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    Quad cap use and other questions



    Posts : 5
    Join date : 2009-03-03

    Quad cap use and other questions

    Post by BruceB on Thu Mar 05, 2009 10:44 pm

    I just received Roy's upgrade board for the ST70, and have a question before I go ordering parts to populate it. Is the original quad cap replaced by C13, C14, C15, C16, or am I missing something? It isn't clear in his manual.

    I have a Curcio PS board in my amp right now (as well as his driver board, but it is coming out). I could take out the PS board and get a new quad cap, as the old one is only on for aesthetics.

    And, what are the values for the other set of caps you can get for C13, C14, C15, C16?

    Just a little confused.

    Thanks for the advice



    Posts : 58
    Join date : 2009-01-19

    Still need quad capacitor or equivalent

    Post by danf on Fri Mar 06, 2009 7:54 am

    Hi Bruce,

    You still need the quad capacitor or an external board equivalent. C13-C16 are used in addition to rather than in place of the quad capacitor. The one change is that you can free up one section of the quad capacitor and parallel that with the first capacitor section after the choke. Have a look at the picture of the underside of a new kit ST70 on Roy's page. Also look at:

    Good luck!


    Posts : 5
    Join date : 2009-03-03

    Re: Quad cap use and other questions

    Post by BruceB on Fri Mar 06, 2009 11:40 pm

    Thanks Dan,
    I thought that might be the case. This will be a slow project, as I'm mot in a hurry to get this done. I have a pair of Welborne Labs 2A3 Moondogs that I normally use. I wasn't too happy with the way the Curcio mod came out on the ST-70. I did it about four years ago, and figured it was time to try this one, which I came accross right after I did the other mod.
    Bob Latino

    Posts : 2644
    Join date : 2008-11-26
    Location : Massachusetts

    Re: C 13,C 14, C 15, C 16

    Post by Bob Latino on Sat Mar 07, 2009 10:15 am


    On the two center caps (C13 and C14) you can go up to 120 uF on each cap. On C15 and C16 Roy recommends that you can go up to 47 uF. Just make sure that you use 450 volt rated caps. The normal power supply caps for the driver board are four 22 uF caps. (88 uF total) With the upgraded caps you will have 334 uF "on board" for the driver board.

    The nice thing about the VTA driver board is that it DOES HAVE it's own DC power storage to run the board. Most replacement driver boards for the ST-70 (Curcio etc.) do NOT have an "on board" power supply and must "rob" DC power from the quad cap or any underchassis quad cap replacement board. When you have the power supply for the driver board right ON the driver board the board can react quicker to changes in the audio signal. Another benefit of this arrangement is that now the quad cap is free to power just the two output transformers. It is amazing that Dynaco got away with just 20 uF on the second section of the quad cap which powers the output transformers. One of the tricks that you can do on an original amp is to add more capacitance to the second section of the quad cap. You should notice an increase in bass response when playing the amp at higher volume levels.


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