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    VTA Octal Driver and SDS Labs Quad Cap Replacement Board

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    achdee55

    Posts : 3
    Join date : 2012-09-24
    Age : 61
    Location : Savannah, Georgia

    VTA Octal Driver and SDS Labs Quad Cap Replacement Board

    Post by achdee55 on Mon Sep 24, 2012 2:20 pm

    Hi everyone,
    I'm new to the forum and a tube amp novice. I have a pair of Mark 3's that I'm trying to restore. I recently purchased the VTA Octal Driver Kit and the SDS Labs Quad Cap Replacement Board. My questions: Are these kits compatible? Has anyone else installed these two together? Are there any wiring considerations to be made?
    Any help would be appreciated. Thank you,
    Harold

    Bob Latino
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    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA Octal Driver and SDS Labs Quad Cap Replacement Board

    Post by Bob Latino on Mon Sep 24, 2012 2:45 pm

    Hi Harold and welcome to the Dynaco Tube Audio Forum,

    The VTA octal driver board and the SDS quad cap replacement board are compatible with each other on the Dynaco Mark III. The SDS board just replaces the quad cap. You should leave the quad cap in it's place with no wires connected. If you do remove the quad cap you will be left with an empty hole in the center of the amp.

    Just follow the SDS board instructions for installation. You have the option of using diodes on the SDS board to replace the GZ34 tube rectifier. Personally, I would omit the diodes and just use a GZ34 tube rectifier like on the stock Mark III.

    For the bias system - You will be using the on board bias system on the VTA driver board and the not the stock bias system. As with the quad cap, I would leave the original bias pot in the amp so as not to have an empty hole showing on the top of the amp.

    Bob

    achdee55

    Posts : 3
    Join date : 2012-09-24
    Age : 61
    Location : Savannah, Georgia

    Re: VTA Octal Driver and SDS Labs Quad Cap Replacement Board

    Post by achdee55 on Mon Sep 24, 2012 3:40 pm

    Thanks Bob, I appreciate the reply. I will follow your recommendations on the original quad cap and the original bias pot. I want to keep these amps as "original looking" as possible. I've been reading some of the posts here, and I will benefit greatly from the experience and expertise that members are willing to share!

    Thanks again,
    Harold

    stewdan

    Posts : 160
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: VTA Octal Driver and SDS Labs Quad Cap Replacement Board

    Post by stewdan on Mon Sep 24, 2012 8:41 pm

    Hi Harold --

    I have Mark IIIs with Roy's VTA Lo Gain (12AU7) Boards mated with the SDS Cap Board and I also have Mark IIIs with Roy's VTA Octal (6SN7) Boards, also mated with the SDS Cap Board.

    There was Forum Post back in March 2012, with the title "octal option??". Use the Forum's Seach Engine to scan for "octal option"

    Take a look at the 2nd page of the Post (actually, Post #24) where Roy posted a wiring diagram which is valid for the Octal and 12AU7/12AT7 versions of the Mark III VTA Board. <<<======= INVALUABLE!!!

    *********************************************************
    The wiring of the SDS Cap Board to a Mark III with the VTA Board is as follows:

    Connection points to the cap board are silk screened on the board for the "4 sections" that basically parrot the Dynaco Quad Cap. I used what Bob suggested and kept the GZ34 Rectifier Tube, so no diodes went to the Cap Board.

    (1) connect the Red/Black wire from the Mark3 Bias line to the BIAS Eyelet of the VTA driver board.

    (2) connect Section 1 of the Cap Board to pin #8 of the GZ34 and one side of the choke.

    (3) connect Section 2 of the Cap Board to the other side of the choke and the red tap wire from the output transformer.

    (4) connect Section 3 of the Cap Board to one end of a resistor, 2200 ohm 2 watt? And run a jumper wire from Section 3 back to Section 2.

    (5 connect Section 4 of the Cap Board to the other end of the resistor, and run a wire from section 4 to the B+ eyelet of the Mark 3 VTA board.

    *****************************************************************
    So, between sections 2 and 3 of the Cap Board you run a jumper wire

    And between sections 3 and 4 you run a 2200 ohm 2Watt resistor.
    *******************************************************************

    The Stock Mark III had 2 wires coming from the Stock Quad Cap to the Dynaco PCB --- solder points 5 and 6.

    The VTA board only has 1 wire coming to the PCB from the Quad Cap, hence the jumpered sections of the Cap Board.

    *********************************************************************

    Hope this info helps. If you have questions, just ask.

    Stew









    achdee55

    Posts : 3
    Join date : 2012-09-24
    Age : 61
    Location : Savannah, Georgia

    Re: VTA Octal Driver and SDS Labs Quad Cap Replacement Board

    Post by achdee55 on Tue Sep 25, 2012 8:37 am

    Thanks Stew,
    Thank you for the detailed wiring instructions. The stock Mark III wiring from point 5 and point 6 is what had me confused. Now I know what to jumper on the Cap board to compensate for the single wire going to the VTA board.

    Thanks again for sharing your time and expertise.
    Harold

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