Hot Rodded Dynaco ST-70

[HF9]

Not a Bottlehead kit but I'm sure a few of the members here will appreciate some pics of a freshly completed Dynaco ST-70

The kit is from Bob Latino of Tubes4HiFi. I snuck in a few little upgrades too, including:

Teflon Tube Sockets and PCBs
Kimber TCSS wiring
Cardas RCAs / Binding Posts / Input wire
Kiwame / Takman Resistors
Mundorf Coupling Caps
Nichicon 100uF Electrolytics
Pilot light
Teflon solder strip for pentode / triode switch wiring
Heavy Duty On/Off Switch
Custom brass input and name labels
IEC Plug

The tube compliment is Sylvania JAN input tubes, Winged C output tubes and Sovtek rectifier.


One thing I realized during this build is that 16 gauge stainless steel is difficult to work with Other that that, it was a fairly easy build and a nice recommendable kit. The sound is lush and very engaging.

 

[Grainger49]

Nice work.  My chrome looks pretty bad.  Maybe I should have painted it?

Does the driver board include the bias caps for the tubes (I know it biases all four outputs and has the pots) or are the bias caps elsewhere?

Final question, what are the RCA jacks next to the IEC connector?  I know the inputs are up front.

 

[Wanderer]

I have been looking at ST-70 mods. I am trying to decide between the Curcio SoB, Welborne and the one you have.

Looks like you did a very nice job. How do you like the sound? What are you using for speakers? 

 

[Hank Murrow]

If you are a lover of Iron, you might be interested in the ST 70 mod kit from K&K Audio.

http://www.kandkaudio.com/poweramplifier.html

It uses iron or nickel input transformers(your choice) for phase splitting and I have heard it........ sounding wonderful.

Cheers, Hank

 

[dbishopbliss]

I have been looking at ST-70 mods. I am trying to decide between the Curcio SoB, Welborne and the one you have.

Looks like you did a very nice job. How do you like the sound? What are you using for speakers?   

I was facing the same decision... I got the board from Triode Electronics.  It was recommended to me by Eddie Vaughn.

 

[HF9]

Thanks gents. Right now I have them hooked up to a set of full range drivers from Tekton audio. The sound is smooth and euphoric, very very enjoyable.

I'm not really a chrome fan, so I sanded down the new chassis and turned it over to the powdercoater. I routed the inputs to the RCAs in the back (using the light blue wire in the fouth pic), the ones up front are just for looks, as is the stereo mono switch that's not plugged into anything.

 

[Grainger49]

I would guess you have no hum.  I wouldn't like my inputs so close to that jumbo power transformer.  I have seen a couple of ST-70s like this.  Do both RCA jack sets come with the kit?

 

[HF9]

Nope, no hum. It is a little disconcerting to look at but the transformer is shielded on the bottom and the wire is shielded and low capacitance. Only the front RCAs come with the kit. Holes had to be drilled/cut for the new RCAs, larger switch, pilot light, solder strip and IEC. Since the chassis is 16 gauge stainless, it takes a bit of effort. The binding posts required a little bit of modification but fit in the existing slots. I used black lacquer coated phenolic boards and drilled them out to hold the posts in place.

 

[Grainger49]

Well, I had to ask about hum.  The chance of it would have kept me from using rear inputs; I'm chicken.  I'm glad it doesn't creep in.  It is more convenient to have the inputs on the opposite side as the speaker binding posts.

 

[Wanderer]

Is the ground of the IEC plug connected? If so what do you connect it to? Chassis? 

Looking at the pics of your amp it is hard to tell. 

 

[HF9]

No, it's not connected to anything. I've read a few different posts where people don't recommend connecting the safety ground to the chassis on Dynacos as it can cause hum and noise.

 

[Doc B]

If the safety ground being connected to the chassis causes noise it is most likely that signal ground is running through the chassis. It has been about 17 years since I had a Stereo 70 so I can't recall specifics of the layout. But if there are signal or power supply ground wires connected to the chassis in more than one location it would probably worthwhile to get it rewired to a single point. Then wire the IEC safety ground to another point on the chassis, nearby. It could end up more quiet this way rather than more noisy.