Dynaco ST-70 – Tubes4HiFi Build


Project Resources
High Voltage Safety Tubes4HiFi Gallery Original Dynaco ST-70 Manual KT88 Datasheet 12AT7 Datasheet GZ34 / 5AR4 Datasheet
Project Notes

While loafing at work a long while back, one of my friends came in and had just purchased a HIWATT guitar amplifier. It got me thinking that I should get a something going with a tube audio amplifier, at least for my office. I looked around for some of the low cost guitar practice amps and figured I could modify 2 of them for a nice Stereo, somehow I came across the Dynaco ST-70 kits from a few vendors (see the links on the right sidebar). They were a bit pricey but they looked very cool, lots of glass showing and polished chassis… how could you not want one. So I started looking on eBay and at the kit places and finally decided to get the Tubes4HiFi ST-70 with all the mods. The mods included upgraded power transformer, extra choke, extra capacitors on the power supply side of things, Triode / Pentode switch and of course their new style VT-70 driver board. I opted to supply the tubes myself to defer some of the upfront cost until I got the ST-70 built.

As luck would have it BING (the search engine) was doing some crazy shopping promotions were I could get 25% off if I bought something from one of their shopping sites. As it worked out eBay was one of them so I purchased the kit from an eBay buy it now, and later (much later) received my 25% cash back for the purchase. I also picked up a used cover for the project as I really like the perforated steel Dynaco chassis covers. The Tubes4HiFi ST-70 is a beautiful device, thick stainless chassis and lots of glass all in a very small form factor.

Once I opened the box I was very surprised to see how well the kit was put together. Every individual component was placed in its own small zip bag with the components value written on it. Everything things was included to make this a successful build, including a coil of wire. All that was needed was a soldering iron and a few tools. Very impressive as it must have taken a long while to put the box of parts together. The transformers were well packed and very heavy. The stainless steel chassis was spectacular, thick, polished and nicely silk screened.

I started the project during my Christmas break and got about 2 days of actual work done on it. The instruction style was very similar to the original Dynaco ST-70’s and well thought out. The sequence was very clear and was pretty easy to follow. Then it happened, I got busy…

The amp sat for about 5 months or so until I got sick of tripping over the box of parts. So I got back on it and finished it off. Total build time was a couple of days when all said and done. I think the only aspect of the build that I didn’t like was the way the PCB mounts to the chassis, it seems that it had to be spaced off the bottom of the case to move the board away from the metal to keep the circuit board traces clear of any shorts. Not a big deal at all, and likely a good thing to allow some venting of the chassis.

Tubes were ordered, I opted for a quad of JJ KT-88’s, and JJ ECC81’s (Need to verify this) for the driver board, and for the rectifier which is normally a GZ-34, instead I opted for the Weber Copper Cap WZ34 due to the increased power from this amp. Things were looking good, just cleaning up a few things and waiting for the tubes!

Tubes arrived and I cleared the bench and started hooking things up. Set bias and balance as indicated in the manual to get things going and flipped the switch. Tubes stated glowing…and glowing and glowing until one of my brand new KT88’s red plated! I flipped the switch as soon as I caught it but it had done some damage to the tube. The bigger question was why???

After letting things cool for a bit and taking the usual safety precautions I started looking at the PCB wiring and found that I had missed connecting up one of the wires from the driver board to the tube. That wire was the driven grid which was just hanging unconnected on the tube and I guess it oscillated itself to death without bias or anything else connected to it. Oh well. I was pretty confidant that the problem was fixed. A new pair of tubes were ordered, wait, wait, wait.

New tubes in and bias and setting refreshed, it all sat quietly and glowing without any red plates. I hooked it up to a set of small bookshelf speakers (Polk Audio Monitor 40 / TSi200) and connected up the iPod directly to the input. I found some music I liked and cranked it up. It sounded amazingly good for small speakers in a very large room. The ST-70 sounded crisp and clean, and was rocking good looking to boot!

I used the amp for a while and reset the bias and patted myself on the back… And so the sickness of tube amplifiers has begun! Following this project I picked up a couple of Dynaco PAS-3’s one stock and one for rebuild and found it a nice match for the ST-70. Did it end their? Nope, Dynaco tube FM tuner, a couple of Dynaco MKIII, and all the home brew projects you can find here on this site.

Hell, keeps me off the streets!

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