New ST-70 Driver board - Blackburn Audio

knowledge base for the classic Dynaco ST70

Postby Sal Brisindi » Fri Jul 11, 2008 8:31 am

burnedfingers wrote:I can see the need to CYA when selling an aftermarket product.


On that note, I recently restored 2 ST-70's for a customer and he will be running the amps without the cages. I am going to write a document stating running the ST-70's without the cage can cause serious burns and electrical shock and I will not be responsible if the owner chooses to take off the cage and make him sign it. I don't need his 6 year old nephew (or whoever) get a nasty shock and me getting sued.

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Postby TerrySmith » Fri Jul 11, 2008 8:39 am

You can also use SS diodes or a Weber copper cap instead of the GZ34, that would free up some power transformer capacity by not using the 5v winding. The Triode or Dynakit power transformer is a very nice upgrade, plus you can sell the original on ebay and get some of your money back!

I noticed the feedback cap connects to the UL tap like stock. If you don't have a UL circuit and wanted to bypass the 1.8k feedback resistor, what size capacitor would you use?
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Postby burnedfingers » Sat Jul 12, 2008 5:18 am

On that note, I recently restored 2 ST-70's for a customer and he will be running the amps without the cages. I am going to write a document stating running the ST-70's without the cage can cause serious burns and electrical shock and I will not be responsible if the owner chooses to take off the cage and make him sign it. I don't need his 6 year old nephew (or whoever) get a nasty shock and me getting sued.

Better add to that SAL.... A disclaimer to insure that you are not held responsible for tube and or component failure. Better yet do what some of us already have done and that is hang up the soldering iron except for personal use.
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Postby burnedfingers » Sat Jul 12, 2008 5:34 am

The Triode or Dynakit power transformer is a very nice upgrade, plus you can sell the original on ebay and get some of your money back!

Some of you are really on the band wagon here. Is there proof the original transformer is overtaxed? Can anyone show the specifications and or underwriters lab specifications concerning transformer temperature? Before you EBAY out a perfectly good transformer it might be wise to see if the specifications have been exceeded. You might also want to consider the usage of the tube cage when firing up this ancient tube monster that is capable of inflecting burns and heat rash when you lay your flesh upon the super hot tubes and or transformer.


Please ! People in the 50's and 60's didn't sue like people
do today ... hot coffee in the lap, kid wearing soft plastic shoes
getting foot caught in the escalator, etc. Product liability
lawsuits were virtually unheard of back then.

We can thank the countless lawyers and judges in the country for this one.
Logic and common sense have been thrown out the window in the hopes of making a fast buck without having to work for it.

So now lets throw out a perfectly good transformer because it runs a little hot. Lets forget that it passes and the Dynaco passed UL regulations ans was considered safe.

Wait!!! Maybe I've finally seen the light ;) we're doing our part to help finance the site. Buy the product and keep the site going even if we don't need the new transformer. ;) ;)
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Postby burnedfingers » Thu Jul 17, 2008 5:27 am

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Postby burnedfingers » Tue Jul 22, 2008 5:46 am

Installed the Blackburn audio board in one of my Dynaco stereo 70's on the 19th and it works flawlessly. The stock transformer runs it without any problems. The highest temperature I recorded was 127 degrees after around 10 hours of running it non-stop The transformer is a later model which has some kind of tar like coating on it. This particular transformer has ALWAYS run hotter that any other unit I have owned.

It went together smoothly and was very straight forward. I found it to be a step up from the original 7199 board. It is very smooth sounding and certainly a steal at around $100 parts and board. While not being as smooth as the Mapletree board I guess one cannot expect it to keep up with a board costing 3 times as much. I do find myself wishing that they offered a similar board to the Mapletree except in PC board format instead of point to point.


I did find that I had to allow the board NOT to touch the chassis in order to eliminate a ground loop. I spaced the driver board under the chassis using 4) plastic nuts and mounted the driver board to the chassis using all plastic hardware in order to insulate. The only modification to the board was to drill out the mounting holes to 6/32 to allow for the 6/32 plastic screws and nuts from Ace Hardware. After this modification the amplifier was 100% quiet.
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Postby burnedfingers » Sat Dec 13, 2008 8:15 am

Has anyone substituted 6SL7's for the cascode V1 and V2.

If so what cathode resistor change is needed?
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Postby burnedfingers » Tue Feb 10, 2009 8:06 pm

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Postby Geek » Tue Feb 10, 2009 10:35 pm

burnedfingers wrote:http://img246.imageshack.us/my.phpimage=dynacospareparts007oz0.jpg

I gave the board a second chance.


I'm getting a "404" Yellow_Light_Colorz_PDT_13
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby burnedfingers » Wed Feb 11, 2009 5:34 am

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Postby burnedfingers » Tue May 26, 2009 4:50 am

There is a Blackburn Audio Driver board on ebay #190310256433 if anyone is looking for one. It has tubes with it ;)
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Postby TomMcNally » Tue May 26, 2009 9:53 am

Too bad he mounted the components on top of the board, near
the hot tubes. That's definitely not good for capacitors.
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Postby burnedfingers » Tue May 26, 2009 3:27 pm

Too bad he mounted the components on top of the board, near
the hot tubes. That's definitely not good for capacitors.

Well they can always be desoldered and mounted on the bottom side of the board. ;)
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Postby burnedfingers » Wed May 27, 2009 5:30 am

Too bad he mounted the components on top of the board, near
the hot tubes. That's definitely not good for capacitors.

Tom,

Please explain why the components mounted on top of the board would pose such a problem. The stock Dynaco board also had its components on top of the board as do most. A look at the Blackburn board and you can see the bottom of the tube is above the top of the capacitors. Doesn't heat rise? If so then the coupling caps will surely last as long as those on the stock Dynaco board. A second reason to mount the caps above the board is so that you still have access to supply the filaments of the tubes as this cannot be done easily with the caps blocking the F6,F5,F2,and F1 holes. With the tube sockets .700-.750 above the PC board I hadn't seen a heat build up problem actually you will find there will be more heat build up on a stock Dynaco board with the tube sockets very close to the components. Have you owned one of the Blackburn boards? I have ;)
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Postby TomMcNally » Wed May 27, 2009 5:49 am

The stock Dynaco boards are being replaced with a multitude of
boards because they have been ruined by heat over the years.
Heat that toasted the phenolic, ruined the resistors, ruined the
capacitors. Heat from tubes and resistors and printed circuit boards
don't mix well to begin with - so it's only sensible to minimize the
problem by keeping the hot parts away from the temperature
sensitive parts.

No, I didn't get a chance to buy a new Blackburn board while
they were available new, but if I had, I would have mounted
the components on the bottom, away from the heat.

This same personal preference keeps me away from the
Aikido All in One Kit where the regulator circuit can't be
flipped to the bottom. Also - with large components top
mounted, it's not possible to mount the board under the
chassis so only the tubes stick out the top for aesthetics
and safety. An amplifier with high voltage exposed isn't
suitable for everyone.

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