Stock ST70 'real world' measured performance

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Stock ST70 'real world' measured performance

Postby Dyna70 » Tue Mar 30, 2010 10:40 am

Hi folks!

Haven't been on here a while (used to post as 'mugsy' but as hadn't used my account for so long I couldn't log in today) but got the Dynaco bug again and just short listing a few parts to get my rebuilt (again) ST70 up and running.

I've always been curious. Many seem to turn their nose up at the original ST70 driver, but 85% of the reasons I have seen they mention bad quality parts and lifted traces on the driver boards. As far as I am concerned this is all to be expected for an amp around 50+ years old!

Taking the above out of the equation, and taking INTO account a brand new driver board with the original design, and good quality components from today's manufacturers, good set of tubes etc etc. has anybody actually measured the stock design ST70s performance?

Reason I ask is despite also owning a VTA board I kind of liked the stock ST70 driver boards sound, though I built my ST70 totally from new/modern production parts, except for a pair of genuine output trannies I got from Ebay. Dynaco themselves boasted about their ST70 on page 3 of the original manual:

"The measured specifications show that the Stereo 70 is an amplifier of unequalled performance, and its listening quality is also unrivalled by any regardless of price."

That's a bloody bold claim, even for 1959 considering the quality of its rivals around at the time from Leak, Quad, McIntosh and others. Marketing speak, or was this really a top of the line performer back then?

I'd love to see the measured performance of the original circuit built with good quality modern parts (inc. updated mains trannie and the 'de rigueur' 80/40/30/20 new manufacture quad caps) to see how it stands up.

Cheers,

- John
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Postby Geek » Tue Mar 30, 2010 10:57 pm

Hi,

Welcome back and nice to meet you!

I look at it this way - if the original circuit was so bad, why did it last so long?

Just like a lot of boutique amps and drivers, there is no one best or worse, each has their own charm :-)

Cheers!
-= Gregg =-
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Postby Dyna70 » Wed Mar 31, 2010 6:21 am

Hi - and thanks :)

Yep 100% agree. I have only ever heard the stock circuit as I built it - from a modern repro board from Dynakits, plus hand matched quality components and the updated mains/quad caps. Sounds great to me and overall I preferred the sound to the VTA board. Maybe not as much extension at either end but made the music more involving and enjoyable.

I'd just be very curious to see how the stock ST70 measures, as I'm sure if it was good then it must still be pretty good now especially with modern parts quality.

- John
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Postby joeriz » Mon Apr 05, 2010 7:28 am

Here are some opinions from this forum from a few years back. See, in particular, the opinion of our esteemed host (3rd post in thread).

http://www.diytube.com/phpBB2/viewtopic ... light=7199

I've asked the same question as the original poster elsewhere and never really received a great answer. Having built a few different versions of the ST-70 over the years I'd have to say that I agree with those who think that the original driver circuit (when built with new parts) sounds very good. In fact, at one point I went from the original circuit (built with fresh parts) to the Welborne mod and didn't feel that I gained anything. Of course, some might say that only prooves that I need my hearing checked but so be it. ;0)

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Postby TerrySmith » Mon Apr 05, 2010 8:10 am

I'd SWAG that the top reasons to replace the original board is:

7199 tubes are getting harder to find and very expensive.
Original boards are burned- lifted traces - warped etc.
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Postby burnedfingers » Tue Apr 06, 2010 4:54 am

I have built a number of them over the years. I have never been impressed with the stock Dynaco stereo 70 because it sounds bad. The bass is loose and it suffers in the mid and high frequency region.

It sounds much better after the power supply has been beefed up. I do have one driver board (7199) that sounds reasonable when coupled with a unit that has power supply mods. This board has had all the components replaced and has matched 1% resistors.

The dynaco starts to shine when the driver board is replaced with a better performing unit.

The Dynaco started out as a cheap tube amp that was designed to compete with other units on the market at the time. The idea was to market something that would return a good profit based on units sold. Maybe its time to quit poking a dead horse and make the needed upgrades. The dynaco will never be anything special as it came out of the box.
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Postby Dyna70 » Tue Apr 06, 2010 8:40 am

"It sounds much better after the power supply has been beefed up. I do have one driver board (7199) that sounds reasonable when coupled with a unit that has power supply mods. This board has had all the components replaced and has matched 1% resistors."

This is what I'm getting at with my original post - people diss the stock ST70s driver board out of hand, when what they are REALLY dissing when they say "it's muddy and vague sounding" is the inadequate power supply (probably with dried out quad cap too!), old and mismatched tubes, the sound of drifting 50 year old carbon comp. resistors and caps and 'on their last legs' circuit boards... As you noted it sounded 'reasonable' when the board had had its components replaced i.e. it was THEN working to spec...

I have only ever heard my own scratch built ST70 with quality hand matched components, NOS pair of matched RCA 7199s, matched quad of EH EL34s and the modern repro. Dynakits 7199 driver board (and also the VTA). I liked it and could hear none of the short-comings many blame on the 7199 circuit. I am used to using Greg Ball's seminal GB150D power amp so I would notice if I was missing anything. It wasn't as extended at the frequency extremes as the VTA, but what was there sounded extremely natural, and involving. Glorious midrange in particular. The VTA almost sounded solid-state & boring by comparison. I still need to try the VTA with better quality caps however.

I've done a lot of research on driver boards and I have to say I have come across many instances of people dissing the stock 7199 board, and then offering their own alternative i.e they have alterior motives. I have also seen a certain designer who shall remain nameless posting on various forums under a different name pushing a certain brand of driver board upgrade...

I'd still be interested in seeing an honest measured performance of a modern build ST70, using the stock 7199 circuit, but decent all new components and the standard upgraded mains tranny/can cap to make things fair when compared to other builds.

Reviews of the ST70 at the time it came out seemed universally to praise the performance from what I can see. And don't forget the ST70 was in competition with Quad IIs, Macs, Leaks, Eico etc etc etc.

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Postby skidave » Thu Apr 08, 2010 12:37 pm

Check out some measurements I made a while back. Here is the thread link: http://diytube.com/phpBB2/viewtopic.php ... highlight=

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Postby 20to20 » Fri Apr 23, 2010 9:45 am

Hello all, newest member here for my first post, on the sound quality of the original 70 board. Just fired up my first ST-70 this week acquired from eBay. It is nearly 100% stock, factory wired. Original OEM muti-cap, and bias caps, selenium rec. The only change I could find was on the board which had the .1 caps changed to Sprague OD's but retained the .1mf rating.

I'd like to address this issue of original sound quality. I was extremely sceptical of what I was going to hear out of it. I had originally bought it as a functional object of art after seeing a Mac 275 and couldn't afford one. I stumbled upon some beautiful pix of the '70 online and decided to go for one.

After seeing a lot of examples of '70's for sale, I feel extremely lucky on getting this one. It has a set of 6CA7 GE fat boys and GE rectifier. After wiping down a layer of greasy goo and using a bit of SoftScrub on the chassis it shines like mercury. Just plain liquid gleem on top. I can't believe it.

It had a couple of problems to fix before I could use it. I tested all the tubes before doing anything. One of the 7199's was shorted H-K and had a maxed out grid leak. So I got the 6GH8A adapter set to use on the board. The second "problem" was on initial fireup I couldn't get the Biaset lower than 1.75v. The GZ-34 tested like new. That bugged me. By chance I had an old RCA 5U4G in my bag of tricks from days gone by and used that to check the rectifier system anyway. Got the Biaset to go centered with that. YeeeeHaaaa!! I let her get good and warm. No humm, no stink! Some of us don't own variacs...

I had to find a preamp of some sort because I don't have a modern preamp in my stereo system. But, I have a good 2 channel mixer so I'm used that for input. Sig/Noise 80 and a couple of modern Technics speakers.

Long storty getting shorter... Sound quality...

I am very suprised at the high fidelity of this amp. After reading all the posts about sloppy lows and rolled off highs. I can only compare this amp to my other main system which is a Sansui G-6000 and CV D-3's refoamed woofers, playing SACD's and DVD-A through an Oppo player, Lp's from a Technics Q w/ Signet cartridge. I think the Sansui is more than an adequate comparison amp. So the bottom line for me is this...

I've been sitting between the speakers for the entire week cycling this on for 8 hrs. and reading the bias, playing a range of rock from Clapton, F. Mac, to Zeppelin from cassette tapes because a cassette deck is what I have at the bench. The fidelity is very comparable to my other system. Highs are just as pronounced and the percussionis just as clean and stiff. The dynamic range and S/N ratio is indistinguishable. I am SO suprised that a tube amp from 1962 sounds so good. And as far as PT temp goes, I have been playing this baby for 3 hrs now and I can lay the palm of my hand on the top of the bell and keep it there. It must be under 120 degrees. It's cooler that very warm shower water. So to sum this post, hello all! I hope to be able to contribute something useful. One last point. I hope that my GE tube set is just a set of Godzilla tubes that are normally out of the bias range. I believe that GE rctifier is OK but I'm going to get another GZ-34 type to see if that gets the bias back down like this 5U4G does. I'm loving this machine for sure. See ya!
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Postby TomMcNally » Fri Apr 23, 2010 9:58 am

Welcome to the forum Mr. 20to20 !

Glad you found a nice one!

One of the easy upgrade/repairs/mods that might help
your bias problem if it hasn't already been done, is to
replace the old selenium rectifier with a simple 1N4007
type diode, and replace the old electrolytics. Many guys
mount the diode on a little terminal strip and you're done.

... tom
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Postby 20to20 » Fri Apr 23, 2010 10:10 am

Thanks! I did expect to have to do those mods and got some parts before lighting this up. I decided on some hardy diodes, perhaps overkill, 1N5399. I have not istalled it yet. Though I did just this morning replace one of the bias caps because on close inspection there was a small blob of black goo surounding a small bare spot on the side of one of the cans. Couldn't tell if the can had split or if it was a remnant of original soldering tip slag on the day the '70 was wired that had burned through the plastic can jacket. 100mf./ 100v. on that one. Didn't seem to matter to the bias setting or noise level, which was 0 to start with.
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Postby dcgillespie » Sun Apr 25, 2010 12:57 pm

Hi John --

I have always been one to place as much emphasis on measured performance as on sonic performance, so I regularly check the "real world" performance of my equipment.

Regarding the measure performance of my rather stock ST-70, each channel driven separately on average delivers:

1. Power output from 20 Hz - 20 kHz = 37 watts RMS.
2. THD at the onset of clipping over this range:
@20 Hz = 2.1%, @ 1kHz = .28%, @ 20kHz = 2.5%
3. IM distortion at the onset of clipping = 41 watts @ .65%
4. Freq Resp. = to 35kHz @ -.5db.

Note that this is while operating from a 121 vac line and biased normally.

With both channels driven, it will produce rated power at mid-band frequencies only: At 20 Hz and 20 kHz, maximum power output with both channels driven is typically 33 watts RMS, and distortion rises appropriately.

With the exception of frequency response, my unit will meet the original stated specifications as published by Dynaco with both channels driven. Note that those specifications do not include a power bandwidth rating for "35 watts continuous" output -- only at 1 db down from 35 watts, at which my unit will meet specifications with both channels operating.

In return for the reduced frequency response, of all the pioneering stereo kit offerings, David Hafler totally understood the complete stability issue in feedback amplifiers. His were among the first to remain completely stable -- even when capacitive only loads are presented. The original design of the ST-70 will remain absolutely stable under all loading conditions -- an accomplishment that Heath or Eico rarely achieved in their equipment. Hafler realized that a frequency response that goes to the moon, but is ultimately unstable is much less desirable than a much more limited one that is completely stable. That is the real beauty of Dynaco equipment, and it manifests itself in the sound. I believe that it, along with ease of construction is why the ST-70 outsold all others. Note that there were never any significant circuit revisions to the basic Dynaco designs, while both Heath and Eico had to make circuit and component revisions to a number of their pieces to address poor stability.

By today's standards you can surely pick at the modest power supply components of the ST-70, or even the much maligned phase splitter driver design. But for the day, his general component quality was considered among the best, while the measured performance of the original design still leaves many of today's "improved" efforts wanting.

If only he hadn't gotten in bed with RCA and their 7199!

Dave
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