The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Link to Dynaco SCA-35 rebuild article - good information

    Share

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Link to Dynaco SCA-35 rebuild article - good information

    Post by Bob Latino on Sat Feb 12, 2011 7:40 pm

    Below is an article written by David Gillespie on his rebuild of a Dynaco SCA-35 integrated amp for improved performance. If you have an SCA-35, the article (in pdf format) is definitely worth a look ...

    Improving the performance of the Dynaco SCA-35

    Bob



    Last edited by Bob Latino on Mon Apr 23, 2012 7:59 pm; edited 1 time in total

    Tazsmonn

    Posts : 18
    Join date : 2009-01-18
    Age : 54
    Location : Richfield (S. Minneapolis) Minnesota

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by Tazsmonn on Sun Feb 13, 2011 6:22 pm

    Yes worth the look. I am not one that believes in hi-bred circuits, that is mixing silicon & vacuum tubes. I'm sure this would improve sound, but I also feel that each tube should have it's own bias. I do agree the original can be improved on. All things considered I will not be replacing my ST35-BCU Bias Control. It has made me ponder a fully mono blocked ST 35. Now that might be the "HOLY GRAIL" 6BQ5/ EL84 PP UL amp!!!
    02.13.11

    symfo

    Posts : 10
    Join date : 2011-01-01
    Age : 55
    Location : Netherlands

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by symfo on Tue Feb 15, 2011 8:10 am

    In Dutch but also nice I think. Don't know the quality........
    http://www.vintageaudiorepair.nl/archief/nlsca35.html

    1973shovel

    Posts : 23
    Join date : 2009-12-13

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by 1973shovel on Mon Apr 23, 2012 7:31 pm

    This is a great article. Thanks for mentioning it, Bob.

    I did Dave's "Enhanced Fixed Bias" mod to my SCA-35 about a year and a half ago. I'll start by saying that I never liked the sound of the stock SCA-35, and I heard several examples. Mine sat on a shelf for many years, because I had other amps, especially the VTA ST-70 which I liked a lot more.

    After reading the article, and finding out that it only took about $10 in parts to complete, I decided to give it a go. Saying "Wow" doesn't begin to cover it. The EFB took my SCA-35 from an amp I never use, to one I now love the sound of, and use frequently.

    I'm building a ST-35 and will install the EFB bias in that as well. I'll mirror Bob's suggestion and ask that you read the article. If you have an SCA-35 or ST-35 with the stock cathod bias, or have changed it to fixed bias (four separate pots, one for each output tube) you have actually hurt the sound of your amp!

    The EFB will give you more power, with less distortion, and sound more musical doing it. Highly recommended!



    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by GP49 on Mon Apr 23, 2012 9:54 pm

    symfo wrote:In Dutch but also nice I think. Don't know the quality........
    http://www.vintageaudiorepair.nl/archief/nlsca35.html

    Link with correct URL:
    Dynaco SCA-35 geintegreerde buizenversterker

    1973shovel

    Posts : 23
    Join date : 2009-12-13

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by 1973shovel on Tue Apr 24, 2012 3:08 pm

    Follow up:

    I was on the forum yesterday reading posts, which is how I happened across Bob's SCA-35 EFB post. Since my mind was on what I was searching for, and not the EFB, I neglected to mention one important fact about the simple bias modification: Output tube life.

    When I decided to bring my SCA-35 out of mothballs for another try, I figured I'd get some good output tubes to judge it fairly. I wrote Jim McShane to ask about the Gold Lion EL-84 's I had read good things about. Jim was concerned about Dynaco biasing the 6BQ5 at 35 mA in the SCA-35 and ST-35. He was hesitant, since he knew the Gold Lions wouldn't last long biased at 35 mA. I happened to mention Gillespie's bias modification (which lowers the bias from 35 mA to 27 mA). His reply was that I could trust whatever Dave Gillespie said, and that he'd be glad to sell me the tubes if they would be biased at 27 mA.

    I bought the Gold Lions (love 'em!) and don't mind running them, since I know they're going to last me a lot longer with the EFB mod.

    Tazsmonn, I can respect your purist take on tube amps. But to me, if it's going to make my expensive output tubes last longer, and sound (and measure) better doing it, then I'm all for it. Oh, and I agree that an adjustment for each output tube is better. That's the way I'll be building the bias supply for my ST-35, using the EFB circuit, but with one pot for each tube.


    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by sailor on Tue Apr 24, 2012 7:09 pm

    I have an SCA35 and like the sound and love the tight Bass. I did find two ways of improving the sound. First I only use it for TV or CD so changing cables back and forth is not a big problem. If you plug the input source to the tape out you bypass a lot of old parts including the selector switch. This improved the sound on my unit. Also I took the knobs off to clean them and followed the instructions when I reinstalled them. It sounded the same as before but I realized that the is no set center on the Bass or Treble control. So to test the output I used a test CD. First I removed the speaker wires and installed large 8 ohm resistors across the transformer outputs. I played the different tones from 20 to 16000 HZ and I measured and recorded the input voltage. Don't accept that your CD player is flat from 20 to 20K it probably isn't. Then I measure the output. What I found out was if you install the knobs like the book says you probably are getting Bass and Treble boost. I adjusted the Bass and Treble until they closely matched the input. Then reset the knobs to center. What was the change? Less but more realistic Bass. louder and more detailed midrange. Overall a better sound for CD's. I still crank up the Bass when I'm on TV. Don't need a sub. If your speakers are efficient.

    rmyauck

    Posts : 9
    Join date : 2012-02-18

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by rmyauck on Wed Apr 25, 2012 4:26 pm

    After reading all of Dave's info and posts on the subject of the EFB mod, one could accomplish the same with a bias PS transformer (Set up like the ST-70) for more $ if you don't want SS. He did it as simple as possible and with the lowest cost the way Dynaco would have done it while having real fixed bias. Space was another reason. As he mentioned it will out preform even adjustable cathode bias for each tube. As mentioned the EFB mod could be set up with more pots so you can adjust a pair or each one.

    Randy


    Sponsored content

    Re: Link to Dynaco SCA-35 rebuild article - good information

    Post by Sponsored content Today at 6:18 pm


      Current date/time is Sun Dec 04, 2016 6:18 pm