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


    Using the SDS Cap board with the VTA Driver

    Share

    Brinkman

    Posts : 14
    Join date : 2009-01-01

    Using the SDS Cap board with the VTA Driver

    Post by Brinkman on Thu Jan 08, 2009 6:54 pm

    Hey everyone.

    I know some people are interested in using combinations of the various after-market ST70 upgrades. Seeing as how both the SDS Capacitor/Power Supply board and VTA Driver are popular choices (and that installation directions for both assume they're being installed in an otherwise stock build), I thought I'd share how to utilize these boards in the same build.

    The first thing to consider is whether or not you're going to use the the bias supply on the VTA board or the bias supply on the SDS Cap board. Since either way can make use of the bias potentiometer feature of the VTA board, there really is no better choice. However, if you plan on auditioning various drivers in your ST-70, it's probably best to use the SDS bias supply since not all driver PCBs include one.

    If you choose to use the SDS Bias supply:
    The following can be omitted from the VTA assembly: D1, C17 & C18, and R39. Otherwise, assemble it per the instructions.
    No parts are omitted from the SDS assembly; once it is assembled, wire up the SDS per the instruction sheet, with the following exceptions:
    Nothing is wired to eyelet "L"
    Eyelet "R" on the SDS board is wired to the "B-" eyelet on the VTA board. If there is no "B-" eyelet on the VTA board, wire it to the right eyelet formerly reserved for R39 on the VTA PCB.
    Eyelet "19" is wired to the eyelet marked "B+" on the VTA board.
    Nothing is wired to Eyelet "20"

    The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap (replaced), red/black wire (already in use), and bias supply (redundant).

    If you choose to use the VTA Bias supply:
    The following can be omitted from the SDS assembly: D1, C9 & C10, and R11 & R12. Otherwise, assemble it per the instructions.
    No parts are omitted from the VTA assembly. Go ahead and assemble it, but do not install or wire it.
    Once the VTA board is assembled, set it aside and begin installing and wiring up the SDS board as instructed with the following exceptions:
    Nothing is wired to eyelet "L"
    Nothing is wired to eyelet "R"
    Nothing is wired to eyelet "Bias"
    Eyelet "19" is now wired to the eyelet marked "B+" on the VTA PCB
    Nothing is wired to eyelet "20"

    The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap which has effectively been replaced.

    Powering Up:
    Follow the VTA board instructions in regard to initial power up and B+. If your B+ is too low (rising to 320VDC and settling well below the specified 400-440VDC), you may have to jumper either R9 or R10 (or both) on the SDS board make the B- settle between 400-440VDC.

    The B- (bias supply) should measure around -50 to -55VDC where R39 meets the bias supply on the VTA board.

    Additionally, you may want to retain the pair of 10K bias pots in the event you choose to revert the ST-70 to stock or come across a driver board that does not feature on-board biasing of the output tubes. Furthermore, the quad cap's sole purpose is now aesthetic, though one could install a Triode/Ultralinear switch in it's stead.

    Good luck.

    Rex Everything

    Posts : 2
    Join date : 2009-01-15

    Re: Using the SDS Cap board with the VTA Driver

    Post by Rex Everything on Thu Jan 15, 2009 1:53 pm

    Is this the same set up for the VTA Mark III boards in use with the SDS set up?

    Brinkman

    Posts : 14
    Join date : 2009-01-01

    Re: Using the SDS Cap board with the VTA Driver

    Post by Brinkman on Thu Jan 15, 2009 5:06 pm

    Wish I could say. Unfortunately, I've never owned a Mark 3 though I really wish I had the extra money to try a pair. That wattage, with separate (improved) power supplies and either a VTA or Poseidon board would be a heck of a lot of fun.

    My best guess is that Dynaco had pretty similar parts layout for the underchassis and since the SDS board does not replace the bias, you'd follow the instructions for the installation outlined above regarding the SDS board in conjunction with the VTA board and using the VTA bias.

    Simply put, all you're doing with the SDS board is replacing the Quad cap. This should be wired to the VTA board at it's B+ eyelet. Perhaps there's an eyelet on the SDS board marked "eyelet 5" or "eyelet 6" (similar to eyelets 19 and 20 on the ST70 driver)? One of these eyelets would be the one to wire the B+ on the VTA board.

    Other than that, make sure the SDS board gets a red wire from the Power transformer, a wire from pin 8 of the rectifier, and both choke wires. These all should be in the directions or marked on the power supply board.

    "The VTA board can now be wired and installed as instructed, ignoring all references to the quad cap which has effectively been replaced."

    You may or may not need to jump the 6.8K resistor on the SDS board (if it's there at all); as I'm not sure what your B+ needs to be at.

    If my instructions are wrong, don't make any sense, or you feel unsure of what to do, please don't proceed. Like I said, I have no firsthand experience specific to the Mark 3 or it's upgrades and I wouldn't want to hurt you or your amps.

    But if everything works out, please post back.

    Best,
    Brinkman

    Rex Everything

    Posts : 2
    Join date : 2009-01-15

    Re: Using the SDS Cap board with the VTA Driver

    Post by Rex Everything on Fri Jan 16, 2009 1:03 am

    Thanks for the reply. I don't have either of the upgrade boards yet so it's going to be a while. I will post up whe I have them and the work is done.

    Sponsored content

    Re: Using the SDS Cap board with the VTA Driver

    Post by Sponsored content Today at 9:08 pm


      Current date/time is Fri Dec 09, 2016 9:08 pm