2. amps-preamps
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Modications to the Dynaco ST 70?

I recently purchased a used ST 70 which was supposed to be stock. It had not been used in a while I was told, but that I should bring it back "Slowly", and after that it should be servicable.

That being said, I was surprised to find 6L6GC output tubes instead of the EL34's. I haven't yet turned it on and all the tubes are Rayethon Japanses types. I will be testing the tubes very soon. The rectifier tube looks like it is clear plastic! a sharp seam running right over the crown of the tube!

I have a few questions:

Using 6L6GC's, could the previous owner run these tubes without modification to the original ST70 circuit? If yes, would it have ruined anything?

Does anyone know if there is a reasonably priced mods available without removing the tube rectifier circuit..I'd like to keep that aspect of the unit. The newer ASL stuff is pretty good, so I don't want to pay that kind of money to mod this thing. I have been thinking of the Triode electronics input board, or the Sound Values input board.

Thanks in advance.
128x128yeast
doug28450
199 posts
 
A fellow named Frank Van Alstine does a lot of mods to the older Dynaco gear. He operates under the name Audio by Van Alstine (AVA). I think I have run across a website for him give your search engine a try and contact him.
ezmeralda11
303 posts
 
Check the older threads, I know someone asked about the same thing. avahifi.com does them as doug said and then I've seen another somewhere that simply replaces the stock board with all updated modern parts. I believes its soundvalves.com. There is a third out there too I've see. The ava mod is a complete circuit overhaul whereas he latter two just bring the older board up to spec again and remedy the worst of its design problems.
sean
6,231 posts
 
I recently inquired about Dyna gear. Someone else was kind enough to post a link to this website, so i'll "pay it forward" : ) Hope this helps.... Sean
>

http://home.indy.net/~gregdunn/dynaco/index.html
woodfield
39 posts
 
I have done modifications and restoration of a few dozen 70s over the years. The AVA is excellent as far as quality and instructions but sounds kind of sterile to me. I would first try rebuilding the original board with spec parts. Pretty easy if you know what they are and have a little soldering know how. Then rebuild the rest of the amp with new under deck parts and good wire. The manual is available at the Curcio website. You should then have pretty close to an original sounding Dyna that is reliable on the cheap. If you want a new board I would try the Curcio without the power supply board or the Triode. I have had excellent luck with both and they do maintain the tube rectification and much of the original sound of the Dyna. I would lean toward the Triode just for the ease of installation. I think someone did a little mod on your 70 to accomodate the 6L6s. At least the bias is probably different. Not positive on that one. You will probably want to put it back to normal and get some EL34s. I have also had the rectifier tubes you describe with the seam across the top. I think they are just cheap 5AR4s but they seem to work OK, Don't worry about it but they do look pretty chincy. Good Luck and Have Fun.
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