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


    Russian K40-Y Caps Recommended Value

    Share

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Russian K40-Y Caps Recommended Value

    Post by BNR_1 on Wed Dec 11, 2013 10:07 am

    So I read often on the recommendation to try the Russian K40-Y capacitors for the Dynaco amps. It provides a good bang for the buck. I do see that the voltage rating should be no less than 400 volts and to give it around 200 hours of break-in time. However, very few instances in the postings are recommended values for the cap. Specifically for the Mark III with the upgraded VTA board what specific value of the K40s are recommended? 0.33? 0.22?

    Thanks

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Russian K40-Y Caps Recommended Value

    Post by Bob Latino on Wed Dec 11, 2013 12:37 pm

    Hi,

     You can really use any cap between .10 uF to .33 uF and not hear any difference in the sound of the amp. I have swapped in and out all values between .10 uF and .33 uF on the K40-Y's in my own VTA ST-120 with no change to the sound that I could hear. Now slightly larger caps will lower the -3 dB point of the amp's low frequency response but we are talking about maybe the 4 Hz to 12 Hz area of the frequency spectrum - well below human hearing.

    Yes - The key thing is to always use a cap with a 400 volt rating or higher. On the VTA ST-70 the four Russian K40-Y caps look at about 250 VDC on the inside lead on the board and -35 to -40 VDC or so on the outside lead - a voltage swing of almost 300 volts. So a 400 volt rated cap is really needed here.

    These caps do need time to break in and 100 to 200 hours looking at voltage will do it .. I like the K40-Y's because once broken in, they give a very smooth (yet detailed) top end and a very nice soundstage in terms of both width and placement of instruments within the sound stage.

    Below is a photo which shows what the K40-Y's look like inside. I cut the metal case away and there is a cardboard cylinder inside that then a "jelly roll" of aluminum foil and oil impregnated paper. The oil is almost like a thin grease .. It's thick ...

    Bob



    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Russian K40-Y Caps Recommended Value

    Post by sKiZo on Wed Dec 11, 2013 3:15 pm

    The oil is almost like a thin grease .. It's thick ...


    KY jelly?  geek 

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Russian K40-Y Caps Recommended Value

    Post by BNR_1 on Wed Dec 11, 2013 3:20 pm

    Thank you Bob for elaborating.

    "You can really use any cap between .10 uF to .33 uF and not hear any difference in the sound of the amp"

    When in doubt split the difference......0.22uF.

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Russian K40-Y Caps Recommended Value

    Post by GP49 on Wed Dec 11, 2013 10:14 pm

    You don't want to go too low on capacitance...if you limit the low frequency bandwidth so that some frequencies do not pass, the result will be a large feedback signal to the first stage, in an attempt to correct the roll-off. Not good.

    Some say larger-value capacitors don't sound as good, all other things being equal. However, since those "other things" are seldom equal, it's hard to judge the truth of this claim.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Russian K40-Y Caps Recommended Value

    Post by arledgsc on Thu Dec 12, 2013 10:17 am

    "You don't want to go too low on capacitance"

    You shouldn't go too high a value either as you don't want to pass near DC voltages that take a long time to settle due to the added capacitance.  This "blocking" distortion can throw off the DC bias until the voltage bleeds.  0.22uF are good values to try.

    DarthBubba

    Posts : 81
    Join date : 2012-05-05

    Re: Russian K40-Y Caps Recommended Value

    Post by DarthBubba on Fri Dec 13, 2013 12:23 am

    BNR_1 wrote:So I read often on the recommendation to try the Russian K40-Y capacitors for the Dynaco amps.    However, very few instances in the postings are recommended values for the cap.  Specifically for the Mark III with the upgraded VTA board what specific value of the K40s are recommended?

    Let's assume that the circuit designer was competent, knew the math, and had the equipment to measure the amp's output.  What's wrong with using the original values?  You certainly do not want to go below the design voltage (irrespective of the part's actual voltage rating) just as a matter of safety, and wandering too far from the capacitor's value (uF) risks throwing off the circuit operating parameters.  Diddle too much with some capacitor values and you end up with an expensive oscillator.  Or a dead rectifier tube.  Is there truly any realism added to the music by being able to amplify record warps?

    I vote for staying pretty close to the designer's value, whether it be on the original Dynaco input board or the VTA (or any other brand) upgrade board.

    Just my two cents.

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Russian K40-Y Caps Recommended Value

    Post by BNR_1 on Sat Dec 14, 2013 11:44 am

    Thanks Darth. You do bring up valid points. This is why I asked these questions. I have no experience whatsoever as to the recommended capacitor values and seek feedback with others that do.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Russian K40-Y Caps Recommended Value

    Post by arledgsc on Sun Dec 15, 2013 10:33 am

    "I have no experience whatsoever as to the recommended capacitor values and seek feedback with others that do."

    What is the standard value for the VTA board? I would just use that value.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Russian K40-Y Caps Recommended Value

    Post by Bob Latino on Sun Dec 15, 2013 11:04 am

    Hi Scott,

    There is no real standard value for the Russian K40-Y coupling caps on the VTA driver board ... Just use any value between .10 uF and .22 uF. Dynaco on their original ST-70 used .10 uF caps .. Right now on the VTA driver board I am using either .15 uF or .22 uF Russian K40-Y. Just make sure you use a cap rated at 400 volts or higher. On the VTA M-125 amps the caps are always 630 volt rated. The reason is those K40-Y caps on the VTA M-125 must handle more current since you have TWO output tubes being supplied by current through each coupling cap instead of one tube like on the VTA ST-70 and ST-120 amps.

    Since 2006, and using over 3000 of the K40-Y caps on the VTA amps, I have seen just ONE failure of a Russian K40-Y PIO cap. These K40-Y's are military grade caps and were build to very high standards.

    Bob

    Sponsored content

    Re: Russian K40-Y Caps Recommended Value

    Post by Sponsored content Today at 4:02 pm


      Current date/time is Thu Dec 08, 2016 4:02 pm