SOLD. Dyna ST-70 project kit (and a long story)


This is basically a collection of parts for putting together a Dynaco Stereo 70, sort of a modern day kit. Some background: this is my 4th ST-70 so I have a long history with the design. I started out back in the early 1970s thinking I could “improve” the design. Solid state rectifiers, big caps, Mylar couplers, metal film resistors, EL-34s in triode, cathode-coupled outputs like ARC used. I am actually embarrassed writing that because the changes were not improvements at all. To make a long story a bit shorter, I eventually concluded the Dyna engineers actually knew what they were doing. The stock circuit is really, really good, but only if it is implemented correctly.

I bought this 4th ST-70 as a rust bucket back in 1999. It was totally stock. I really didn’t need it because I had gone solid state back then. I had a Classe CA-300 driving Thiel 3.6 speakers. The Thiels were a wicked load and needed all of the Classe’s 600 watts/ch (at 4 ohms) to sound decent. Nevertheless, I connected the ST-70 to the Thiels just out of curiosity, and Lo and Behold the stock ST-70 sounded better than the monster transistor amp. Of course, it had power limitations. When I turned up the volume above conversation levels, it softened transients and basically just didn’t go any louder. However this convinced me, once and for all time, of the superiority of a well-executed tube circuit.

Fast forward a bit. I wanted to see if I could improve the sound without giving up any of the Dyna’s wonderful musicality. I replaced the stock circuit board with a Triode Electronics replacement board that uses 3 tubes—an EF-86 for each channel and a 12AU7 or 12BH7 phase splitter shared by the two channels. That may sound like a radical change from a single 7199 per channel, but it turns out the Triode Electronics schematic is identical to Dyna’s design but is simply implemented with separate tubes. One-half of the 7199 is a pentode with nearly identical characteristics as the EF-86, and the other half is a triode nearly identical to a 12AU7. So the new board just separates the two sections into different tubes. This turns out to be a brilliant solution to several problems. Not only are good 7199s hard to find today but it turns out Dyna’s design depends on premium versions of that tube to meet its performance specs. Using an EF-86 allows the amp to perform as intended more predictably and more consistently. It also allows you to roll different tubes more easily to get the sound you want.

Anyway I ended up selling that Triode Electronics board to a friend, but I later decided to build another ST-70 to have as a spare amp so I bought another Triode Electronics board along with the SDS power supply board mainly so I wasn’t dependent on sourcing more electrolytic cans. The rust bucket chassis clearly wouldn’t do so I bought a new chassis and bottom plate (I think they’re stainless steel) which came with various binding posts and input jacks. However, as the pictures clearly show I never got around to putting the thing together. I did solder the tube sockets and a number of resistors to the circuit board but that’s as far as I got. Now, after more than a decade sitting in the closet, it’s time to face facts. I am not ever going to finish it so it needs to move on to someone who will complete it and then enjoy it.

Here’s what’s included:

* Original Dyna chassis with all transformers and choke. The OPTs are the more desirable early version with cloth leads.
* New chassis and bottom plate. There’s no tube cage but it looks better without it anyway.
* Triode Electronics board for EF-86/12AU7 circuit, partially assembled.
* SDS power supply board with parts for assembly.
* Instruction manuals for audio and power supply boards.
* I will include JJ E34L tubes (quad), EF-86 (pair), 12BH7. These are used but should enable you to get started. 5AR4 rectifier is not included.

The audio board does not have coupling caps. You will need to supply your own. You will also need octal tube sockets for the new chassis. It’s possible there are some other odds and ends missing but the ones mentioned are the only ones I know about.

Price is $475 for the whole package, plus shipping. US sales only. I will cover PayPal fees. Photos will follow.
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Junior Member
That's a great deal for someone that wants to build a ST-70. All the good stuff is included.