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    SCA-35 question

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    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    SCA-35 question

    Post by daveshel on Sat Feb 06, 2016 9:11 pm

    Where the heck is the phono board? You have 2 oblong boards that look like the ones in the ST-35, then a larger square one that looks kinda like the linestage board in the PAS. But the PAS has a separate phono board.

    Just wondering because I'd like to build one someday. Although those hum adjust controls are a little scary.

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: SCA-35 question

    Post by MontanaWay on Sat Feb 06, 2016 9:14 pm

    the two oblong boards are the power amp and preamp, an all in one pcb, the other is the phono preamp.
    Hum adjust is because the original unit uses AC even for the preamp tubes. If you get one of the newer power supplies, I believe filament for the phono & preamp tubes are DC, and AC for the EL84 etc..

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: SCA-35 question

    Post by daveshel on Sat Feb 06, 2016 11:07 pm

    Are you sure that's correct, Montana? Those boards are very much like those in the ST-35, and it seems like they would have needed additional components for the tone control circuitry.

    So I did some more searching, and this article http://www.tnt-audio.com/ampli/dynaco-sca35_e.html says the phono is somehow combined with the linestage board. How they pulled that off is beyond me - they accomplish with 2 tubes what it takes the PAS 6 tubes and 2 boards to do.

    In any case, my research has cooled my jets on wanting to build one. Doesn't seem like there would be an easy way to bypass the tone controls. And, as that article points out, it doesn't sound as good as a PAS/ST-35 combo.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: SCA-35 question

    Post by PeterCapo on Sat Feb 06, 2016 11:37 pm

    The low level inputs go to PC-11 (low level inputs, only).  Then, the output of PC-11, as well as the high level inputs via the selector switch, go to the various front panel switches and pots.  So, the SCA-35 does not really have an active high level stage – it’s passive in this regard.  After the balance control, the audio signal goes to the power amp boards, which are similar but not exactly the same as the Stereo 35.

    Some have successfully bypassed the SCA-35 tone controls and, I suppose, the other front panel pots and switches.  I think the trick is that you need to emulate the impedance the power amp section “sees” looking back into the circuit before it, and the signal level would have to be kept correct, too.  IIRC, the 7199 (instead of 12DW7 in Stereo 35) is set up for a good deal of gain to compensate for signal losses through the passive circuitry between the selector switch and the power amp section - but, if someone thinks otherwise on this point, please say so.

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: SCA-35 question

    Post by daveshel on Sun Feb 07, 2016 12:28 am

    PeterCapo wrote:IIRC, the 7199 (instead of 12DW7 in Stereo 35) is set up for a good deal of gain to compensate for signal losses through the passive circuitry between the selector switch and the power amp section - but, if someone thinks otherwise on this point, please say so.

    That would explain why the amp board is so close to that of the ST-35 except for the 7199.

    This amp is pretty remarkable in terms of the shortcuts they used to meet the low original price.

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: SCA-35 question

    Post by MontanaWay on Sun Feb 07, 2016 11:47 am

    as far as tone controls are concerned on the SCA-35, Dynaco came up with a pretty cheap solution. They used a PEC (packaged electronic circuits) for each channel, which had all the components for the tone controls enclosed within. Kinda looks like a giant ceramic capacitor with a bunch of leads coming out of it. I guess one can also call them integrated circuits, in this case, all analog!
    These PEC's were directly connected to the tone control pots. They even used PEC's at the phono preamp input for Hi-LO-Ceramic phono cartridges, as the SCA-35 had individual phono in RCA sockets for each of the three types of phono cartridges.
    No doubt it worked, using these PEC's, but it was a cheap way of achieving tone controls etc..
    I would think, having spent some time looking at the SCA-35 circuit and build, any upgrades and or mods, one would be better off to remove the tone controls and end up with a 'classic' line preamp stage.
    For its time, the phono and preamp/amp circuits were quite OK, and there are some very good modern upgrades available, that will breath new life into an old SCA-35, giving you a nice integrated tube amp, pretty much the size of a PAS3.
    So I would think it is definitely worth while picking up an old SCA-35, and as long as the transformers are all still good, which are of course the most expensive parts of the amp, it would be a nice amp to upgrade!

    In case you're curious as to what a PEC looks like, these are the PEC's used in the SCA-35

     photo PEC_zpsuov84al9.jpg

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: SCA-35 question

    Post by daveshel on Sun Feb 07, 2016 7:53 pm

    I was thinking more along the lines of building one from all new parts from Kevin and Dave, but it sure adds up quick. Without looking up the price of the parts to stuff Dave's boards, I'd guess it might be a little north of $800, skipping the tone controls. I guess I could skip the cover and save 55.

    It all reminds me that I really don't need an integrated.

    corndog71

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    Location : It can get windy here

    Re: SCA-35 question

    Post by corndog71 on Mon Feb 08, 2016 11:04 am

    daveshel wrote:I was thinking more along the lines of building one from all new parts from Kevin and Dave, but it sure adds up quick. Without looking up the price of the parts to stuff Dave's boards, I'd guess it might be a little north of $800, skipping the tone controls. I guess I could skip the cover and save 55.

    It all reminds me that I really don't need an integrated.

    I too contemplated rebuilding an SCA35.  Like you found it was a bit more trouble than anticipated.  This is particularly true if you want that original chassis.  Personally, all of the compromises of the design turned me toward just building an ST35 and keeping with my separates.  The idea of an integrated amp sounds good but ultimately it's meant to be a compromise.  With the mods that I wanted I needed a brand new box and layout anyway.  My first attempt cost me around $400.  Add a nice new box, connectors, a fancy volume control and you could probably get in around $500.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: SCA-35 question

    Post by sailor on Tue Feb 09, 2016 6:05 pm

    I have a factory built SCA35. It has a Phono stage controls and an amplifier stage. There is no line stage in the SCA. If you are running only a CD player you can plug the CD into the tape out and this will bypass everything up to and including the selector switch. You can then remove the input wires from the amp stage and run wires direct from the volume control to the vacated holes. Very important you must not lower the value of the volume pot as it is in parallel with the input resistors on the amp boards. The 7199 was required to get enough amplification to drive the output stages. If the output boards are changed, the 12DW7 has enough amplification to drive the output stage if the tone controls are bypassed.

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