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Review and Measurements of Dynaco ST 70

amirm

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#1
This is a review and detailed measurements of the vintage Dynakit Dynaco Stereo 70. It is on kind loan from a member who bought it used online and repaired it. Previous owner hand upgraded it but could not get it working.

Given that it is 60 years old, one can forgive the rust and dust:

Dynaco Dynakit ST-70 Power Amplifier Audio Review.jpg

Some of the online samples I have seen are a lot more crusty.

The unit I received came from matched pairs of tubes from Apex. While each pair had the same parameters, the ones for each channel varied quite a bit. As you will see, this is reflected in the measurements.

This unit has an upgraded driver board:

Dynaco Dynakit ST-70 Power Amplifier pre-amp Audio Review.jpg

There is a single bias control for each pair of tubes which is not optimal. I went ahead and adjusted both for 1.56 volts as stated in the manual.

I am not sure what output transformer tap is used. I tested it as is with my 4 ohm load. Hopefully the owner will advise what it is wired for (didn't want to open the bottom).

Despite having just a handful of tubes, the unit generates good bit of heat.

The configuration is push-pull output with a power rating of 35 watts.

Amplifier Measurements
As usual, I start with my dashboard with the input voltage adjusted to get 5 watts of output:
Dynaco Dynakit ST-70 Power Amplifier Audio Measurements.png


As we see there is 8 dB difference in distortion and noise between channels. FFT shows a jungle of spurious tones mostly caused by the high mains frequencies. Ultimately though, the distortion products set the SINAD (signal over noise and distortion) with second harmonic around -58 dB and third at -61 dB (for channel 2).

Averaging the SINAD for two channels we get 59 dB. I took the time to update my power amplifier SINAD table now that we have good bit of data:
Best Audio Amplifiers 2019.png


Kind of shameful how bad transistor designs have become to leave such a small gap between them on the right and the Dynaco ST-70.

Frequency response is good for the audible band:
Dynaco Dynakit ST-70 Power Amplifier Frequency Response Audio Measurements.png


The mismatch between channels shows up here too. The channel in red has much steeper drop at both extremes.

Intermodulation distortion is initially dominated by the mains noise until distortion sets in:
Dynaco Dynakit ST-70 Power Amplifier Intermodulation Audio Measurements.png


The ST-70 is rated at 1.3 volts for rated power but I found that it severely distorted there. So much so that I could hear the transformer audibly singing the input tone. :) So for above testing I kept the max input voltage at 1 volt or it would skyrocket in distortion.

We see that effect in power versus THD+N:
Dynaco Dynakit ST-70 Power Amplifier Power Audio Measurements.png


We have internal distortion causes prior to severe clipping at 31 watts. I am not sure how useful the power is between 10 and 31 watts though with that rising distortion. We are talking 1.3% distortion+noise at rated power.

Signal to noise ratio is specified at 90 dB and we miss that by some margin:

Dynaco Dynakit ST-70 Power Amplifier SNR Audio Measurements.png


This tells me that the mains hum and noise is aggravated by the power drawn from the power supply. Maybe better power supply filtering is in order?

Conclusions
I hope to perform some listening tests on the ST-70. For now, I say it is not that bad considering what we have in front of us. Wonder how good it can be made with the type of instrumentation I have now versus what used to be available and what people typically use. Cleary we can push the weak channel to be as good as the other as a minimum.

I love the simplicity of tube amps like this. There is just so little to them. If I were less busy, I wouldn't mind playing with one to get it to optimal performance.

Anyway, it doesn't make sense for me to give or not give recommendation.

------------
As always, questions, comments, recommendations, etc. are welcome.

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#2
Thanks so much for reviewing this!! This amp was my entry into hi-fi. I've since moved past wanting to fiddle so much, but I'll always have good memories.

I modded mine with an excellent Triode Electronics board and power supply upgrade, which seemed to make a big difference.

Yours tested better than I'd expect!
 
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#3
Yes, very cool review. I grew up wanting an ST 70 (never got one, however I do have a different tube amp now), and always wondered how it actually performed.
 

cjfrbw

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#4
When my old VFET Sony TA 5650 croaked in the late 80's, I decided to try tubes again and got a pair of vintage ST70's. from an antique recycling store on Shattuck Avenue in Berkeley. Over time, with my sucky electronic skills, bought, built and installed two upgraded 12ax7 input boards. I got tired of the output chokes failing, so I just replaced them with a couple of large resistors.

I still have a bunch of 7199 tubes in my tube closet/dustbin/audio morgue with some el34's. Some of the el34's are Mullards, which I gather have gone up in value, but used to be standard at the electronics stores. I used one ST70 for the front and one ST70 for surrounds.

I used them with Large Advent speakers and then some Klipsch Heresies in the front, when the foam bit the dust on one of the Advents.

I loved the upper bass and lower midrange blat from those amps, ultimate tube-ey albeit distorted joy machines. I got tired of messing with them and sold them for peanuts on ebay.

The cap filtration on them is trivial for the demands of the amp, and I would imagine better power supply management would bring up the specs, but would it lose that loosey goosey charm?
 
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SIY

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#5
What are the tubes on the replacement input stage board? And do you know offhand who that board is from? There were a few decent upgrades, many more lousy ones.

I've built and modified perhaps a hundred ST-70s over the years. The stock version (with dried out electrolytics and leaky paper caps, as well as noisy carbon comp resistors replaced) is actually reasonably good. It was a solid and smart design.
 
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#7
What are the tubes on the replacement input stage board? And do you know offhand who that board is from? There were a few decent upgrades, many more lousy ones.

I've built and modified perhaps a hundred ST-70s over the years. The stock version (with dried out electrolytics and leaky paper caps, as well as noisy carbon comp resistors replaced) is actually reasonably good. It was a solid and smart design.
This is my amp, I bought it on Craigs list from someone that tried to install the driver board but could not get it working. The driver board is a
direct replacement of the stock board i.e. identical circuit and identical component values, not sure where it is from but it looked OK and the components I checked were new and right on spec. The tubes are 7199 as per original circuit. Looking at test results I would not be surprised if one of the 7199 were going bad... 7199's are hit or miss and very expensive these days.
 

amirm

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#8
What are the tubes on the replacement input stage board?
One is a Delta tube and the other GE. I swapped them between channels but
didn't make a difference.
 

cjfrbw

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#9
I'm impressed that the distortion is 'only' 1.5 percent at the 43(?) watts.
 

amirm

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#10
FYI the RCA connectors are pretty bad. Spent 15 minutes trying to figure out why one channel was so weak, only to be a loose connection there.

Also, during the testing, I managed to get it to go into oscillation. It was stuck at 35 watts and stuck there regardless of input. Power cycle made it go away. I was running a frequency sweep so it may have gone into oscillation then. Don't want to stress it by trying it again. :)
 

dreite

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#11
Nice patina on that one. :)
I haven't repaired 100.....more like about 12, but the common problems were easily repaired. It seems a lot of users removed the tube rectifier and replaced with a solid state rectifier. (Not a good idea.)
If you own one of these and it's still stock, leave it stock. Very cool old amplifier.

Dave.
 

amirm

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#12
I haven't repaired 100.....more like about 12, but the common problems were easily repaired. It seems a lot of users removed the tube rectifier and replaced with a solid state rectifier. (Not a good idea.)
The tube rectifier in this one is Mullard and British made.
 

cjfrbw

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#14
Yes, El 34's, those are the ones I have, Mullard, my bad calling them el84.
 
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#15
I'm impressed that the distortion is 'only' 1.5 percent at the 43(?) watts.
One is a Delta tube and the other GE. I swapped them between channels but
didn't make a difference.
Hi Amir.. since you are trouble shooting my amp for me can you try swapping the power tubes from one side to the other to see if the weak channel follows :)
 

DKT88

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#16
Yes, El 34's, those are the ones I have, Mullard, my bad calling them el84.
yeah I deleted that post because it sounded critical, I meant to say I make the same mistake mixing up 34s and 84s since I've had amps with both. Here's some information on the ST-70 including a repair guide. The guide mentions channel imbalance and says that if swapping the output tubes doesn't diagnose the problem it may be something in the local or global feedback loops. http://www.curcioaudio.com/st7upg_3.htm
 
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SIY

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#17
It seems a lot of users removed the tube rectifier and replaced with a solid state rectifier. (Not a good idea.)
You can do that if you up the voltage ratings on the filter caps (if you don't, prepare for smoke on start-up). And it reduces sag under load.

Power supply regulation is useful as well, taking out a lot of the spurs around the test frequency. The selenium bias rectifier has to go as well, but that's more of a maintenance thing than an actual modification.
 

anmpr1

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#18
A year or two ago I built two monophonic Dyna Mk IV clones (sourced from dynakitparts dot com). Basically a monophonic 70 (add another rectifier and power transformer on two chassis). The big concession to 'modernity' is the input tube: the original 7199 is no longer made, so it is replaced with a 6GH8A. The only other change I made are updated speaker terminals.

In spite of late 1950s specs (the design engineers were David Hafler/Ed Laurent), I am happy with the sound. Interestingly, it is quite powerful using fairly efficient (8 ohm) period speakers (JBL L100), and plays as loud as I could want, in my living room. In fact, subjectively, it is really as strong as my more powerful Japanese solid state amplifier (measured at 140 watts @ 8 ohms, 210 watts @ 4 ohms, 360 watts @ 2 ohms). I would not recommend it with super low impedance speakers, however during the '60s, and in normal-sized listening rooms, Dynas were often paired with AR3 and similar acoustic suspension speakers.

From an historical perspective, Dyna designs were usually respected, second or third place to much higher priced Marantz and McIntosh gear. Because of their ease of assembly and relatively low cost, Dyna amps were often 'upgraded' by amateur and pro hobbyists. Audio Research sold a mod kit; for a time Gordon Holt touted a set of Paoli modified Mk IIIs as his reference; and Harvey Rosenberg of NYAL Futterman OTL fame (infamy) started his tube life with Dyna. By the early '70s, solid state had pretty much put an end to mass marketed kit based tube amps, although I believe the Dynaco PAS-3 tube preamp stayed in the catalog a while longer.

I would never argue that these old machines are 'accurate' in today's context. That said, in these days of 'black boxes' there is nothing that is much more fun than rebuilding an old Dyna. Although prices are rising considerably, tens of thousands are still out there, just waiting to be found and refurbed.
 

restorer-john

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#19
That's a fabulous FR (the good channel) for a 60+ year old tube amp into 4 ohms IMO. Must be some really good OPT TXFs.

A bit of work on the filtering, regulation and tightening up the tolerances to bring the channel balance back into line and it'd be a fun project.
 

DDF

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#20
I've built and modified perhaps a hundred ST-70s over the years. The stock version (with dried out electrolytics and leaky paper caps, as well as noisy carbon comp resistors replaced) is actually reasonably good. It was a solid and smart design.
SIY, is there a driver board you'd recommend that ditches the hard to find 7199? I have a pair of clean MKIV's collecting dust for the last 30 years.

I may be the only guy on ASR that owns both amps at opposite extremes of the SINAD graph. :)
 
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