Advice needed, buying ST-70, where to look at?

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Advice needed, buying ST-70, where to look at?

Postby Dynaco_WJW » Sat Jun 18, 2011 2:35 am

Next week I go and see a ST-70 amplifier. Since this is my first ST-70 I don't know a lot about it. If everything is OK I will add it to my Dynaco collection.

Can you help me with the buying process of a ST-70? Where to look at? I saw that the cover is a bit different. It has no 'Dynakit' label but just 'Dynaco'. The cover is attached with screws. I saw the power transformer is PA-521 so suitable for 230V~. But on the back I can't see 230V~ written. Is this strange? The circuit board is altered I can see. How about this version of ST-70's? How many versions are made? I read something about different makes of the output transformers. With cotton wound wires and plastic wires?

As you understand, all information is welcome. Where to look at especially.

Thank you very much.

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Postby dcgillespie » Sat Jun 18, 2011 5:43 am

WJW -- While some physical details of the ST-70 did change over the years, the actual circuit design of the unit never did.

Your unit appears to be a slightly modified stock unit, with the 230 volt power transformer "option". I place the word option in quotations to distinguish it from being a "version". There was really only one version of the ST-70 produced under David Hafler, in that it's circuit design never changed during that time. Being a product of the USA, the vast majority of those produced were supplied with a power transformer suitable for operation from a 117 vac power source, but an alternate power transformer was available to accommodate 230 vac operation. It could have been built like that at the factory as an offering for the European markets, special ordered as a kit in the states, or later changed out by someone to suit their needs. In any event, the alternate transformer was available, but I doubt that Dynaco would change the silk screening on the chassis to accommodate it. They would most likely employ a sticker to be placed appropriately on the unit to indicate it has the alternate power transformer option installed.

As for changes over the life of the ST-70's production run, they were very minor, never electronic in nature, and typically related to either component availability, or slight cosmetic changes.

The specifications of the OPT transformers never changed, but the manufacturers did. The differences were most often noted by different type wires used, and even different part numbers being used. Of course, that spawned a world wide debate over which transformers were better, but to date, any evidence of any differences is opinion based only, as nobody has been able to show any measured performance difference between any of the transformers based on which manufacturer it came from. The transformers on your unit appear to be the original item produced for Dynaco, with the A-470 part number stamped on them.

The front sticker changed in size and styling, as did small details of the cover. Older units used two screws on each side of the unit to hold the cover and bottom plate in place, later units used three on each side. The address of Dynaco as silk screened onto the chassis changed over the years as well.

Capacitors were provided by different manufacturers over time, and of course, everybody has their favorite maker, but the values never changed over the course of its run.

Some of these changes were a product of Dynaco moving its location, and some from it eventually being sold to Tyco. Even later, a ST-70 II appeared from the buyer of all remaining Dynaco stock (Stereo Cost Cutters) after Tyco formally shut operations down. However, the bottom line is that the original ST-70 basically remained unchanged in all that was important over the entire course of its production run, whoever was producing it.

Other than that, your unit has had the original coupling caps bypassed with (obviously) larger (physically and electrically) values at some point by a previous owner. Other components appear to be bypassed or changed as well. Only you can determine if you like the effect of this or not. From a technical standpoint, it is always a concern to raise the value of coupling capacitors significantly in a feedback amplifier, as then low frequency stability can become an issue. Dynaco amplifiers were always known as being quite stable, as this was an issue that Hafler openly championed in the day, and built into his designs. Those with a lesser understanding of these concerns who hap-hazardly change these values in the quest of better sound can unknowingly create more problems than they solve. This is simply offered for you to beware of. Of course, the original schematic is widely available to convert the unit back to its original form, and numerous aftermarket boards with all different manner of tube configurations are available as well.

The ST-70 is widely reported to be the highest selling stereo power amplifier of all time, surpassing many other marks put together. If you get this one, congrats on expanding your Dynaco collection!

Dave
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Postby Dynaco_WJW » Sat Jun 18, 2011 12:20 pm

Dave -- Thank you very much for your comprehensive response! Now I know a lot more and what is important for me this moment that it gives me confidence in buying this amplifier. Quite expensive, but I think it is worth it. The ST-70 is quite rare in the Netherlands I guess. Sometimes only 117VAC amplifiers are offered. What is important for me is that it is original and not built with components from different amplifiers. The state of the chassis is important and of course the transformers.

I will bring it back to the original state. There are some changes like wiring and components like the coupling capacitors. These are big indeed. Maybe they would give the sound 'more air' with bigger capacitors ;)
I received the phone number of the person who did the changes. I can ask what he exactly changed and for what reason.

I saw ST-70 with 3 tubes on the pre amplifier board. I just found out that this is a different design with triodes. I don't have the intention to install a board like that because I don't know what the advantages are. Are there advantages? Indeed I saw a lot of available boards. If I will replace I have to find out what board is best suitable. Pity that these boards are not available anymore at Audioregenesis.

I placed a advertisement with the question "wanted: Dynaco tube amplifiers". Two people responded. One of these said that his ST-70 has the 'better hand wounded" transformers. Maybe someone told him, or it is a selling trick.
Next friday I will probably buy the ST-70 what can be seen on the photo's.

Thanks again Dave for your answer. FYI the SCA-35 with the EFB™ still sounds fantastic!
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Postby TerrySmith » Sat Jun 18, 2011 5:47 pm

Nowadays you can buy brand spanking NEW ST-70 from three different places, each with their own personality. Depending on what someone is asking for an original and the added cost to properly rebuilt it can come very close to the price of a new one.

As for new ST-70's all come with stainless steel chassis, larger power transformers, better circuit boards, etc.
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Postby Dynaco_WJW » Sun Jun 19, 2011 4:21 am

Terry -- Thanks. Can you tell me more about these three places? One of these is Dynakitparts I suppose? What is the difference? What do you mean with the different personalities?
Shipping costs from the US to Europe is very high. To ship a capacitor (for ST-35 or ST-70) is about $35,- so for a complete ST-70 kit it is much higher.
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Postby burnedfingers » Sun Jun 19, 2011 4:41 am

If it were me looking to buy my first Dynaco I would buy a NEW kit. My personal opinion is that just about anything sounds better than the original Dynaco driver board. As first introduced the Dynaco kit was marketed as a inexpensive way to get into a tube amplifier. The output transformers do sound good and they are probably the only redeeming feature of the Dynaco stereo 70 amplifier. The 70 can be made to sound good by using an add on power supply board, individual bias adjustment, a better power trnsformer just to name a few things. The original driver board does need to go. Just adding new coupling caps with no other modifications doesn't do hardly anything.
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Postby TerrySmith » Sun Jun 19, 2011 9:14 am

One is the http://www.dynakitparts.com/store/ , it's basically buying a bone stock ST70 except it is all new. This one uses the hard to find 7199 driver tube.

Next is http://store.triodestore.com/ ST70 kit that had a different driver board, a capacitor board underneath to replace the can cap.

And there's Bob Latino's http://bob01605.50webs.com/ version, probably the easiest to build and has a much better driver board than the other two. This one would be my choice if I had never worked on a ST-70 before.

All three have larger power transformers and stainless steel chassis and are high quality kits.
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Postby rmyauck » Sun Jun 19, 2011 6:24 pm

Maybe start with Mark III (A-431) output transformers for greater bandwidth especially run at lower power. They have the same primary impedance 4.3K as the ST-70 (A-470) transformers. The reproductions are said be just as good as the original ones. Probably better as some have said on these forms some old ones have not matched up.

Look in the HI-FI on this form index for Geek's thread on the Koren Local Hero board's as they have No Global Negative feedback in the design. Should have great sonics. You can get 11Hz to 46KHz at full power (40W). You can use the bigger tubes too. He sell's powers supply boards with improvements too.


For chassis you could make your own to save shipping cost and keep the mono block idea for sonic improvement's , weight savings and easier to work on design. You can buy reasonably priced cast aluminum chassis from Hammond too which are light to ship and easy to drill & cut etc. Look at the finished Budgie. But maybe it has to be all Dynaco in looks.
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Postby rmyauck » Sun Jun 19, 2011 7:10 pm

You might be able to save considerable $ by buying your Output transformers in Europe as I think there are many great winders over there too.. Lundahl of Sweden makes nice C-core ones for good prices. 5K would be a good input impedance for lower distortion. Allen Wright of Vacuum State Electronics of Switzerland specified the LL1663 PP 40W 5K Primary 8 ohm output for his well liked PP amps with no Global feedback. They sell for $300/Pr in the US. You can buy covers for them.

AudioNote also sell's suitable 25W PP Output Transformers for 110 Euro's ea, but they don't have UL taps if you want that. They should be suitable because they are 50-100% overrated for power.

If you can't find affordable PS transformer over there Edcor might be an affordable choice.

Check or ask on diyaudio for transformer info too.

Then you can build a ST-70/Mark III clone with a bit of European flavor for affordable $
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Postby Dynaco_WJW » Tue Jul 05, 2011 1:57 pm

In the meantime I bought the ST-70. It really looks fantastic! Thank you very much for your help. I just began the restoring process and I have already some questions.
I would like to open a new post for this ST-70 and discuss the problems(?) I find during restoring this beauty.
In this post I also will explain why I have to restore the amplifier.
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Postby Sal Brisindi » Thu Jul 07, 2011 8:35 pm

TerrySmith wrote:One is the http://www.dynakitparts.com/store/ , it's basically buying a bone stock ST70 except it is all new. This one uses the hard to find 7199 driver tube.

Next is http://store.triodestore.com/ ST70 kit that had a different driver board, a capacitor board underneath to replace the can cap.

And there's Bob Latino's http://bob01605.50webs.com/ version, probably the easiest to build and has a much better driver board than the other two. This one would be my choice if I had never worked on a ST-70 before.

All three have larger power transformers and stainless steel chassis and are high quality kits.


I just completed building a new ST-70 kit from Dynakitparts.com with the Bias Balance controls. Great quality, even the <BR>
printed circuit board is the original brown color Dynaco used back then.

In this photo I used 6GH8 tubes with the 6GH8-7199 adapter that comes with the kit to test the amp but my customer had 2 NOS Dynaco 7199 he installed in the amp.

Sal

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Postby Dynaco_WJW » Sun Jul 10, 2011 10:20 am

Interesting this Bias Balance Control Upgrade. Is somewhere in a schematic to be seen how it works?

I like the circuit board. It looks vintage! Thanks for this posting Sal.
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