Dynaco ST-70 Project

SwiftEnthusiast

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I bought a Dynaco ST-70 project from Salectric in the Emporium here about three years ago: Sold - Dyna ST-70 project kit (and a long story).

I finally found the time to complete this project. The time became available suddenly when a larger project ahead of this got delayed (some home improvement work - key materials delayed). So, I dove into this and was mostly done before I thought about a build thread. So this will be more of a project summary post rather than a build thread.

The project included most everything needed to build an ST-70, including some nice upgrades. A replacement chassis, original cloth wire transformers, a partially assembled SDS Labs driver board, a SDS Labs power supply board, most of the components needed to build the boards, and a set of used audio tubes to start with. To complete this project, I added a few things to the parts collection such as a rectifier tube, a few missing resistors, the driver board capacitors, new pots with longer shafts plus knobs (old pots were included, but I just preferred having knobs), some sockets, some wiring, some solder, and some time.

Dynaco ST70 Build 01.jpeg

Dynaco ST70 Build 02.jpeg

Dynaco ST70 Build 03.jpeg

Salectric thought the replacement chassis was probably stainless steel. I am still not sure myself, but agree with that assessment (probably stainless). It holds a magnet, which allows either possibility. The underside finish looks like raw stainless steel to me. The outer surface appears like polished metal at sufficient distance, but it has a light texture up close. My initial guess was that maybe it is polished stainless steel with some type of clear coat.

As you can see in the photos, the replacement chassis had a little oxidation. There are also some light minor scratches here and there which are not as easy to see. So, I hand polished it a little using some fine polish intended for automotive paint. This substantially reduced the oxidation, to the point where it is only slightly visible in the right light and at the right angle (and my eyes cannot see it at all once everything was later assembled). It didn’t really help with the light scratches, but I didn’t have high hopes on that anyway. I may have been able to improve things further by starting with more aggressive compounds, which I do have on hand. But I am not looking for highly polished perfection - just aiming for something that looks nice and sounds great. Some wear and patina are fine.

The polishing turned the rag black, which I think suggests the outer surface is raw metal rather than clear paint. If it weren’t for the textured appearance, I would think it was just polished stainless. But since it does have that texture, I have no idea. Could it be some sort of plating? Obviously, I don’t know. If anyone else has any ideas what it might be from these clues, I would be interested to learn more.

Dynaco ST70 Build 04.jpeg
I figured I might need to disassemble, clean up, and repaint the transformers. But before starting down that path, I wanted to see how well the existing finish would clean up. It cleaned up better than I anticipated and I ultimately decided to stick with what remained of what I believe may be the original finish. This image shows the transformers after a few passes of Rustoleum rust dissolver (small amounts on a rag).

Dynaco ST70 Build 05.jpeg
Here are the mounted transformers after a couple passes of Brownwells Oxpho-Blue (again, small amounts on a rag). I know it’s not for everyone, but I really like how the transformer finish turned out. As a side note, I did try this same process on that original chassis included with the project. That rusting was much more substantial and it was still quite unsightly afterwards (significant pitting, the larger contrast against the steel vs black paint, etc). So, after salvaging the serial number tag from it, I recycled the old chassis so the remaining atoms can go on to serve some other useful purpose.

Dynaco ST70 Build 06.jpeg
Assembly and wiring in progress.

Dynaco ST70 Build 07.jpeg
Wiring and assembly nearly completed, just need to install the bottom cover.

The assembly was pretty straight forward, aside from a couple minor clearance issues.
  1. When I went to install the driver board, I found that the leads of two resistors closest to the front edge of the board, R21 and R24, contacted the chassis and prevented the board from seating. Additionally, the board position was off a little to one side of the chassis opening, enough to leave a small gap on the other side. I resolved both of those issues by slotting the mounting holes in the driver board. The board could only be shifted rearwards a small amount before it interfered with the bias measurement holes, so I also sanded a little bit off the rear board edge in that area. These tweaks were enough to allow mounting the driver board, but the clearance between the resistor leads and chassis were still a little tighter than I would like. So, I decided to also mount those two resistors on the underside of the board, with the leads trimmed to stop well short of the top board surface.
  2. After completing assembly, visually double checking everything, and double checking key connections with my meter, I was ready to power it up. I went to install the fuse and found the next clearance issue. The rear fuse terminal moves with the internal spring loaded fuse contact and needs enough clearance to allow sufficient range of motion to install and remove the fuse. And one of the power supply capacitors was interfering in the last bit of that range. It was fortunate to catch this during the build, even if it was right at the very end. It was still relatively easy to resolve as that point. If I had built this with the fuse installed in the holder, I think it would have cleared and I may not have realized this clearance issue until some later date if/when the fuse blows. Anyway, I resolved that, with some extra margin, by uninstalling the fuse holder, trimming a bit off the end of the rear terminal, and reinstalling the holder.

Notes about some other minor variances you may be wondering about…
  1. That is a terminal strip mounted CL-90 thermistor near the power switch to limit inrush current in case I want to try out a socketed solid state rectifier in the future.
  2. The terminal strip just forward of that serves as a junction point for the inductor as my inductor wires were not quite long enough to reach the power supply board. Additionally, while the audio transformer red wires were long enough to reach the original capacitor location (and this terminal strip), one was not long enough to reach the new power supply board location. So they were tied in here as well.
  3. The red and black power transformer wire lands on a makeshift single position terminal strip. It is made from a stainless steel corner brace, insulated shoulder washers intended for transistor mounting, and some stainless hardware. The purpose is to extend this wire which is only long enough to reach the original termination point, far short of the new power supply board location. This was a last minute course change I made with parts on-hand, as I was originally just going to splice, solder, and heat shrink this connection (as I did for the feedback / 16 ohm tap connections). I figured this would allow easier changes in the future without potentially having to further shorten this wire that was already pretty short. I had also considered a simpler option of using a plastic stand off as you can see in the first wiring image, but I was not sure how that would hold up over the long term with heat from the amp.
Dynaco ST70 Build 08.jpeg
Here is the amp assembled and back on its feet.

Dynaco ST70 Build 09.jpeg
Here is the amp in its new home with tubes installed. Those are the EL34s and 12BH7 that came with the project. The EF86s and rectifier are used Mullards I sourced separately. It is on a variac that reduces the supply voltage from about 125 Volts to about 115 Volts. This is in my living room - I will try to post soon in the intro / systems section with some information about the rest of my system.

Dynaco ST70 Build 10.jpeg
And here it is at night. That is a 3 second time lapse, which is what my iPhone does by default in this circumstance. A beautiful distortion of how it appears in real life.

I haven’t had a lot of listening time on this ST-70 yet, but I am pretty happy with what I have heard so far. :)
 
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Looks great.
I have not heard of the SDS Labs board before.
Any info on it ?
 
I have not heard of the SDS Labs board before.
Any info on it ?
That's Sheldon Stokes' board if memory serves - Dynaco ST-70 Amplifier
I am not entirely sure, but I do believe that is correct. Both of these boards appear consistent (schematic, board layout, etc) with the documentation at the SDS Labs site Thermionics linked. And that is what I ended up using to complete the driver board assembly as the included printed Triode driver board instructions seemed to be for a similar, but different, driver board. My best guess at this point is that perhaps Triode was selling these SDS Labs driver and power supply boards back then. The current description of the ST-70 power supply board Triode offers now mentions it is an updated version of the classic SDS board. Perhaps a similar story with the driver board.


Looks great.
Beautiful ST70!
Thank you!
 
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Just an aside to the SDS board story. Sheldon Stokes, when I asked him about ordering boards about a year ago, told me that he put the designs for many of his boards on OSHpark where they can be ordered directly from them. Or, the design files and Gerbers can be downloaded and sent to other board fab companies. I have ordered several of his boards directly from a board fab company. I using some of his Dynaco MK3 cap boards in my restore of a pair of MK2's.

Here a link to the beginning of a search for his projects:

John
 
I am not entirely sure, but I do believe that is correct. Both of these boards appear consistent (schematic, board layout, etc) with the documentation at the SDS Labs site Thermionics linked. And that is what I ended up using to complete the driver board assembly as the included printed Triode driver board instructions seemed to be for a similar, but different, driver board. My best guess at this point is that perhaps Triode was selling these SDS Labs driver and power supply boards back then. The current description of the ST-70 power supply board Triode offers now mentions it is an updated version of the classic SDS board. Perhaps a similar story with the driver board.
Yep. They did this with a few of Sheldon's projects.
 
What an incredible build and restoration! I really miss my ST-70, it was also carefully restored and sounded fantastic. I should never have sold it.
 
Thank you MikeT. and JohnVF!

I have been enjoying listening to my ST-70 when I can here and there. I have it driving a pair of Meadowlark Swifts. Sounds really good to my ears.

This is my first ST-70, although I do have an SCA-35 that had served that same role (main amp in primary audio system) for a couple years or so. I bought that one in working order and have not had to do anything to it yet... but it suddenly developed a moderate hum problem a while back. Swapping in spare tubes made no difference and I didn't have the time to dive in any deeper, so I took it out of service until I could investigate further. Wondering if the cap might be failing (I believe that amp is mostly original). It will be next up on the workbench, aka my dining room table. :)
 
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Hey swift, now that your SCA is up and running, how do you feel it stacks up against the ST70?

(I have a guess but I don't want to move the needle one way or the other)
 
Hi @asilker. Good question. So, I now have a Dynaco SCA-35 (Dave Gillespie boards), a Dynaco ST-70 (SDS Labs boards), and a First Watt J2 (stock) on hand. I hope to make time to listen to all three in the current iteration of my main system in the next week or so. I will then follow on this question and your prior question about the ST-70 vs J2. For the moment, I have the SCA-35 in my main system and really enjoying it.
 
Hi @asilker. Okay, here is a first installment of an answer to your question. :)

While the ST-70 sounded good to my ears and I think I could be happy with it as my only amp, I found that I preferred the sound of my Dynaco SCA-35 and First Watt J2 in my system (driving Meadowlark Swifts). To my ears, the main difference I notice with my ST-70 relative to the other two amps is in the reproduction of vocals and acoustic instruments. The ST-70 is just a little less convincing and transparent. I cannot pinpoint that further at the moment, but I think my impression probably results from a collection of minor differences (imaging seems a little less precise, tonality variances in the midrange, etc).

The output levels produced with the lower power of the SCA-35 and J2 are sufficient for my needs and I don’t believe I am playing the ST-70 any louder. So, its benefit of higher output power is probably going to waste on me with my current system.

Along those lines, I recently rewired the ST-70 output stage tubes in triode mode (disconnected the UL feedback and tied that screen grid pin to the anode via a low value resistor). Although that reduces the maximum output power, the reduced output power should be on par with my Dynaco SCA-35 based on what I have read from several sources (including this ST-70 article by Dave Gillespie). I read about this change in various articles and threads I came across on the internet. They are easy to find via search engine, but just let me know if anyone needs links. Some recommend using the next lowest output tap (4 Ohm tap for speakers that are 8 Ohms nominal, etc) in conjunction with this change. And some also recommend disconnecting global feedback. I have been trialing those revisions as well. Disconnecting the feedback was too far in that direction for me, so I have been trialing varying degrees of reduced feedback instead. It does seem there is probably a good compromise somewhere on the spectrum of reduced feedback. There are a number of permutations possible with these variables and I am in no rush to work through them all, so it may be some time before I settle on a particular configuration. And it is also too early for me to say much about resultant subjective quality, but I can at least say that my ears are telling me this is improving those areas where I felt my ST-70 fell a little short of my other two amps.

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Revised the underlined portions of a sentence above to convey what I actually meant and added a reference to the most credible source (the others mentioned similar numbers, usually a little more optimistic).

Added this FYI. The link below is for instructions from Curcio Audio Engineering which detail the changes involved in triode strapping the output stage. I actually found this document later in my journey, but it is the clearest and most thorough documentation of these changes I have found to date.


"Dynaco Stereo 70 Output Stage Triode Conversion Instructions" from Curcio Audio Engineering
 
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Very interesting report.
I have a tubes 4 hifi octal board waiting for to be built and try in my ST70.
I hope to get to it after the holidays.
 
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Also, interesting comments. When I was deciding which tube amp to build as a first project I kept seeing too many contrasts between EL84 amps and EL34 w/ a clear preference to enhanced mid-range vocals for the EL84 to consider it just a passing opinion set. While I'm only comparing my ST-35 w/ an early Mac SS and old Halfer, I find it highly satisfying in the female vocals and piano jazz material.
 
@SwiftEnthusiast
By the way, I purchased both boards from Triode Electronics although at different times. The audio board design dates back to the old ownership of Triode Electronics (the Ned Carlson days). What I always found interesting is that the 3-tube circuit is actually identical to the stock Dyna circuit; it is simply implemented with different tubes----an EF86 for each channel and a shared 12AU7 for the phase splitter compared to Dyna's 7199 which combines in a single tube a high-gain section similar to the EF86 with a lower-gain section similar to a 12AU7.

The power supply board was a Sheldon Stokes design sold by Triode Electronics.
 
@MrEd
I recently restored a pair of Mk2’s with an octal driver board using 6SN7’s. It sounds really good but needed a couple of mods. It appears to be a copy of the VTA ST70 circuit. The feedback is very low and the bias circuit is poorly filtered. You can’t increase the feedback too much without having a low input sensitivity. I got it up to about 7dB which is still a low feedback. The output noise was directly affected by the bias supply filtering. PP noise canceling only works to a point.
 
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