Bottlehead Forum
General Category => Technical topics => Topic started by: dbishopbliss on December 28, 2010, 09:03:28 AM
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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.
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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.
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Dynaco doctor also has some nice boards.
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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.
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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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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-
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You may get some useful information for you build from my custom ST-70 build page.
http://wardsweb.org/audio/ST70/ (http://wardsweb.org/audio/ST70/)
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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.
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ward--that is simply excellent
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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.
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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/
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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.
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Triode's transformers are great too! If you need them. Also, they are better than the originals.
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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 ;)
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www.diytube.com has this ST70 driver board available:
http://diytube.com/phpBB2/viewtopic.php?t=813
HTH
/ed B in NC
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Its been almost 4 years since I started this thread and I'm finally getting around to upgrading my amp. I had the boards already, but I just ordered the parts to populate them. Next Mid-Atlantic Bottleneck Meeting I will bring the amp.
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I've been doing a little readying (dangerous I know). Lots of people say that adding a grounded power cord to the ST-70 causes ground hum loops. Has anyone done this?
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I have built the VTA ST-70 and use a grounded power cord. Mine is dead quite. Hope this helps.
kind regards
Michael
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I have built a Triode ST-70. Also has a grounded IEC, and no problems here either.
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I've rebuilt 4 ST-70s and all got grounded power cords and no hum. Two of those were almost scratch builds with Paully.
A frequent source of hum is the 10 Ohm resistor that floats the input's grounds from ground.