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    Gain of the ST 35 versus the SCA 35 boards

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    sailor

    Posts : 269
    Join date : 2011-04-04

    Gain of the ST 35 versus the SCA 35 boards

    Post by sailor on Tue Jun 12, 2012 3:38 pm

    I know that the gain is higher in the 7199 based SCA board over the 12DW7 based ST board. The question is does anyone know by how much in DB.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Gain of the ST 35 versus the SCA 35 boards

    Post by sailor on Sat Jun 16, 2012 12:54 pm

    I guess no one knows the answer to my original question so here is a little different question someone should know the answer too.
    What was the gain of the original Dynaco ST70 with 7199's and what is the gain of the ST 35 with 12DW7's. I have a ST70 so if I know the gain of both amps I will have an idea of how the ST 35 compares to the SCA 35.
    The reason for my question is I have a SCA35 that I am using to drive very efficient speakers. I love the sound, [the best bass I have ever heard], except I know I am losing a lot of detail to the front controls and the 7199's are a major pain. I am trying to decide if I should replace the selector and volume and bypass everything else. I might change out the boards for the ST35 boards if there is enough gain. I may or may not leave the phono stage.

    Flyquail56

    Posts : 24
    Join date : 2009-05-04

    Re: Gain of the ST 35 versus the SCA 35 boards

    Post by Flyquail56 on Sun Jun 17, 2012 10:00 pm

    The insertion loss from the tone controls in the SCA-35 is about 17db. The power amp board of the SCA-35 has about 17db more gain than that of the ST-35 to compensate for the tone controls. So, deleting the tone controls and changing to the ST-35 circuit will net you essentially zero difference in gain. I have one such modded SCA-35 and one stock. If you don't need tone controls, this is a very worthwhile project for you.

    If you are planning to use the phono stage, it is somewhat mediocre in stock form, but can be changed to a tweaked version of the PAS phono stage. I wrote a step-by-step for the phono stage in Dynaco Doctor forum a few years ago. I can send it to you if you're interested.

    Best regards,
    Mike

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Gain of the ST 35 versus the SCA 35 boards

    Post by sailor on Mon Jun 18, 2012 5:36 pm

    Mike,
    Thank you, the info you posted is very helpful.
    I figured the phono stage was lacking. I would be interested in a copy of your phono mods although I am not spinning records at this time. I may want to go ahead and redo the phono board in the future. Do you need my email address or can you post to the board?

    Flyquail56

    Posts : 24
    Join date : 2009-05-04

    Re: Gain of the ST 35 versus the SCA 35 boards

    Post by Flyquail56 on Mon Jun 18, 2012 10:22 pm

    Here are the SCA-35 phono stage mods. It is really not much more than changing it to an improved PAS phono stage. I have seen several different RIAA feedback values recommended over the years, all claiming to be the best. I have chosen the values that Joe recommended for the PAS a few months ago with what I think give excellent results. So here it is; you’ll need the schematic to follow along. Also, this procedure will disable the (now obsolete) tape head input.

    Values given are for left channel, all resistors are ½ watt.

    R1 – 1.2M, no change unless PEC is removed from phono input jacks, then change to
    47K
    R2 – 150K, no change
    R3 – 5.6K, change to 47K
    R4 – 120 ohm, change to 1K 5% carbon film or 1% metal film
    R5 – 120 ohm, change to 1K 5% carbon film or 1% metal film
    R6 – 1.2M, no change
    R7 – 47K, change to 100K
    R8 – 390 ohm, change to 1K 5% carbon film or 1% metal film
    R9 – 18K, replace with jumper wire
    R10 – 100K, remove, do not replace
    R11 – 47K, replace with jumper wire
    C1 – 0.1 mfd, replace with 10K resistor
    C2 – 0.022/400V, replace with same value new cap
    C3 – 0.1 mfd/400, change to jumper wire
    C4 – 330 pF, remove, do not replace
    Replace wire from eyelet 11 on circuit board to lug 2 on selector switch with 0.1/400V capacitor.
    Remove wire from eyelet 3 to lug 4 on selector switch.
    Make up a new feedback network consisting of 2 resistors and 2 caps. Solder a 95K resistor in series with a 2M resistor, both 1% metal film. Then solder an 820 pF/500V silver mica or polystyrene cap in parallel with the 95K resistor, solder a 2700 pF/500V cap in parallel with the 2M resistor. Solder the 95K end of this network to eyelet 11, and the 2M end to eyelet 3. Be aware that space is really at a premium. I soldered these networks on the bottom of the board, but in doing so, you must ensure that there is no chance of shorting against any other wiring.

    That’s it! You are now ready to enjoy a much better sounding phono stage. My concerns about hum were unfounded, even though the unit uses AC on the heaters. No hum with the volume fully up when using LS3/5a speakers. If you’re using very high-efficiency speakers, the Sovtek 12AX7LPS might be a worthwhile investment. But I have used 70’s Mullards and new production Chinese 12AX7s with excellent results. Gain is reduced by about 10db from the original, but is still adequate.

    Beyond what I have already mentioned specifically, resistor and capacitor types are very subjective, so I’ll leave that issue alone other than to point out again that space is at a premium. I have used Illinois MWR metallized polyester and CDE metallized polyester coupling caps with good results, and they don’t take up a lot of space.

    Any further questions, please feel free to ask.

    Best regards,
    Mike



    Last edited by Flyquail56 on Mon Jun 18, 2012 10:23 pm; edited 1 time in total (Reason for editing : Removed some reduntant information.)

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Gain of the ST 35 versus the SCA 35 boards

    Post by sailor on Tue Jun 19, 2012 12:45 pm

    Thanks Mike for your time and very usable information.
    sailor/Dave

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