yet another ST70 amp

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yet another ST70 amp

Postby mesherm » Mon Mar 07, 2005 11:18 pm

This latest project was another ST70 amplifier. This time I decided to build it as originally designed with just a few minor changes. I started with a new stamped metal ST70 chassis aquired on eBay. I sent the chassis to a local plating outfit and had it copper plated to a bright mirror polish.
The PCB is the original circuit but a quality board made with modern materials from Vintage Electron. I loaded the PCB with all 1% metal films, silver micas, and Auricaps. The PT was the PA060S from Uncle Ned as was the filter choke. The filter cap was the same 80/40/30/20 I have used before. I had originally planned to use Uncle Ned's OPTs also but they were out of stock so I secured a set of NOS Dynaco A470/454326 on eBay. They have plastic leads and I suspect they are the later ones wound in Japan. I also bought two NOS RCA black plate 7199s and a Mullard GZ34 (with hole in guide pin). I wired the amp per the original manual except with increased filter capacitance. I used the capacitor in 30/80/40/20 configuration to be on the safe side (thanks to PSU Designer II).
How does it sound? Well it sounds great. I wish I had a bigger room so I could crank the volume more. I swapped a set of Sovtek 7199s and 5AR4 in for the NOS tubes and didn't notice any diminished sound although I think I like the RCAs and Mullard better (could be the price I paid). I CAN tell you that the tubes reflect great in the high copper polished chassis with the lights down low.
I have satisfied myself that the venerable ST70, as designed, with good components can hold its own against rebuilds with designer driver boards.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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excellent work!

Postby Shannon Parks » Tue Mar 08, 2005 7:22 am

Hey Mesherm, can you send us a pic?

The ST70 is truly an excellent amp and some masterful engineering, IMHO. Those who bash the stock ST70 don't get it. If there was a good supply of NOS 7199s out there I probably wouldn't have done my driver board.

BTW, I've tested those late model A470's and they test identical to the earlier ones - same saturation point, etc.

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Postby mark@marktwain » Tue Mar 08, 2005 9:50 am

What did it cost to have that chassis copper plated? I checked with our local shop about plating the end bells for my st-35 outputs and power transx and about fell over. He wanted 30 bucks a pair. Yellow_Light_Colorz_PDT_10 Jeeze I wonder what chrome would have cost 300.00?
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Postby mesherm » Tue Mar 08, 2005 10:37 am

The copper plating cost me $100 cash and took about 12 days. They polished the outside to a mirror finish but the inside is ugly. I had my choice of brass, nickle, chrome, plus various antiqued finishes. I used some car paste wax on it before I started, thinking to protect the finish from fingerprints and stuff. I will snap some pics as soon as I get the driver board cover I ordered. I built two driver boards, one with the Auricaps that I installed and one with less expensive polypropylene caps for a backup. Like my ears could tell the difference anyway... Yellow_Light_Colorz_PDT_09
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby mark@marktwain » Tue Mar 08, 2005 1:12 pm

That's a pretty good price, I bet the dude I talked to just didn't want to mess with em. I used a copper top plate on my st-35, the pics are in this thread. http://www.diytube.com/phpBB2/viewtopic.php?t=544
I tried the same approach you did with the wax and then I read if the wax has carnuba in it it will cause the copper to oxidize. Well sure enough before I completed my amp the copper had begun to tarnish. I've tried clear laquer on copper before and I just don't like it. It looks so much better just polished. I used brasso and it looked great for a month or so and began to tarnish again. So now last weekend I polished it again with semichrome polish. We'll see if this stuff lasts any longer, if not I'll just polish it every month or so, no big deal.
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Postby mesherm » Tue Mar 08, 2005 8:53 pm

Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby mark@marktwain » Tue Mar 08, 2005 10:31 pm

Beautful! I just absolutely love polished copper. Where's my shades? Yellow_Light_Colorz_PDT_11
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Postby Shannon Parks » Wed Mar 09, 2005 1:18 pm

mesherm wrote:Pics of ST70 amp with copper plated chassis


AWESOME!

I am so jealous.

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