html

 

[SIY]

Mike is a good guy and does a nice job of providing nostalgic components which follow the originals closely. But that's different than "highest performance using contemporary design and materials."

 

[rebbiputzmaker]

No, it isn't. It's a combination pentode-triode. The pentode first stage is where you get all the gain for the input and driver because the cathodyne phase splitter has slightly less than unity gain, so a triode like a 12AX7 won't work as well here.

edit: Basically, you need to drive the output stage to about 30-35V for full power out, so with a gain of 100 in the first stage (which isn't going to happen without a CCS plate load, 70 is more realistic), you could only get 9dB or so feedback- and not even that in the top octave or two.

Yes that is correct I was thinking and mixed up the 7247 the st35 driver. A better circuit.

 

[SIY]

Yes that is correct I was thinking and mixed up the 7247 the st35 driver. A better circuit.

You can get away with that in an EL84-based amp since the EL84 only needs 8-10V of drive. That circuit would perform very poorly trying to drive EL34.

 

[Bald1]

Always wanted tube equipment from Dynaco as McIntosh was simply out of my budget back in the day. But as a military member moving every 2 to 3 years, tube equipment risked damage I didn't want to be saddled with. So in 1978 I opted for the solid state ST410 kit which most considered the best sounding of the series given its lack of DynaGuard circuitry. I still recall getting a reservation and showing up at a Nakamichi sponsored equipment testing day at a major stereo dealer in Vallejo, CA. The techs were very impressed with the test results on my unit to the point where one made an offer to buy it. Mine (I still have it in working condition but it is in storage) served many decades including driving Magneplanar MGIIIa speakers.

 

[rebbiputzmaker]

You can get away with that in an EL84-based amp since the EL84 only needs 8-10V of drive. That circuit would perform very poorly trying to drive EL34.

Yes...

Back in the day I also had a GSI design st70 6dj8s driving 8417s (high Gm) the last power tube designed.

 

[DDF]

For those interested in upgrading their dynas, here are cut-and-paste notes I took during a bygone era, back when my ambition still outpaced my sense and my time: pre-html era advice on modding ST-70s. No doubt many suppliers are long gone but I hope this is of some interest

 

[snip3r77]

Does it mean that tube amps are crap since it has distortion?
BUT it irks me that everyone say tubes are the best?

 

[DKT88]

For those interested in upgrading their dynas, here are cut-and-paste notes I took during a bygone era, back when my ambition still outpaced my sense and my time: pre-html era advice on modding ST-70s. No doubt many suppliers are long gone but I hope this is of some interest

I see a lot of familiar names in the big file. I was studying the same stuff in the 89/90 time frame.

 

[sergeauckland]

Does it mean that tube amps are crap since it has distortion?
BUT it irks me that everyone say tubes are the best?

Tubes are not best, or even better, but they are different, and for those who judge hifi by what it sounds like, may well prefer the sound, and therefore for them it's better.

I don't think tube amplifiers are crap, just that they distort more than SS amplifiers, have a higher output impedance, a less extended frequency response and generally higher noise, consume more power and are more expensive to make and heavier to ship but, the better examples can still measure well enough to be transparent, albeit with care over what load they are driving.

It seems to me that a tube amplifier that's transparent is just pointless, as then it would sound like pretty much any SS amplifier, so why bother? (looks?) So, that leaves tube amplifiers being chosen because they sound (and mostly look) different, and that now becomes a matter of taste, not audio engineering.

S.

 

[SIY]

For those interested in upgrading their dynas, here are cut-and-paste notes I took during a bygone era, back when my ambition still outpaced my sense and my time: pre-html era advice on modding ST-70s. No doubt many suppliers are long gone but I hope this is of some interest

Thanks for that. As one might expect, there's a mix of good solid advice with audiophile lore and nonsense. It can be tough to separate that out.

My own general comments: if you don't REALLY understand the feedback system and how to do compensation, don't fool with that. Triode mods on the output stage will degrade performance unless there's a total redesign. As soon as someone mentions fashion capacitors or the magic words "cathode stripping," walk away.

 

[anmpr1]

Tubes are not best, or even better, but they are different, and for those who judge hifi by what it sounds like, may well prefer the sound, and therefore for them it's better.

I don't think tube amplifiers are crap, just that they distort more than SS amplifiers, have a higher output impedance, a less extended frequency response and generally higher noise, consume more power and are more expensive to make and heavier to ship but, the better examples can still measure well enough to be transparent, albeit with care over what load they are driving.

It seems to me that a tube amplifier that's transparent is just pointless, as then it would sound like pretty much any SS amplifier, so why bother? (looks?) So, that leaves tube amplifiers being chosen because they sound (and mostly look) different, and that now becomes a matter of taste, not audio engineering.

S.

There's so much ink on this. In the context of the ST-70, below is transcribed from an interview with David Hafler, the man responsible for the Dynakit:

Why did you decide to go into transistors, and who designed your transistor equipment?

People were clamoring for them. They wanted them. I would say, in terms of listening, there wasn't any essential difference between tubes and transistors. If there was a difference it wasn't because of tubes versus transistors, it was because they may have been trying to drive wrong size speakers with the wrong size amplifiers. Ed Laurent did much of the design. And Erno Borbely was also involved in the final details.

What are your thoughts about consumer audio, today?

There is a big rip off going on. Companies are selling extra high-priced equipment that has no benefit except a high profit to the company that sells it. I don't think that many of these fads that come along are true advances.

 

[Jaimo]

For those interested in upgrading their dynas, here are cut-and-paste notes I took during a bygone era, back when my ambition still outpaced my sense and my time: pre-html era advice on modding ST-70s. No doubt many suppliers are long gone but I hope this is of some interest

Thanks for posting this - looks like a very useful guide.

 

[invaderzim]

Does it mean that tube amps are crap since it has distortion?
BUT it irks me that everyone say tubes are the best?

Your definition of 'best' means most accurate representation of the input signal at any volume output and their definition of 'best' means most enjoyable for them to listen to.

If we all liked the exact same sound and had the same listening spaces that affected the sound the exact same way then there would be one amp sold for every power/price point and one speaker sold for every price point. And we'd all seal ourselves in anechoic chambers to listen to the music. Instead in the real world we all have different preferences, listening rooms and listening habits.

 

[Burning Sounds]

Here are a few measurements from a Curcio mod ST70 I found on the web at Canuk Audio Mart. Measurements done with an AP System One.

Power Output Per Channel: 31 watts continuous into 8Ω, at 1% THD+N

Frequency Response: ±0.15 dB from <10Hz to 20KHz, 8Ω
+0, -3dB dB from <10Hz to 123KHz

THD+N @ 1W: 0.043% (left ch), 0.048% (right ch)

Hum and Noise: >91 dB below rated power.

Channel Separation: >91dB at 1KHz (below the amp noise floor)

 

[snip3r77]

Your definition of 'best' means most accurate representation of the input signal at any volume output and their definition of 'best' means most enjoyable for them to listen to.

If we all liked the exact same sound and had the same listening spaces that affected the sound the exact same way then there would be one amp sold for every power/price point and one speaker sold for every price point. And we'd all seal ourselves in anechoic chambers to listen to the music. Instead in the real world we all have different preferences, listening rooms and listening habits.

Not really. I use a tube amp myself I love the 'distortion'.
I want to undertand why most tube users like 'distortion' which is the opposite of accurate.
if this is true, how can tube amp be 'measured' ?

 

[SIY]

I want to undertand why most tube users like 'distortion' which is the opposite of accurate.

I never took a survey, so I can't speak to what "most tube users" want, but for my personal sample-of-one, I engineer my designs to have low enough distortion to be unobtrusive. I understand the appeal of effects boxes, but am uncomfortable with effects that can't be adjusted or turned off.

 

[anmpr1]

Honest to god, I don't know what to think half the time. You read some of these designers, very smart and creative men, and they say they can hear differences among brands of resistors and capacitors they use in their circuit. Some claim to hear differences in a meter of cable. Or less. For others it's the power cord that plugs into the wall socket. Why is it, then, that when you hide the brands and match the levels, no one can reliably tell anything? Why is that? And these are supposed to be items that 'blow away' what came before! I've read reports of test results where people hear 'differences' in the same component (when they thought they were switching between different gear).

And why is it that each new and improved amp design is superior? Are the newest Harman Levinsons so much better sounding than what he was making in the late '70s? Or is it just marketing? Back in the day, Mark himself couldn't tell you why his preamp sounded better. You just took his (and some magazine reviewer's or audio salon jockey's) word for it. Everyone knew that the revised ML-1 was better than the JC-2, which was not nearly as good as the newer ML-7, but if you wanted the best, you bought two ML-6A. Just keep adding a thousand dollars to each purchase, and Mark would guarantee your happiness. That is, until Mark sold out, and then you had to have a Cello. At least with that, you got some tone controls with a fancy name (Audio Palette--who thought up that? Genious.) Tone controls which, by the way, were sonically bad when he was selling stuff under his own name. LOL

It's almost enough to make a sane person get another hobby. Other than solid body (but not acoustic--where a case can be made) electric guitars ('tone' wood, poly v nitro finish) I don't know of any other hobby out there with this kind of gross subjectivity. Do people claim that a Leica lens is better than a Nikon? Maybe they do. I don't know.

PS: I use tubes because I built my gear, and have some pride of ownership over it. I also like to look at the warm glow, especially at night. That said, at reasonable listening levels, with sensitive speakers, I can't tell the difference between it and my SS amp, if I am honest with myself. Maybe the low end is not as 'tight' with tubes, probably. But my speakers are a little loose on the bottom, so if I wanted to tighten it up I'd get new speakers, and not a new amp. Why would I worry about an amplifier when speakers make the most difference?

 

[invaderzim]

Not really. I use a tube amp myself I love the 'distortion'.
I want to undertand why most tube users like 'distortion' which is the opposite of accurate.
if this is true, how can tube amp be 'measured' ?

I'm with SIY. I like my tube amps but not for distortion. The definition of 'tube sound' seems to generally be mushy and distorted but a well designed amp that is playing within its reasonable power limits can sound clean and detailed.

I built this one: http://diyaudioprojects.com/Tubes/KT88/ and it sounded pretty good but it was always just a bit off until I followed the tweeks shown in the Kegger/Blueglow build. When I added the negative feedback, which is designed to reduce distortion, the amp came alive and is really enjoyable to listen to. I'd read so many things about how negative feedback was bad that I thought I'd be taking it back out of the amp right away but was pleasantly surprised. If I crank the volume too much past 3/4 the distortion becomes audible but I really never go past halfway on the volume so it is of no concern for me.
With a well setup subwoofer that has speaker level inputs and properly placed speakers it is just a joy to listen to. Made even more fun by the fact that I built it and the glow is fun also. I've built SS amps but they just aren't as much fun. I just finished a LM3886DR that was set to replace the tube amp until I did the negative feedback modification on the tube amp, then it was no longer a contest as to which sounded best to me.

 

[Burning Sounds]

PS: I use tubes because I built my gear, and have some pride of ownership over it. I also like to look at the warm glow, especially at night. That said, at reasonable listening levels, with sensitive speakers, I can't tell the difference between it and my SS amp, if I am honest with myself. Maybe the low end is not as 'tight' with tubes, probably. But my speakers are a little loose on the bottom, so if I wanted to tighten it up I'd get new speakers, and not a new amp. Why would I worry about an amplifier when speakers make the most difference?

Yes, exactly this. My first incarnation of my Curcio ST70 almost 30 years ago (the build, including circuit board layout was in an early issue of Glass Audio) was my introduction to using a multimeter, a variac, understanding how valves (tubes) and power supply regulation works. I don't have any formal electronics background, so it was the start of a new journey for me. I designed the case for my recent rebuild using a CAD programme, something that I had never used before. So I have a soft spot for my old valve gear and satisfaction in having built it myself. It makes audio fun and I don't want to lose sight of that.