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  • Modified ST35

    So for over a year now I've been itching to get my hands on a classic Dynaco SCA-35 or ST35 EL84-based amplifier. As always money is limited and I have a life so I never get around to chasing these down on ebay or anywhere else for that matter. A few months ago I decided I was just going to build one myself.

    Dynakitparts.com sells terrific Dynaco replicas and they even offer the ST35 but I wanted to try something different. I did however pick up the needed transformers from them.

    Diytube.com also has produced a modified ST35 design and even leaves available for free the full instructions and parts list. The main deviations from the stock circuit are:

    1. Replaces the 12DW7/7247 driver tube with a 12AX7 for the voltage amp stage and a 12AU7 stage for the phase splitter.

    2. Splits the common cathode connection into 4 seperate cathode circuits.

    3. Boosts capacitance in the power supply.

    I was preparing earlier this year to get that kit but procrastinated as I'm wont to do. And then I found Dave Gillespie's recently published EFB modification. Enhanced Fixed Bias does several things to improve the measured and audible performance of these old amps as well as reduce distortion and extend tube life. His paper on it can be found here: http://tronola.com/html/dave_s_lab.html#NewLook Some of you may remember I posted about this before.

    So I wanted an amalgamation of these 3 circuits but was highly intimidated by the thought of doing it myself. Which lead to more procrastination.

    Then I discovered audioregenesis created an EFB enhanced power supply PCB for the SCA-35. I really wanted to get it but at the time Canada was going through some internal drama with their post office and nothing was getting shipped at the time.

    So I kept obsessing over these multiple circuits and picking them apart in my head and on paper and ultimately I figured out how to do it myself. And the end result:


    On the Right is the Bottlehead Quickie Preamplifier with Ipod Touch as source.


    Top view.
    It was quite a challenge figuring out a whole new layout and ground configuration but I've been a good student of Bottlehead, Gary Dodd and other tube circuit designers and ultimately winged it.



    It sounds better than I could've imagined! I hear incredible bass and detail as well as an impressive soundstage. It captures the rawness of guitar distortion bringing to life classic guitar performances. It's really quite amazing for providing only 17.5 watts per channel. Of course when pushed it will eventually reveal its limitations but with relatively efficient speakers and mated with the Bottlehead Quickie preamp it makes for a small, yet fantastic tube stereo! The best part is that I was able to put it together for under $400!

  • #2
    Very nice work and very creative approach. Are you going to post the schematic? I'm very happy for you that it sounds so good!

    John

    Originally posted by corndog71
    So for over a year now I've been itching to get my hands on a classic Dynaco SCA-35 or ST35 EL84-based amplifier. As always money is limited and I have a life so I never get around to chasing these down on ebay or anywhere else for that matter. A few months ago I decided I was just going to build one myself.

    Dynakitparts.com sells terrific Dynaco replicas and they even offer the ST35 but I wanted to try something different. I did however pick up the needed transformers from them.

    Diytube.com also has produced a modified ST35 design and even leaves available for free the full instructions and parts list. The main deviations from the stock circuit are:

    1. Replaces the 12DW7/7247 driver tube with a 12AX7 for the voltage amp stage and a 12AU7 stage for the phase splitter.

    2. Splits the common cathode connection into 4 seperate cathode circuits.

    3. Boosts capacitance in the power supply.

    I was preparing earlier this year to get that kit but procrastinated as I'm wont to do. And then I found Dave Gillespie's recently published EFB modification. Enhanced Fixed Bias does several things to improve the measured and audible performance of these old amps as well as reduce distortion and extend tube life. His paper on it can be found here: http://tronola.com/html/dave_s_lab.html#NewLook Some of you may remember I posted about this before.

    So I wanted an amalgamation of these 3 circuits but was highly intimidated by the thought of doing it myself. Which lead to more procrastination.

    Then I discovered audioregenesis created an EFB enhanced power supply PCB for the SCA-35. I really wanted to get it but at the time Canada was going through some internal drama with their post office and nothing was getting shipped at the time.

    So I kept obsessing over these multiple circuits and picking them apart in my head and on paper and ultimately I figured out how to do it myself. And the end result:


    On the Right is the Bottlehead Quickie Preamplifier with Ipod Touch as source.


    Top view.
    It was quite a challenge figuring out a whole new layout and ground configuration but I've been a good student of Bottlehead, Gary Dodd and other tube circuit designers and ultimately winged it.



    It sounds better than I could've imagined! I hear incredible bass and detail as well as an impressive soundstage. It captures the rawness of guitar distortion bringing to life classic guitar performances. It's really quite amazing for providing only 17.5 watts per channel. Of course when pushed it will eventually reveal its limitations but with relatively efficient speakers and mated with the Bottlehead Quickie preamp it makes for a small, yet fantastic tube stereo! The best part is that I was able to put it together for under $400!
    Tweak City Audio WAF-1, Paradigm PS1000, Jolida FX-10 tube amp, Samsung Blu-ray, Beresford TC-7520 DAC, Apple Airport Express (living room)

    Hawthorne Audio 101515 OB Trio DIY speakers, DIY Bottlehead Stereomour 2A3 amp, DIY 6CL6 SPUD amp, Rythmik 370 OB Plate Amps (in stereo), Beresford TC-7520 DAC, Macbook Pro/iTunes & Decibel FLAC player for Mac, Sony S370 Blu-ray (listening room)

    Other stuff: Usher S-520, JohnBlue JB-3, MiniWatt, Glow Amp One, Nuforce Icon Mobile, Sennheiser HD595

    Comment


    • #3
      Posting a single schematic could be a bit tricky as I worked off of 3 schematics. I'll see what I can do. It would be nice if there was a program for drawing electrical schematics.

      Comment


      • #4
        I am very impressed :applause:.

        Comment


        • #5
          Originally posted by corndog71
          Posting a single schematic could be a bit tricky as I worked off of 3 schematics. I'll see what I can do. It would be nice if there was a program for drawing electrical schematics.
          There are good demo programs from the web for schematics. One is Eagle from www.cadsoftusa.com. Not sure if it will do tubes.
          Better living through Audio Nirvana!

          Comment


          • #6
            Very nice! Which EL84s are you using? My favorites are Tungsram.

            Comment


            • #7
              Originally posted by 1BionicEar
              Very nice! Which EL84s are you using? My favorites are Tungsram.
              Currently I'm using a matched quad of Valve Art tubes from
              http://www.tubesandmore.com

              I really liked their EL34 tubes in my ST70 and Mk IV amps. Plus you can't beat the price. I think I paid $37 for the quad of EL84s.

              In a few months I want to get a full set of Genelex Gold Lions for the amp just for the hell of it.

              It may lack a lot of power, but on a pair of bookshelf speakers like my AV123 X-LS', it just rocks the room in a way none of my other amps do. The speed and dynamics of the music really come through in a way I hadn't expected!

              That reminds me... I've been wanting to try it with my X-Statiks.:biglaugh:

              Comment

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