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


    ST 70 Cap voltage rating

    Share

    Dingojazz

    Posts : 49
    Join date : 2010-09-03

    ST 70 Cap voltage rating

    Post by Dingojazz on Thu Mar 02, 2017 10:53 pm

    Hello again,

    As a follow up to prior post, and since I will be replacing .1 uf caps at C1 and C2 in the st70 with the VTA 12AU7 board, this question: do the replacement caps need to be rated at 630 v like the originals (as I suspect), or could I get by with 400 v K40 PIOs?

    Thanks,

    K.
    avatar
    Peter W.

    Posts : 362
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST 70 Cap voltage rating

    Post by Peter W. on Fri Mar 03, 2017 8:42 am

    I believe that there is over 400V on those caps. And even if it were (slightly) under 400V, you do not want to be running caps so close to their rating. Go with 630V caps.
    avatar
    Bob Latino
    Admin

    Posts : 2525
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST 70 Cap voltage rating

    Post by Bob Latino on Fri Mar 03, 2017 11:23 am

    There is less than 200 volts on C1 (left channel) and C2 (right channel). The most recent schematic that I have from Roy states "136 V" at that point in the circuit. A 400 volt rated capacitor is more than enough to handle that part of the circuit.

    Bob

    Dingojazz

    Posts : 49
    Join date : 2010-09-03

    Thanks, Bob. It appears that

    Post by Dingojazz on Sat Mar 04, 2017 12:16 am

    C-3 and C4 may see a little higher voltage, -about 300V (?).  
    Would that call for a 600V rating, or would 400V be ok there as well?

    Thanks again,

    K.
    avatar
    Bob Latino
    Admin

    Posts : 2525
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST 70 Cap voltage rating

    Post by Bob Latino on Sat Mar 04, 2017 12:29 pm

    Dingojazz wrote:C-3 and C4 may see a little higher voltage, -about 300V (?).  
    Would that call for a 600V rating, or would 400V be ok there as well?

    Thanks again,

    K.

    On the newer VTA boards since early 2012, there is no C3 or C4 on the VTA driver board? C3 and C4 were eliminated when Roy went to a CCS (constant current source) in the tail of the phase splitter part of the circuit.

    If you are referring to the 4 main coupling caps (C5, C6, C7 and C8) on the VTA ST-70, consider that these 4 caps see about +275 volts DC on the inner lead and -40 VDC on the outer lead of each cap. This is a voltage swing of about 315 - 320 volts. A 400 volt rated cap here works just fine .. A 630 volt rated cap is not really needed on the main coupling caps ..

    Bob

    Dingojazz

    Posts : 49
    Join date : 2010-09-03

    Cap Rating

    Post by Dingojazz on Mon Mar 06, 2017 12:43 am

    Yes, I guess it's a pre-2012 board, as it has C3 and 4. I'm leaving the existing Solens in there for now, though I will watch them to see how they hold up.

    I have replaced the Solens at C1 and 2 with Russian K40s PIOs. They sound nice, though early in break in. C5 - 8 were already 400v K40s.

    Thanks Bob et al.,

    K.

    Sponsored content

    Re: ST 70 Cap voltage rating

    Post by Sponsored content


      Current date/time is Mon Jun 26, 2017 8:16 am