ST-70 Rebuild

[BlueMDPicker]

I wanted to give the Dynaco a facelift, without turning it into a completely different amp (ie modded driver board, inputs/output connectors, or completely different power supply circuit.) So, I've been accumulating bits and pieces for awhile and started on the project this week.

Cap voltage points were tested, all resistors were removed and values checked, and then everything was reassembled on a new chassis that is authentic in every respect. All NOS hardware, input jacks, and speaker terminal strips went into the chassis. The PC-3 is a Vintage Electron, identical to the original, with better board composition and much better tracings and solder pads. I re-used all the original point to point wiring (again, wanting to maintain the authentic sound versus modern dialectric coating and silver plated "audiophile" conductor.) The only non-authentic components are the orange drop coupling caps, and a diode by-passing the selenium rectifier. I hope to have it pumping music by tomorrow, but took a break to show a before and after. It's been a wonderfully fun project. Like a vintage car, there's no better way to understand it inside out than a rebuild.

Edit: sorry for the large pic. Hopefully this one is better.

[RuSsMaN]

Looks awesome!

Ready for another one? Even cleaner than the first? ;)

Cheers,
Russ

[BlueMDPicker]

Thanks Russ.

No, not quite ready for another one. I have three build/rebuild projects on the bench. I'm seriously looking at scratch building a Mullard 520 based integrated on the old ST-70 chassis. What I've learned designing the front end of my little high tone guitar amp is pulling me in that direction.

I have a couple of very nice Counterpoint SA-100 tube/mosfet amps that I really need to find a good home for as well. That's the last of my (semi) SS gear. I like the tube gear a lot, and I can troubleshoot, fix and tweak it with a high level of comprehension.

[organ]

Looks awesome! That ST-70 looks brand new. Did you follow the original intructions when rebuilding the amp or did you have a schematic? I'm using the same tube sockets as you. They're very cool but a pain in the ass to re-solder all the connections. Good to see you're staying with the original design. I was thinking about using one of the upgrade boards but I think I'm going to order a replacement. Where did you order yours from? We better stock up on NOS 7199. I'm going to re-do the power supply caps on mine though. Going with the Curcio external caps which increases capacitance. Many say the quad cap was the major weak point in the ST-70 and going bigger gives more dynamics and bass.

Are you going to triode strap this guy? I'm VERY interested to hear how they sound with your Abby's or even RF-3. That amps sounds so sweet in triode with crazy ass imaging.

Let us know how it sounds now that it's completely rebuilt.

Maurice

[BlueMDPicker]

Hi Maurice,

I have an original ST-70 assembly manual that I picked-up from eBay. I followed it religiously. It was amazing to me that all the original resistors on the PC-3 checked out within spec. And, the bias resistors at V2 and V7 are perfect and within 1% of each other (original values for these are hard to find and, without the original values, bias setting for the amp has to be changed.)

The PC-3 replacement ($25) and NOS connectors and hardware packages ($36) came from Vintage Electron in Chicago. They bought out all the existing Hafler/Dynaco parts inventory several years ago. Ebay handle is vintagelectron.

The new chassis ($100) came from a fellow named Scott (eBay handle scott0527). I agonized over this aspect for quite awhile, thinking I could somehow restore the original (I rebuild old cars and the cardinal rule is rebuild.) But the new chassis is a carbon copy of the original and I'm glad I decided to go that route.

The new tube sockets came from Angela Instruments. From a purist perspective, I suppose I should have gone with brown, bakelite sockets. But, I wasn't satisfied with the loose pin contact that the old style sockets provide.

I agree that the ST-70 can surely be improved upon with increased filter capacitance. But, then, I wouldn't have had as close a match to the ST-70 original sound -- which was the goal. I'm not doing triode strapping on this amp, consistent with the same philosophy. If I build the Mullard 520 circuit on the old chassis, I will incorporate switching for that in the design (and even -- dare I say it -- a remote :D )

I'll let you know how the "new" amp sounds. My 16 year old daughter just got her learner's driving permit and has decided I should get my head out of the amp this weekend and go driving. So, I'm side-tracked for awhile.

Mike

[Spawndn72]

Man, that is just too cool. I would really like to do something along those lines one of these days. Looks great Blue.

[BlueMDPicker]

Thanks Spawn -- it's a fun rebuild project.

Okay, it's passed the smoke test and (for those of you who know the ST-70) the VOM says it all:

[faster100]

looks great Mike!! now ya need a tester for all those el34's :D

[BlueMDPicker]

Thanks Cliff. I'm thinking about the tube tester, but haven't had a chance to read through the link you posted.

The "new" amp sounds great and much improved.

There were two broken/repaired traces on the old PC-3 driver board (it was actually installed on top of the chassis with scotch tape under the edge to insulate tracings), a bunch of bird turd cold solder joints throughout (I think this was someone's first kit project), the tube sockets were loosy-goosey, and the input RCAs were pretty grungy. So correcting all that that accounts for a lot.

If you get a chance to pick-up a Dynaco ST-70, with good iron, I'd encourage you to. They're a real milestone in audio history. With a little TLC they make for world class amplification.

Final shot in the audition chair pumping out some John Mayall -- sweet!

[faster100]

alright Mike take your time and pm me when ever ya want if you have ???'s

I had a st 70 and sold it to buy my sda 1b's... to say the least i kick my ass for selling it, It was IMO mint condition with manual and original schematic fold out... I'm starting to get the tube amp burn again, but florida rooms with no A/C are to hot for 4 el 34's to run in :)

[ND13]

Beautiful work.

BTW, how would one get a hold of one of those kits? Are they still available new?
Would love to build one, especially if I can't work out a deal with this fellow I know who just acquired a Con John tube pre and tube amp he picked up at an estate auction last weekend.

[BlueMDPicker]

Thanks, much appreciated.

The Dynaco Dynakit ST-70 is history. However, there are a few kit amplifiers that follow the design faithfully, with this being one of the best I've seen.

No affilliation

http://cgi.ebay.com/ws/eBayISAPI.dll...777335035&rd=1

[RuSsMaN]

Or you can buy an original, with stamped tannies / cloth leads, and great chrome/cage - loaded with Mullards, from a fellow forum member for less.

;)

Cheers,
Russ

[ND13]

Originally posted by BlueMDPicker
Thanks, much appreciated.

The Dynaco Dynakit ST-70 is history. However, there are a few kit amplifiers that follow the design faithfully, with this being one of the best I've seen.

No affilliation

http://cgi.ebay.com/ws/eBayISAPI.dll...777335035&rd=1

Thanks for the info.

[BlueMDPicker]

PFM!

[BlueMDPicker]

Despite my goal of keeping this amp as close to stock as possible, I began experiencing biasing issues about 90 days after my rebuild. A quick roll of the EL34s into other gear eliminated them as suspects, so I worked my way through the bias circuit hoping it was just failing resistors or caps. It wasn't. It was the aluminum can, stacked cap.

After toying with the idea awhile of just replacing it (hard to find, but I found one), I opted to leave it in place and bypass it, while also replacing the entire grid bias circuit, with an SDS Labs cap board. Sheldon Stokes did an excellent job with the circuit design and layout. And, while it's touted as a "drop-in replacement" for the biasing circuit and power supply cap, it's actually quite a little project.

I bought the amp used and, like most people (I would have done the same), the original builder trimmed excess lead length from the power transformer, output trannies, and choke. So, I had to do quite a bit of patching extension leads and heat shrink insulating. The board mounts inside the chassis on threaded couplings attached to the right channel output trannie mounting bolts. It's a snug fit and I also had to re-work some of the wiring and resistor mounting on the rectifier tube socket. Results? Well worth the effort.

The board increases capacity from the original 30-20-20-20 to 60-160-60-60 uF at 500V, and also replaces the bias supply electrolytic capacitors (with high quality metal foil), resistors, and the diode (which had already been inserted as a replacement for the selenium rectifier.) Either 5AR4/GZ34 tube rectifier or solid-state diodes may be used. Of course, I stuck with tube rectification.

The added capacitance really takes this amp to another level of clarity and dynamics that I honestly wouldn't have imagined. As soon as I let it all burn-in for awhile, I'll post a pic of the board installed. Meanwhile, here it is (running a quad of original Tesla, blue glass EL34 snagged from madmax) and still looking damn near stock except for the Spragues on the driver board (a necessary evil ;) .)

[HBombToo]

Simply Awesome!

[RuSsMaN]

Looks great, bet it sounds even better. A lot of guys 'back in the day' did Orange Drops - I think it looks VERY authentic.

Cheers,
Russ

[Shizelbs]

Wow, that looks wonderful. Great work man.

[BlueMDPicker]

Thanks guys, much appreciated.

When I was a kid, this was the kit I always wanted to build but never got around to. I have so much time and money in it that I think I'll apply for a SS number for it, and claim it as a dependent. :D

If/when we get together at BDT's or Russ', I'll drag it along for those who may not have had any exposure. It mates up fantastically with the RTA8Ts I got from Mark.

Oh yeah, here's the installed cap board pics:

[madmax]

VERY nice!
madmax

[BlueMDPicker]

Thanks Chuck. These Teslas are a very good match with this circuit and the rest of the tube compliment. After a few rolls, I found that I prefer the JJ/Tesla E34Ls in the Manley 75+75 with the Mullard front-end.

I also rolled several high quality 5AR4/GZ34 through it. Anyone who thinks rectification doesn't impact sonic signature can drop by for a demo. The NOS Sylvania fat bottle (pictured in the amp) has a dual cathode and makes this thing purrrrr.

The driver board is now sporting 2 NOS Hammond 7199 from the organ/leslie parts vault. I found 4 stored in a box, with 8 metal-base 6550 Sylvania, a couple of weeks ago. I remembered stashing the power pentodes back when I had two Leslie setups, but not the 7199s! :) Damn, they're pretty (the 6550s)!

[BobMcG]

Excellent!!!

[amulford]

Outstanding work, Mike.

I can't wait to see it in person.

[organ]

That's awesome! Congrats on a killer restoration job!

Can't believe the ST-70 can pump out more dynamics with the cap upgrade board. My system can already pull off concert level dynamics, if I can squeeze more by the cap upgrade, I'm going to crap my pants. That will be my next upgrade in my Dyna. I'm thinking about the external one from Curcio.

How does your Sylvania compare to a Mullard ribbed plate? I love the bloom and warmth from my Mullard.

Maurice

[jtgranby]

Mike
Where did you buy the adapter going into the speaker wires? The adapter with your PS Audio cables then to the back of the Amp.
Thanks
JT

[BlueMDPicker]

Hi Jim,

A friend of mine in the Boston area made a batch and sent me a couple of different configurations. I have another set with all four binding posts on one box and four leads to small spades (perfect for the old terminal strips like the Dynacos, Macs, Marantz, etc.) You're welcome to it if it will work for your application. The leads are a fuzz short for the ST-70 layout. But, if that's what you need it for they could be lengthened easily.

[Kenneth Swauger]

Hello Mike,
Very nice job! It's great to see the classics still being used. Is the new chassis nickel plated?
Regards, ken

[BlueMDPicker]

Hi Ken,

Thanks! I don't believe this chassis is nickled. As I recall, and it's been awhile, it didn't have the heft a plating job usually imparts. I believe its just a high luster chrome.

This amp is now the heart of my main system for digital sources. It is totally engaging and possesses pure luck of the draw synergy.

Mike

[Kenneth Swauger]

Thanks, Mike. I still have a factory built ST-70 that I use to test crossovers that Polk owners send. I bring it up slowly on a Variac since it's completely stock. I replaced the tubes ten or so years ago and treat it with TLC. Beautiful amp.
Take care, Ken