Hello Everyone! This is my first post here and I'm hoping to share my journey of building an "ST-70". I put "ST-70" in quotes, as nothing on this build is original from an ST-70. About 8 years ago, I wanted to get into tube amps and had pretty much zero knowledge on the subject. After a few months lurking around in forums, it seemed like the Dynaco ST-70 was a solid option for a first-time owner/DIYer. The next step was trying to figure out where to get parts, which found me on eBay for quite a while. At random, I stumbled upon one of Mr. Latino's auctions for a new blemished SS 16 gauge chassis and VTA board - the project officially began. I purchased all components new, from online retailers and other eBay sellers.
The first step I had was assembling the VTA board, which was straight forward. All components were purchased from Digikey or eBay and were soldered in place with no issues.
After this, I hit a long stall when I returned to school. Three degrees later (expecting 4th shortly...yeah..I know), I emerged ready to continue on the project by searching for transformers and small mechanical components like jacks and switches. It seemed like I could spend a bunch of cash on new Dynaco transformers, roll the dice on used ones, or take an alternative path. Alternative paths are always more fun...so...for a power transformer, I went with a Tomiko PT-ST-70 Mk III toroidal. Now, I originally sourced this thing back in 2010..ish as a Mk II version. However, since the project was shelved, all I had was a dead bookmark on eBay and the seller's name. I reached out to the seller who was able to sell me the newer version off-site, which was really cool. Another eBay purchase from overseas was able to supply a nice cover for it which required two additional holes to be drilled into the chassis. The output transformers are a pair of Edcor CXPP60-4.2K with 4/8/16 ohm taps. For tubes, I went with matching JJ EL34II, 12AT7 and a GZ34. I then opted to get everything gold-plated that could be: tube sockets, jacks, binding posts. The local hardware store was well stocked with brass hardware, so all components are attached using brass pan heads which included removing the original hardware on the Edcors and swapping it with equivalent brass.
Assembly took place at work after hours. We have a nice electrical room with all the wire and tools you need. I also have access to a fully equipped machine shop, which made things much easier. To start, I needed to find a way to mount the toroidal. I found a piece of scrap aluminum and machined it to shape, leaving room for the wires to pass through.
The wires were separated and passed through a flexible PVC conduit which also prevented rubbing on the aluminum panel or steel chassis. The toroidal was then mounted to this plate using the supplied hardware. Two holes needed to be drilled into the chassis to allow mounting of the toroidal cover (the back is kinda free floating).
Backing plates for the jacks were laser cut from black acrylic. The binding post plates were etched at a lower power on the outside to indicate 8/4 ohm hookups. The etching was filled with white paint to give a clean label.
Assembly instructions basically followed the VTA guide to a tee, while taking care to make sure the correct wire colors on the new transformers were correct. I didn't have any of the original pieces, so instead of the whatever-lug terminal strip, I installed a ground bar where all ground connections could be screwed into place.
I won’t lie. Initial start-up was absolutely terrifying. I realized after finishing that I still lacked thorough knowledge on the EE side of things, but had some faith that I followed the instructions and translated wire colors correctly. I expected to flip the switch and see a large arc and my project go up in smoke. Accordingly, I plugged the unit into a power strip, then grabbed a 3 foot wooden dowel and clicked the power strip on....nothing...no sounds, no magic smoke. I then used the dowel to flick the switch on the back of the amp...nothing...silence, no magic smoke. Then, the 3 tubes on the driver board began to glow faintly. Eight years folks! Eight years I waited for this moment! With a grin going ear-to-ear, I called an Uber and shuffled this thing home.
Following Bob's instructions, I installed the EL34 tubes, set the bias, and sat back with a giant...GIANT grin. It works. It actually worked!
The next thing was plugging this thing into a source and speakers. The speakers in this case are a pair of Paradigm Monitor 7 V1. I lack a solid pre-amp and my previous NAD 3155 has quite a bit of hum. Maybe a PC? That was horrific, the ST70 was humming in sync with the PC fan. The cleanest signal I could get without a hum was an old Samsung Galaxy S3. *sigh*. I plugged that into the front jacks, and it was dead silent until I loaded Spotify up.
I'm not going to sit here and report nonsense feelings of openness or tone over my setup. So instead, I'll simply say that it sounds amazing. There is zero hum, zero noise, no distortion. I'm absolutely blown away. If it wasn't for the faint glow, you'd have no idea this thing was running. My first step into the world of tube amps could not have gone any better. Bob, thank you for selling me the chassis in 2010 and answering my random questions over the years. If it wasn't for your support this thing would still be a pipe dream.
Edit: I got the NAD to simmer down! Now using the 3155 as a preamp with tone control and phono hookup
The first step I had was assembling the VTA board, which was straight forward. All components were purchased from Digikey or eBay and were soldered in place with no issues.
After this, I hit a long stall when I returned to school. Three degrees later (expecting 4th shortly...yeah..I know), I emerged ready to continue on the project by searching for transformers and small mechanical components like jacks and switches. It seemed like I could spend a bunch of cash on new Dynaco transformers, roll the dice on used ones, or take an alternative path. Alternative paths are always more fun...so...for a power transformer, I went with a Tomiko PT-ST-70 Mk III toroidal. Now, I originally sourced this thing back in 2010..ish as a Mk II version. However, since the project was shelved, all I had was a dead bookmark on eBay and the seller's name. I reached out to the seller who was able to sell me the newer version off-site, which was really cool. Another eBay purchase from overseas was able to supply a nice cover for it which required two additional holes to be drilled into the chassis. The output transformers are a pair of Edcor CXPP60-4.2K with 4/8/16 ohm taps. For tubes, I went with matching JJ EL34II, 12AT7 and a GZ34. I then opted to get everything gold-plated that could be: tube sockets, jacks, binding posts. The local hardware store was well stocked with brass hardware, so all components are attached using brass pan heads which included removing the original hardware on the Edcors and swapping it with equivalent brass.
Assembly took place at work after hours. We have a nice electrical room with all the wire and tools you need. I also have access to a fully equipped machine shop, which made things much easier. To start, I needed to find a way to mount the toroidal. I found a piece of scrap aluminum and machined it to shape, leaving room for the wires to pass through.
The wires were separated and passed through a flexible PVC conduit which also prevented rubbing on the aluminum panel or steel chassis. The toroidal was then mounted to this plate using the supplied hardware. Two holes needed to be drilled into the chassis to allow mounting of the toroidal cover (the back is kinda free floating).
Backing plates for the jacks were laser cut from black acrylic. The binding post plates were etched at a lower power on the outside to indicate 8/4 ohm hookups. The etching was filled with white paint to give a clean label.
Assembly instructions basically followed the VTA guide to a tee, while taking care to make sure the correct wire colors on the new transformers were correct. I didn't have any of the original pieces, so instead of the whatever-lug terminal strip, I installed a ground bar where all ground connections could be screwed into place.
I won’t lie. Initial start-up was absolutely terrifying. I realized after finishing that I still lacked thorough knowledge on the EE side of things, but had some faith that I followed the instructions and translated wire colors correctly. I expected to flip the switch and see a large arc and my project go up in smoke. Accordingly, I plugged the unit into a power strip, then grabbed a 3 foot wooden dowel and clicked the power strip on....nothing...no sounds, no magic smoke. I then used the dowel to flick the switch on the back of the amp...nothing...silence, no magic smoke. Then, the 3 tubes on the driver board began to glow faintly. Eight years folks! Eight years I waited for this moment! With a grin going ear-to-ear, I called an Uber and shuffled this thing home.
Following Bob's instructions, I installed the EL34 tubes, set the bias, and sat back with a giant...GIANT grin. It works. It actually worked!
The next thing was plugging this thing into a source and speakers. The speakers in this case are a pair of Paradigm Monitor 7 V1. I lack a solid pre-amp and my previous NAD 3155 has quite a bit of hum. Maybe a PC? That was horrific, the ST70 was humming in sync with the PC fan. The cleanest signal I could get without a hum was an old Samsung Galaxy S3. *sigh*. I plugged that into the front jacks, and it was dead silent until I loaded Spotify up.
I'm not going to sit here and report nonsense feelings of openness or tone over my setup. So instead, I'll simply say that it sounds amazing. There is zero hum, zero noise, no distortion. I'm absolutely blown away. If it wasn't for the faint glow, you'd have no idea this thing was running. My first step into the world of tube amps could not have gone any better. Bob, thank you for selling me the chassis in 2010 and answering my random questions over the years. If it wasn't for your support this thing would still be a pipe dream.
Edit: I got the NAD to simmer down! Now using the 3155 as a preamp with tone control and phono hookup
Last edited by bennytehcat on Sat Sep 08, 2018 7:24 pm; edited 2 times in total
