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    Can You Identify The Quadcaps On This PCB



    Posts : 23
    Join date : 2013-10-16

    Can You Identify The Quadcaps On This PCB

    Post by EddieGnz1 on Fri Nov 29, 2013 7:12 pm

    Do you know how to identify capacitors on an ST-70?

    which capacitors on this ST-70 kit are the replacement of the Quadcap?

    PCB Photo

    here's a list of the parts in the above board;

    Parts List

    Which one replaced the 80?
    which one replaced the 40?
    which one replaced the 30?
    which one replaced the 20?

    Kind thanks,

    Bob Latino

    Posts : 2382
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Can You Identify The Quadcaps On This PCB

    Post by Bob Latino on Fri Nov 29, 2013 7:57 pm

    C1 and C2 together replace the 40 section
    C3 and C4 together replace the 80 section
    C5 and C6 together replace the 20 section
    C7 and C8 together replace the 30 section

    Although the first section (C1 and C2) are together 800 VDC rated (two 400 volt rated caps in series), the last three sections on that cap board are only rated at 500 VDC. The original Dynaco quad cap was rated at 525 volts - so - what that cap board from Triode Electronics (designed by Sheldon Stokes) gives you is larger capacitance than the original quad cap or the 80, 40, 30, 20 uF replacement caps at the expense of LOWER VOLTAGE ratings on the last three sections.



    Posts : 381
    Join date : 2008-12-05

    Re: Can You Identify The Quadcaps On This PCB

    Post by PeterCapo on Fri Nov 29, 2013 8:40 pm

    I have one of these.  It might not be strictly a Sheldon Stokes design.  My impression is that it might have been designed by Triode USA when Matt Ihnen (later of Blackburn Audio) was at Triode.

    I was concerned about the lower voltage rating, especially for the second section.  But, after assembling it into my new Stereo 70, I measured continuous voltage on those nodes well within the working voltage ratings for the electrolytics, especially using the new Dynakitparts PT that is wound for 122 VAC and with a thermistor in the PT primary.  I ran my line up to 130VAC with the variac and the voltages on those nodes were still well within the working voltage ratings.  If anyone is using an original PT, I suspect you’d still be within voltage tolerance, but with an original PT and a line voltage in excess of 125 VAC, it would be a good idea to put one or two thermistors in series with the PT primary, like an Ametherm SL22 12103, to drop the line a bit.

    Also, each one of those electrolytics in the second, third and fourth sections has a working voltage rating of 250VDC and typically a surge rating of 300VDC, which means each of those sections actually has an overall surge rating of 600 VDC, which I would think should provide enough headroom for a turn-on surge - and if you have a thermistor (or two), a turn-on surge may not even be an issue.

    I did change the values of the stock resistor pairs in parallel with the capacitor pairs.  Those stock resistor values are fine for bleeding the electrolytics when the amp is shut off.  To enhance protection for equalizing the voltage across the series-connected electrolytics, the resistor values should be adjusted downward; I looked up the leakage formulas for the electrolytics and took it from there.  As a result of adjusting the resistor pairs in parallel with the electrolyitcs, it was then necessary to adjust the 6.8k and 22k B+ resistors to get the voltages correct at the nodes.  Works great.


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