Dynaco ST-70 Upgrade

dbishopbliss · 19959

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Offline dbishopbliss

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on: December 28, 2010, 09:03:28 AM
I just picked up a ST-70 (like I need another project), but the price was right so I figured why not.  It has the original driver circuit with 7199 tubes.  There are a number of upgrade boards out there that vary considerably in price.  Does anyone have any experience with upgrading a ST-70?

The deal also included a PAS-2 preamp and an EICO HFT-90.

David B Bliss
Bottlehead: Foreplay I, Foreplay III, Paramour I w/Iron Upgrade, S.E.X. w/Iron Upgrade
Speakers: FE127E Metronomes, Jim Griffin Jordan/Aurum Cantus Monitors, ART Arrays
Other: Lightspeed Attenuator, "My Ref" Rev C Amps, Lampucera DAC


Offline HF9

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Reply #1 on: December 28, 2010, 09:52:25 AM
Dynacos are great to restore.

I'm currently trying to get a PAS 2 up to snuff. I got one that had been completely defiled by a "modder", so pretty much everything is getting replaced. I've sanded down the chassis and it's at the powdercoaters getting taken care of.

Tubes4HiFi makes a nice Input / Output board to replace the nickel plated stock. http://www.tubes4hifi.com/PAS3.htm

Michael NiPomo on ebay makes a sleek front panel if yours is in bad shape: http://cgi.ebay.com/CUSTOM-BLACK-ANODIZED-FRONTPANEL-DYNACO-PAS-PREAMP-/150510252272?pt=Vintage_Electronics_R2&hash=item230b1c30f0

Classic Valve makes a very solid looking regulated PSU board : http://www.classicvalve.ca/dynaco.html

There are tons of replacement boards floating around on ebay on the cheap. Audioregenesis makes some really nice ones if you don't mind that they're already populated: http://www.audioregenesis.com/products/PAS/index.html (I personally think that takes the fun out of it, but that's just me ;) )

You'll also need a 6 position, 2 pole switch to replace the strange one that came stock.







My DIY Audio Electronics Blog: DIYAudioBlog.com


Offline ssssly

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Reply #2 on: December 28, 2010, 10:03:46 AM
Dynaco doctor also has some nice boards.



Offline JC

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Reply #3 on: December 28, 2010, 10:26:12 AM
I just paid the price for a couple of 7199s.

First item, for safety's sake, was to replace the Selenium rectifier in the negative bias supply circuit with a generic silicon one.  You don't want to run the risk of breathing the results of a Selenium diode going "poof", I am told.  I just mounted a little 3-terminal strip to the screw that holds the Selenium stack in place to mount the silicon diode.  Since that bias supply is important, I replaced the caps and resistors in it while I was at it.

A new multi-section cap for the main supply was to be had at Antique Electronics Supply.  They were actually manufacturing a drop-in replacement.

The plating on the input connectors is usually pretty grim, so I replaced those with a couple of cheap gold-plated ones from RS that fit into the same phenolic insulator that the old ones were in.

A three-wire power cord, again for safety's sake on an exposed steel chassis with lots of Voltage running around inside.

New film-and-foil coupling caps on the PC board.  Some metal-film resistors where they were appropriate.  New ceramic tubes sockets all around.  Some new grounding.

I know this isn't what you asked for, but I wanted to hear what it sounded like in more-or-less its original configuration with a few updated parts.

I was not disappointed.

Jim C.


Offline HF9

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Reply #4 on: December 28, 2010, 10:40:32 AM
Dynaco doctor also has some nice boards.
I forgot to mention Curcio Audio (Dynaco Doctor), it's a great resource for the original manuals and replacement bits: http://www.curcioaudio.com/dynadr_3.htm#DYNACO%20PAS%20PREAMP%20%28Models%202,%203,%20and%203X%29

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Offline howardnair

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Reply #5 on: December 28, 2010, 01:05:28 PM
tubes for hifi also has a very nice  psu board  and input/ output rca  board for the pas2-the input /output board only has 7 rca's instead of 9 much more room i liked that very much--i went with new stock boards for mine- instead of the upgraded boards--i kept the tone controls-i would get a  new selector switch -i used prp resistors and a mix of soniccraft and jupiters on the phono and line stage--used soniccraft on the panel-i also used relcaps on the phono board --you will need to do the riaa eq  phono board mod --just type that in your google and it will come up-if the faceplate of the pas2 is brass you can refinish that by sanding it down on the upper part with 600 and 1000 -and repaint the bottom  strip and lightly sand the lettering then some spray lacqeur-will look brand new-- for the st-70--there is tubes for hifi--triode electronics--dynakitparts.com and of course the dynacodoctor--then the choice of upgraded driver board or stock-



Offline Wardsweb

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Reply #6 on: December 29, 2010, 02:25:05 AM
You may get some useful information for you build from my custom ST-70 build page.

http://wardsweb.org/audio/ST70/



Offline Grainger49

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Reply #7 on: December 29, 2010, 03:58:48 AM
Curcio Audio, The Dynaco Doctor, sells a replacement for the multi-cap that fits where the bias supply sits.  It gets rid of the old bias supply and the Selenium rectifier.  I would go with that for sure.  Leave the old multi-cap there for stock looks.



Offline howardnair

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Reply #8 on: December 29, 2010, 07:51:03 PM
ward--that is simply excellent



Offline Listens2tubes

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Reply #9 on: January 03, 2011, 12:13:00 PM
ward you are an artist. Most St-70 mods have simplistic additions to te original chassis that often look "add-on". Your design is original and sexy.

Neal P.


Offline Paully

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Reply #10 on: January 05, 2011, 02:11:28 AM
Don't forget Triode Electronics.  They have tons of stuff too.  I used their Power Supply board this time around for my most recent Dynaco rebuild.

http://store.triodestore.com/



Offline dbishopbliss

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Reply #11 on: January 05, 2011, 03:15:08 AM
I think I'm going to go with the Triode Electronics SDS power supply board and the EF86 Driver Board.  Although, I really should finish the other projects I have on the bench before I start this one.

David B Bliss
Bottlehead: Foreplay I, Foreplay III, Paramour I w/Iron Upgrade, S.E.X. w/Iron Upgrade
Speakers: FE127E Metronomes, Jim Griffin Jordan/Aurum Cantus Monitors, ART Arrays
Other: Lightspeed Attenuator, "My Ref" Rev C Amps, Lampucera DAC


Offline STURMJ

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Reply #12 on: January 05, 2011, 05:25:26 AM
Triode's transformers are great too! If you need them. Also, they are better than the originals.



Offline HF9

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Reply #13 on: January 05, 2011, 06:37:20 AM
Although, I really should finish the other projects I have on the bench before I start this one.
Sounds like you need a second bench ;)

My DIY Audio Electronics Blog: DIYAudioBlog.com


Offline Dyna Saur

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Reply #14 on: January 05, 2011, 03:31:17 PM
www.diytube.com has this ST70 driver board available:

http://diytube.com/phpBB2/viewtopic.php?t=813

HTH

/ed B in NC

ed brown