diytube.com

I was lucky to buy a Dynaco ST-70. In my country you won't find these ST-70 a lot. You also have to be lucky with the state it is in. Also the power transformer has to be suitable for 230VAC.

This is the amplifier:



I could see that this amplifier was modified, but the chassis, the transformers and the cover looked very good! My goal is to bring this amplifier back to the original state, so the modifications didn't bother me a lot.

When I looked at the bottom I was a little bit surprised what I saw. In this state I thought it was not a good idea to connect it to the grid so I started to disassemble the components. With a vacuum de-soldering tool (Weller) I removed a lot of solder.


I have removed everything right now to clean the chassis. Here is a picture where everything wasn't removed yet.



My first question is about the chassis. On some parts on the chassis I see very small 'cracks'. With a magnifying-glass it looks as if a worm is busy just beneath the surface. Does this mean that there is a layer of lacquer on the metal? How did Dynaco deliver their chassis? Only polished steel or with a layer?

To save money, Dynaco just nickel plated the chassis,
and left out the protective chrome plating.
Folks like Sal Brisindi have done amazing polish jobs on these
chassis.

http://www.diytube.com/phpBB2/viewtopic.php?t=919

I couldn't find Sal's link.

With a sharp knife I opened the 'worm' on a place where it can't be seen. In fact it is rust. Luckily it is hardly to be seen and the chassis looks good.
Here is a picture of the 'worm' and the chassis. I polished it a bit with polishing wax.







Of course it is possible to nickel plate it again but then you will loose the fantastic silk screening. When you look at this with a magnifier glass you see the fantastic red color what is transparent.



I found on pin#2 of the GZ34 a unsoldered wire! All these years it functioned apparently.




Next is to clean all the components and start rebuilding the ST-70. I will use the circuit board which I cleaned. I have to find a store for metal oxide flame-proof resistors but these are hard to find here.

The cleaned PC-3. Some print wires were cut by the former 'tweaker' but are easy to repair.

There are better driver circuits available, like Shannon's board
in this forum.
If you plan to use the original Dynaco circuit, you may want
to consider this.

http://www.classicvalve.ca/dynaco.html

How's your 7199's? A new board might get you low cost easy to find driver tubes too, unlike the 7199. You can cut traces and rewire sockets to run 6GH8A, if you want to keep the original circuit. AudioRegenesis is also coming out with a board to run the 6GH8A. You probably saw that as you bought preamp boards from them. Some resistors should be changed to run that tube and he's making those changes I believe as he has a link to that info. I think that's one reason why the market developed for different driver boards.

@Quad & @rmyauck

That is indeed a possibility to buy new boards. I have several new 7199's in stock (RCA). George Audioregenesis doesn't have the PC-3 boards anymore. This market is a bit saturated. For the SCA-35's I have one with 7199 PC-10 boards and one with 6GH8A PC-10A boards.

For the ST-70 I like to keep it original, but on the other hand I would like to have a good sound of course. My idea is first to restore the original board, listen to the music and after a while look what possibilities there are for the ST-70 like another pre-amplifier board and/or power supply.

I had a stereo 70 driver board once that I had used 1% resistors and had done a mod. I don't remember what the mod was now but it seemed to make a difference in the low end sound. Anyway this board did manage to make a superior sounding 70.

Since there are thousands of 70's on the market and owned the 70 really doesn't command a premium price nor is it a collectors item. I would suggest a newer type of driver board instead of the original.

With respect to the finish...you cannot polish a turd. The chassis when manufactured received a nickle finish that does not hold up good. Your options would be to send it to a plating shop to have it stripped and re plated or to purchase a new chassis.

I'll toss in my twonie...

As for finish, a sandblast from the local auto shop, primer and paint is what a lot of people do, or powdercoat.

As for sound, every driver board is different and has its own charm. New caps go a looooong way, IMO, more than resistors with improving sound.

(and thanks for the plug, Quad )

Cheers!

@6bg6ga -- What driver board do you suggest? Since I've seen many driver boards for sale, I don't know the differences.

I'm lucky that the chassis of my ST-70 looks very good, there are only very very small 'worms' (see picture in this posting). When I should nickle finish it, I will loose the silk screening. This silk screening is very beautifull. The laquer what is used is a little bit transparant. I know that George from Audioregenesis did silkscreening for SCA-35 but I don't know if it is possible for the red transparant laquer what is used for the ST-70. In fact, the amplifier is 40 years old or older. So, it doesn't have to be shiny and new.

I'm curious what driver board you suggest. How about the Selenium rectifier? Do I need to replace this diode?

I see several Chinese PC-3 boards. What is the quality of this? Does anyone have experience with this? I must say that the boards from classicvalve look very nice.

quote:
6bg6ga -- What driver board do you suggest? Since I've seen many driver boards for sale, I don't know the differences.

If it were me changing out my first driver board I would visit Geeks site and purchase his board. (the one with the 6SL7's on it) If I were to make a second choice I would go with the Mapletree board. I've tried a number of them everything that has been mentioned so far I have tried.

I lot of good information from people that have had their hands in a number of 70's.

As mentioned the chassis can be sand blasted and re painted. It can be stripped and chromed. It can be stripped and copper plated. The 70 is open to anything and will end up a winner with a few modifications. Power supply is one. Loose the selentium rectifier and replace with a 1N4007 diode for the bias supply. Replace the caps in the bias supply. Add individual bias adjustment for each of the output tubes.

I would highly suggest that you email Greg (Geek) and talk over your plans for the 70 and listen to what his experience has to say.

Quad wrote:To save money, Dynaco just nickel plated the chassis,
and left out the protective chrome plating.
Folks like Sal Brisindi have done amazing polish jobs on these
chassis.

http://www.diytube.com/phpBB2/viewtopic.php?t=919

Thanks,
I have used the good old original formula Brasso polish and a lot of hard rubbing to clean and polish the chassis. Of course the chassis has to be in good condition as your chassis is in.

I have replaced chassis's purchased from Dynakitparts.com for those chassis that had no hope with many rust and pitted spots for customers.

Sal

@Sal. I also used Brasso. This works fine.

At this moment I started rebuilding the amplifier. I have to wait for a new can capacitor, components for the circuit board and components inside the chassis.

I have a question about the lifetime of the coupling capacitors. Originally 'Black Cat' is used. I also have some NOS Sprague capacitors. Is it a wise thing to replace the Black Cat's? If yes, replace them by Sprague NOS or new capacitors? What brand should these new capacitors be? (price about $10,- a piece)

These are my NOS Sprague capacitors:


The original Black Cat capacitor beside the Sprague capacitor.

DO NOT use those old caps, even if they are NOS!! Those have to be at least 50 yrs old, and most likely leaky or shorted.

For general replacement the "orange drop" is about the best out there. I personally like the 716P series. Whatever cap you buy just get new fresh stock! Also get 630v rated caps, 400v is marginal.

There are two types of non-animate "Black Cats", the capacitors and the firecrackers, and neither of them will be any good after fifty years

Of course, no one at the Dynaco company EVER thought that their amps would still be in use half a century later, and would be the object of so many modifications and upgrades. The design of the driver boards reflects this, they were "just good enough", but not to last for multiple decades of service. Fortunately they designed the transformers so very well

/ed B

Well the label on the Sprague caps says "unconditionally guaranteed" !
Trade the old bitches in for some new ones!