Project: eBay ST-70 with Rats nest inside.....

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Project: eBay ST-70 with Rats nest inside.....

Postby Kyle Korum » Sun Oct 23, 2011 1:15 pm

*UPDATE* It was discovered that this amp has a 1st gen Welborne Labs kit hastily installed inside.

Hello all, long time lurker -1st time poster here.

Anywho, I just purchased an ST-70 on eBay, and as you will see in the following pictures; it's had an interesting past. I'm currently investigating it's config, trying to do some reverse engineering to try to decide what to keep and or change to make it playable for me. The things I know right now:

1. Apparently, this unit was owned by either Paul McGowan of PS Audio *or* Arnie Nudell of Genesis Technologies? -according to seller.

2. Driver and Bias boards must be prototype, or early run of some type. No markings.

3. Driver circuit is a 6U8A - 5751 combination. 6U8A's are PRO/COMM, 5751's are RAM Tube Works.

4. Finals are Tesle E34L's. Rectifier is a Sylvania 5AS4 .

5. Input looks to have been modified to a low impedance drive.

If one of these Veteran Gurus here can help identify where these boards originated from -that would be of great help!

Now, for the pics:

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I'll post more pictures as I dig deeper into this mystery piece. For now, thanks for looking ;)
Last edited by Kyle Korum on Sun Jun 07, 2015 2:54 pm, edited 4 times in total.
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Postby TomMcNally » Sun Oct 23, 2011 1:24 pm

Wow ... what a mess. I think I'd rip it all apart and start over.
The cap board should be ok. Hard to say what the driver is
(or used to be) a nice diytube ST-70 board would work well.

That's a late model (70's) ST-70, hopefully the transformers
are good. Hopefully you got a good deal.

... tom
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Postby DeathRex » Sun Oct 23, 2011 5:51 pm

Oh my goodness that looks scarry. Whoever worked on it, obviously had his/her hands full. Yea like Tom said.

I guess those power supply capacitors are in series to ground, so for 2 they would be rated at 900VDC at 50uf. Cause a 450V capacitor would be a little close to what the ST-70 runs at. That 5AR4 is a 5AS4/5U4; the voltage will rise alot faster with it than a 5AR4. Also it's 5V at 3A, might be much better to have a Chinese or Sovtek 5AR4 in.

Those WIMA capacitors should be good, but they just don't look right inside a ST-70. Rather have the ST-70 almost stock with just a little bit more power supply capacitance, like a pair of JJ 50/50uf 500V capacitors. Replace the original 30, 20, 20, 20 with 50, 50, 50, 50, all you need. CE manufacturing makes a 40, 40, 20, 20 in one can that might be able to be installed back on top for $35 at tubesandmore.com. That and Shannon's board + parts and you'll be set.

Best to replace that power cord too. AES has a 2 prong polarized and unpolarized cheater cords you can get.
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Postby EWBrown » Sun Oct 23, 2011 8:13 pm

Best to replace that power cord too. AES has a 2 prong polarized and unpolarized cheater cords you can get


Or just get a a 2 wire extension cord, and lop off the "female" end, and you have a nice six foot long, 2 wire power cord for pretty cheap, and it is readily available. I've "guillotined" more extension cords than I care to freely admit (666)

Best to use no smaller than 18 AWG, for most amp projects.

/ed B
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Postby Geek » Sun Oct 23, 2011 10:32 pm

Lots of those 6' "guillotineable" extention cords for $1 in the dollar store. Some are even 16 GA.

Cheers!
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Postby Kyle Korum » Mon Oct 24, 2011 7:55 pm

I made a ton of progress today. It turns out the Boards are of Welborne Labs origin. I emailed their tech dept. about my newly procured ST-70, with the Welborne Labs Kit + hack install + wacky-weird input config, along with a few photos of the current "situation". I received an email from Steve@welborne shortly thereafter:


Hi Kyle

Thanks for your email.

That is our original version of the ST70 mod, however it looks to be quite the nightmare of an assembly job! Our assembly manuals always state to keep your wiring short and neat but it appears the assembler didn't read that paragraph LOL. We refer to this as a "Spaghetti Job".

Spaghetti jobs are prone to noise pickup and of course, if something is amiss, it's difficult to track down the error due to the wires crisscrossing in every which direction. There may be some mods done to our mod but once again, it's had to discern what is there due to the poor wiring.

Yes there is the main board and then the smaller pcb is the bias network.

If it was my piece of equipment I would yank everything out of there and start over. If you are not a DIY'er, and purchased the amp hoping it would make music right out of the package then your best bet would be to find a local skilled technician that can make some sense out of what's there. Or alternatively you can send it to us.

Definitely recommend a redo on the wiring and that rusty power supply choke needs to be replaced. Or, if yanking everything out to start over, you might consider the latest version of our mod. www.welbornelabs.com/st70.htm

Regards,

Steve


I emailed him back, stating that I'd planed on redoing this spaghetti job, but that I was a bit apprehensive about attacking the job without parts diagrams and a schematic. He emailed back with an attached copy of the assembly manual. -Yes! (d)

I was very impressed with their customer service on the "tech support" aspect, and their newer kit design looks like a winner as well. If I end up feeling happy with the performance of this version when completed; I might consider a newer version kit for my ultra minty '61 :))

Check it out here: DYNACO ST70 MODIFICATION KIT

I'll keep you guys updated with pictures and progress as time allows, but for now -I have some work on my hands here... (lol)
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Postby EWBrown » Mon Oct 24, 2011 9:20 pm

Lots of those 6' "guillotineable" extention cords for $1 in the dollar store. Some are even 16 GA.



Just make sure that the wire in these "cheapies" is actually copper - I have seen some of these with unsolderable wire in them. yeah, there's a reason why they are only $1... =:o Dunno, it's probably steel or aluninum stranded wire in some of them (sick)

This isn't the first time I've encountered "unsolderable" wire, I've also seen it used on some of the cheaper computer PSU DC witing bundles - yeah, those DC wiring bundles have been a major source of my project building wire.

At least 90% of these have copper wire, some of the lesser "no-name" brands of PSUs will have the "useless" wire, and some of these also do not have the input AC line filtering components, the board has the pads for the inductors and caps, but they are just jumpered out, and these PSUs are hellish RFI noise generators.

/ed B
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Postby burnedfingers » Tue Oct 25, 2011 5:02 am

I'd put it on ebay as fast as I could unsolder it. I would then purchase Geeks board and install it.
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Postby Geek » Tue Oct 25, 2011 5:25 am

EWBrown wrote:
Lots of those 6' "guillotineable" extention cords for $1 in the dollar store. Some are even 16 GA.



Just make sure that the wire in these "cheapies" is actually copper - I have seen some of these with unsolderable wire in them. yeah, there's a reason why they are only $1... =:o Dunno, it's probably steel or aluninum stranded wire in some of them (sick)


Yes! And here's a 90% foolproof way to check in store...

- When bent, does it spring back? Steel. Compare a clothesline for learning the feel.
- When bent, does it stay? Aluminum.
(most dollar store importers REALLY check against this, they could wind up in the 'klink' in .ca for that)
- Does it feel like every other cord, especially those with the 'Leviton' label? Copper!

Cheers!
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Postby DeathRex » Tue Oct 25, 2011 8:30 am

Knowing now it's a Welborne, I'd probably stick with it if you can. Hate to spend another $200 on a mod that is maybe 0.00001% better. I've been following Welborne on DIYAudio for a couple of years, and a bunch lately because he's designed several GU-50 amps. He knows his stuff, but like any good engineer his amps and mods are overly complicated, at least too complicated for me.

I likes my amps like I likes my women, cheap and easy. $)
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Progress!

Postby Kyle Korum » Sun Dec 18, 2011 6:15 pm

Hello all,

No, this project didn't die, it has been going at a snails pace though.... :/ Anywho, progress has been made. EVERYTHING was stripped down to bare chassis.
Chassis is in "fair" shape, with it's minor pitting in the finish if you look very closely. I just cleaned it up with Mothers brand metal polish, followed by a wd-40 wipe down to get rid of the compound, then, a coat of fine auto wax to help protect the remaining finish. It looks presentable again, IMO.

Next, I took apart the "Iron", sandblasted the end bells, and sanded the lam stacks by hand. A multi-step paint process was employed, using Engine Enamel because lets face it; these freakers get more than slightly warm. I am super duper impressed with the results, take a look!

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Progress, part deux...

Postby Kyle Korum » Sun Dec 18, 2011 6:33 pm

Starting to add components, and associated wiring.

A goodies box, from our friend at Dynakitparts:

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Sockets are of the ceramic type, with some pretty impressively robust gold plated connectors:

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Trafos placed ontop the chassis just for sake of photos:

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I had to wrap the octal sockets with 33+ tape, to bush them. More trimming of tape was done:

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Postby DeathRex » Sun Dec 18, 2011 8:21 pm

If would be nice if the big black cover could go underneath. Kinda homely covering up all that nice chrome.

But then I like the aluminum cap can too.
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Postby Geek » Sun Dec 18, 2011 8:40 pm

Rockin' great job so far!!!! :))

Cheers!
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Postby snitch56 » Mon Dec 19, 2011 10:38 pm

EWBrown wrote:
Best to replace that power cord too. AES has a 2 prong polarized and unpolarized cheater cords you can get


Or just get a a 2 wire extension cord, and lop off the "female" end, and you have a nice six foot long, 2 wire power cord for pretty cheap, and it is readily available. I've "guillotined" more extension cords than I care to freely admit (666)

Best to use no smaller than 18 AWG, for most amp projects.

/ed B


Another good source is old vacuum cleaners (I typically see at lease one out on the curb during garbage eve in my neighborhood every other week). The power cords are typically 2 prong polarized 16 or 17 AWG copper.
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