ST-70 modifications

I've done several of the Triode Electronics mods on a couple of ST-70s and also a set of Mk-IIIs. These were all full mods where not only was the cap board replaced but the driver board as well. I just finished a gut-rehab of a ST-70 which now powers my second system. The modded amp runs 6L6GC outputs instead of EL34s.

The Triode mods are well thought out and documented and they have a great deal of flexibility. You can use a variety of output & driver tubes (as in my example above) and you can even run the driver board, the outputs or both in triode mode.

In short, the Triode Electronics design is a first class option.

PS, the instructions for the Triode mods are available on their web site if you search a bit.

I've also worked on a Curcio modded amp. It was good but I prefer the Triode option.

I have experience with both the Curcio mod and more recently with the Alan Kimmel kit that Ron Welborn sells. I've been extremely impressed with the Kimmel constant current design, but it really changes the character of the amp. If you like the romantic quality of the stock ST-70, I'd recommend the Triode kit. You'll get much better bass control & stability, and much better parts quality than the original Dyna parts.

The Kimmel design makes the ST-70 sound like a modern amp, much more extended highs and tight bass. It is still rich in the mid-range, but a lot less lush sounding than the original. Mine has Tung-Sol 6u8a's and black plate RCA's in the driver board, with Gold Lion KT-77's and Mullard rectifier. Frankly it sounds fantastic with appropriate efficient speakers. But it is NOT a kit for the unskilled, although I hear the latest version has better instructions. You need to be able to read schematics and should know something about tube amp construction. If I do another one, I would definitely do the Kimmel again, but I'm a masochist... YMMV.

One more kit you might look at, and that is the driver board offered by Mapletree Audio Designs. The MAD board is said to be very nice sounding, but I have not heard it myself, altho I have a MAD octal tube preamp I built which I like a lot.

there's a pretty good dyna board on AA you should check out. Lots of folks have trodden these paths. If you need help, just ask.

HTH

K&K Audio has a mod package for the ST-70. It converts the amp to 10 watts triode. Might be worth looking into.

The K&K Audio mods for the ST-70,sound amazingly good. I had one in my system for a couple weeks.

Last year I built a bone stock ST70 kit from dynakits.com I also bought a VTA board along with triode kit to convert it to after getting a baseline on the stock amplifier.

I own a lot of highly regarded tube amplifiers, and with the modern resistors and caps and decent tubes (RCA 7199 & JJ E34L), the stock amplifier is so captivating, I've never felt the needs to implement the upgrades I've already paid for.

For what it's worth, I'd recommend rebuilding the driver board with modern parts and see how you feel at that point.

Just to add a little more detail to my request, I am not an experienced DIY'er. I have made interconnects and refoamed speakers but that is about it. I am pretty comfortable with my ability to solder and follow instructions and take my time on projects, but I have never built or rehabbed an amp or preamp. So along with the quality of sound, I need something relatively simple to install.....I would prefer a completed board as opposed to a blank board with all the parts and instructions. I think I need to replace the main driver board and the quad cap. Thanks again for your input.

Contact Oritek audio, Stuartbmw3. I just got back my modded ST 70 last week. Ori replaced the whole board, disabled the ultralinear mode, and made it a Triode(single-ended)with 18 watts. He uses no feedback. The live feel of the amp is fantastic! His price is the best also(You should hear his pre/dac, which beats many $3000 preamps.). It's all discrete components. In fact, I tried to put a review(sort of)of his Stereo 70 over the weekend on Audiogon. You can't go wrong with his amp. Please give him some time to work his magic, as he usually doesn't offer this work. As I remember the Stereo 70, this doesn't sound like it. It is very detailed(more than my Nuforce's), dimensional, fast-it just sounds more like music than what I am used to.

I had owned quite a few modded Dyna. One which I was fond with is the (defunct) GSI mod. It had 6DJ8/6922 in the driver board. I had modded an ST70 with the Triode driver and power supply cap boards. It sounds quite nice, balanced and detailed. I initially used Kimber coupling caps but have switched to Audiocap Theta bypassed with the Russian FT1 teflon, which is a few steps up. However, when compared with the Fisher SA-16 and Pilot SA-232 (both EL84 amps) modded with the same Theta/FT1, the EL84 amps easily trump the ST70 in my system, even with reissue Genalex KT77 or JJ KT77 in the Dyna. My ST70 does not have stock Dyna OPTs though, it has Magnequest/Kimber OPT. That said I can recommend the triode boards as they do have excellent documentations.

Quoc -- you gotta love those vintage EL84 amps! I have the Pilot SA-232 also and it is really sweet. The Eico HF-81 has had some good press lately. Mine has been in the family since my uncle built it in 1959, still sounds great!

But for the original poster, stay away from the Welborne kit if you are not skilled at reading a schematic. I hear the new ones have better instructions, but I'd be wary of it. I'm anxious to try the K&K mod sometime. I love Kevin's phono pre.

Another ST-70 board that has been well-reviewed, and is a simple drop-in installation is the one offered by Maple Tree Audio in Canada. It uses octal tubes, which are right in line with the lovely classic sound of the Dynaco. On the other hand, I would strongly recommend a power supply upgrade at the same time you do an input board. Check out the Dynaco forum on AA where you will see lots of help and advice.