Dynaco SCA-35

Discussion in 'Tube Audio' started by vintageboy, Aug 3, 2010.

  1. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    I just got a tube amp Dynaco SCA-35 and I am new with the tube amps.

    A friend told me that is has to have the "Bias" set, I googled and I found a lot of info but the amp I have doesn't have the bias pot like most of the amps have. Another guy told me that some amps are with fixed bias so is there anything I have to do on the amp? I am asking because it is in mint and barely used, everything original and I don't wanna ruin it!

    Thank you
     
  2. Phototone

    Phototone Super Member

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    Any old tube amp should have the capacitors replaced, including the power supply electrolytics. I believe the SCA-35 is self-bias..does not have a bias setting pot. However, you need to use fairly well matched pairs of output tubes (6BQ5). That amp can be a real sweet sounding one.
     
  3. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    The tubes are original Dynaco so they are the proper ones for the amp! Everything is original like I said! I have the brochures and the manual book. I guess it is fixed since it doesn't have the bias pot!

    I guess the capacitors are still good because it sounds good. It is barely used. I have heard an amp with old caps so I know how it will sound. The one I have sounds pretty good so I just worried about the bias thing.

    Changing the caps is an easy thing and I might change them when I have free time!
     
  4. DanO55

    DanO55 Radar - BSFR

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    I would, the sooner the better.
     
  5. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    I will do it soon. Thank you guys
     
  6. Phototone

    Phototone Super Member

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    To be correct technically, "fixed" bias is the term used to indicate a bias that you "can" set with a pot or control. "Self" bias is the term used when there is no way to adjust bias.
     
  7. spkrman57

    spkrman57 wrongsideofthetracks

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    It uses "cathode bias".

    There is one resistor connected between the cathode of all 4 output tubes and ground.

    Nothing to adjust!

    Ron
     
  8. Tom Bavis

    Tom Bavis Audiophool Subscriber

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    Upping the cathode resistor from 90 to 100 Ohms (10W or so) will reduce heating and increase output tube life, at the sacrifice of a small amount of output power. 6BQ5s were CHEAP when this was new...
     
  9. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    The SCA-35 is a nice EL84 push-pull amp; not at the top of the heap, but very musical and live-with-able :) There's one in my living room even as I type this.

    It was a low-low-end kit/product, though, which is good and bad. On the bright side, as with all (AFAIK) Hafler/Dynaco designs, the output transformers are very good (best part of the amp). The preamp section (except for the MM phono preamp, of course) is entirely passive; the two 12AX7s in the phono preamp may be removed if you're not going to use the phono preamp (it's not very good sounding); this saves a little stress on the power transformer and doesn't seem to raise voltages in the amp unacceptably.

    The problem with Dynaco's economy lies mostly in that ├╝ber-cheap cathode bias scheme (one cathode resistor and electrolytic capn for all four output tubes): the four output tubes must be very closely matched or the bias on the tubes will be all mucked up. One surprisingly inexpensive solution (or ameloriation) is to add an aftermarket power supply board that uses pairs of cathode resistors and electrolytics. Under this condition, one need only match pairs of outputs (a bit more achievable, 'specially if one wants to run vintage tubes). I used the board from http://www.audioregenesis.com/ It is inexpensive, easy to install, very well documented, and the support of the proprietor was excellent. No affiliation, just a satisfied customer (and just sayin').

    There are also output boards that are designed to use more readily available tubes than the 7199s used in the original design. I have no experience with these (yet).

    Tom's advice, of course, is also good if yo want to stick with the single cathode resistor topology.

    Hope these ramblings are helpful.
     
    Last edited: Aug 4, 2010
  10. JonL

    JonL AK Subscriber Subscriber

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    And increased power supply capacitance will give the little amp some additional low-end headroom. There are several aftermarket boards that replace the can caps, rectifier diodes, and cathode resistors. Highly recommended.

    Edit: I see Mhardy covered the bases while I was typing!
     
  11. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    :)

    I like the board I recommended because it is stunningly inexpensive!
     
  12. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    Now everything is so clear! Thank you guys a lot! Big help here :)

    http://www.audioregenesis.com/ is a very good site! I think I will buy from there! ( Saved in the bookmarks )
     
    Last edited: Aug 4, 2010
  13. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    IN the interest of fairness and completeness, there are multiple vendors for upgrade/restoration Dynaco parts and boards. That said, I was completely satisfied (EDIT: actually, I was thrilled!) with audioregenesis' product and service.
     
    Last edited: Aug 4, 2010
  14. DanO55

    DanO55 Radar - BSFR

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    Like their add says "if you only do one upgrade to the SCA-35 this should be it". I too am a satisfied customer from this company.
     
  15. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    You might well have been the person who recommended it to me :)
     
  16. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    So what is the exact part from the site to upgrade?
     
  17. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    http://www.audioregenesis.com/products/SCA-35/images/scacap_r03.jpg
    [​IMG]

     
  18. BinaryMike

    BinaryMike Pelagic EE Subscriber

    It's called the "Power Supply Capacitor Board." I have one of these, as well as a pair of the PC-10A output boards, but no time to work on the project! More news later!
     
  19. vintageboy

    vintageboy Spinning vinyl like there is no tomorrow

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    Thank you again, It is not expensive at all. About $50 with shipping.
     
  20. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    It took me about four months to get around to installing mine :)
     

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