Dynakit Stereo 70 Refurb - Printable Version
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+--- Thread: Dynakit Stereo 70 Refurb (/showthread.php?tid=10341)
RE: Dynakit Stereo 70 Refurb - Joeztech - 11-15-2014
As a follow-up, I am currently restoring a second ST-70 and also painted the end bells of the transformers in the same way as the first unit. However, I started assembling the unit before I let the Instant Chrome paint cure long enough and they have gotten scuffed as I worked on the amplifier reassembly. I also had tried spraying them with some clear acrylic lacquer and discovered that the lacquer ruined the look of the paint, so I had to scrape and sand them once more and repainted. I also wrote Duplicolor and asked if they had any clear coat that could be used with the paint that would not ruin the appearance. They replied that, no there is no clear coat paint that will not mess up the look of the paint - bummer!
If you do use the Instant Chrome paint, be sure to let it cure for at least a week or more in warm room etc. before reassembly. The paint remains soft longer than most other types of spray can paint.
You can see the new ST-70 effort at:
http://s1213.photobucket.com/user/rubal182jhn/library/2nd%20Dynaco%20ST-70?sort=6&page=1
Joe
RE: Dynakit Stereo 70 Refurb - thirtiesradio - 12-22-2014
Looking great
How do they sound? I've never heard a 70 in person.
RE: Dynakit Stereo 70 Refurb - Joeztech - 12-23-2014
They sound great. The first one I did is populated with JJ EL34 outputs and they have a nice soundstage for stereo. Typically they are very quiet with very little hum and noise. The Van Alstine modification was made to the feedback and input stages of the amp. This reduces the low end somewhat and I wound up modifying the input coupling cap to .047 instead of .027 or .033mFd. This brings the bass capability back up but only to a reasonable level with loudness compensation for low level listening. For cases where really loud reproduction is desired, the loudness compensation needs to be turned off and only the normal bass and treble adjustments should be made. The original input circuit was direct coupled and the amplifier does not need to try to reproduce frequencies below 15Hz. Adding Van Alstine's band limiting circuit to the front end cleans up the muddy bass these were known for when new. More filtering in the power supply also helps the low end. When finished the result is much more life like and to my ears sounds better than the solid state amplifiers I had been using for years (including Marantz and Onkyo 80 and 100W per channel power amps).
6GH8A gain and driver stage tube are excellent replacements for the OEM 7199 tubes. The PC board has to be modified or adapter sockets at $24 apiece need to be installed to use them. I decided to just modify the board since good 7199 tubes are now quite expensive.
This latest unit was populated with JJ KT77 output tubes which have slightly more bass output than the JJ EL34 tubes, but only by a small amount. I made the same modifications to the front end and feedback as mentioned above. This last unit was a gift to a friend for Christmas. He and his wife are enjoying great sounds now.
Merry Christmas and Happy New Year!
Joe
RE: Dynakit Stereo 70 Refurb - thirtiesradio - 12-23-2014
Very nice Joe!
I've read a lot about the 70 and know there are many mods that can be done. They seem to be a very popular amplifier.
I've read where the 6GH8's are a good replacement for the 7199's. As you know, many tubes like the 7199's are getting up there in price. Many people think they sound as good or better too which is a big plus
I did a similar driver swap in a set of Heathkit W5M amplifiers many years ago using 6CG7's in place of the 12Au7's. In that swap there was a big change in sound. I used them for about 3 or 4 years. The W5M's could be hard on tweeters as many Williamson designs were. I found out there were mods to help prevent this, but I had bought and restored other tube amplifiers by then and ended up selling them.
I'd like to listen to a ST70 sometime.
Merry Christmas!
RE: Dynakit Stereo 70 Refurb - Phlogiston - 04-18-2015
I have a ST 70 and come to think of it a ST35 and a MKIII all sitting in the HI-FI collection closet. Don't have room to run all of them. Have talked to Sue about adding on to the museum for a 50s, 60s and 70s Hi-Fi display - not to enthusiastic response.
Anyway I rebuilt the ST 70 with one of those remake filter caps. Like said, not to impressive ratings, but it works. I think that next time I would restuff the original with caps of my liking.
As to the "chrome" paint, I invested in a 5 gal. nickel plating set up ( and a copper one, but you don't need that). If you are doing a lot of these a small kit would do fine. Build your own DC supply. It takes very little current - a battery is too much.
Anyway they are nice units. I could set up a 5.1 surround sound with mine - just kidding. Did I mention that I got them all for $19? But that is another story.
