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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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Bob Latino
Midwestside
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    VTA ST 70 Russian PIO cap discoloration

    Midwestside
    Midwestside
    Midwestside


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    Join date : 2019-03-20
    • Post n°1

    VTA ST 70 Russian PIO cap discoloration Empty VTA ST 70 Russian PIO cap discoloration

    Post by Midwestside Sat Jul 06, 2019 12:58 pm

    I recently blew a rectifier and had an EL 34 go bad.
    So checking some connections and checking a few things,
    I noticed one of these PIO caps on the pre board was discolored.
    The red wire there , next to the cap with the tape, once upon a time had a tiny nick in it, perhaps from a soldering iron melt.
    At first i thought the cap had been burnt somehow, but it looks too uniform and nothing else is scorched.
    The discoloration is a bit darker than represented in the picture. I am now thinking , perhaps a staining from its contents inside?
    Wondering if its a 'bad one' and if so, should i replace?
    Is anyone able to speak to what this might be?




    VTA ST 70 Russian PIO cap discoloration Photo_10
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

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    Post by Bob Latino Sat Jul 06, 2019 1:30 pm

    Hi John,

    All the Russian PIO caps are pretested by me before they go out to the kit builder. Sometimes a Russian PIO cap may have a mark or some type of external blemish on the outside of the cap. Those green K42-Y PIO caps must measure .220 uF + or - 10% before they are shipped out with a kit. I doubt that the external darkening on the end of that cap had anything to do with your EL34 tube going bad. It looks to me that the end of your soldering pencil was near the cap end and the heat from the soldering pencil burned the green paint on outside of the cap.

    That said > If you want a new cap for that position, Email me and I can send you out a new .22 uF K-42Y PIO cap for that position.

    Bob
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    Midwestside
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    • Post n°3

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    Post by Midwestside Sat Jul 06, 2019 1:42 pm

    Bob Latino wrote:Hi John,

    All the Russian PIO caps are pretested by me before they go out to the kit builder. Sometimes a Russian PIO cap may have a mark or some type of external blemish on the outside of the cap. Those green K42-Y PIO caps must measure .220 uF + or - 10% before they are shipped out with a kit. I doubt that the external darkening on the end of that cap had anything to do with your EL34 tube going bad. It looks to me that the end of your soldering pencil was near the cap end and the heat from the soldering pencil burned the green paint on outside of the cap.

    That said > If you want a new cap for that position, Email me and I can send you out a new .22 uF K-42Y PIO cap for that position.

    Bob

    Ah, I bet you are right. I may not have caught this when it happened and assumed this happened in the meantime while using the amp.
    IF this is the case, im probably good.
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    Roy Mottram
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    • Post n°4

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    Post by Roy Mottram Mon Jul 08, 2019 1:43 pm

    I see that you do not have those capacitors insulated from the PCB. They should always be fully wrapped in heat shrink insulation or electrical tape.
    If the metal body contacts the circuit board they can short out components on the PCB.
    They do have a thin layer of paint on them, but it doesn't take much with the sharp edges of tube sockets and resistor leads to poke thru that.
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    Peter W.
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    • Post n°5

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    Post by Peter W. Tue Jul 09, 2019 10:27 am

    I am sorry, but installing a Russian-made part, very nearly 60 years old, in an expensive amp, where spectacular failure may result in spectacular costs just does not make sense in what passes for my mind. I do not care how often they may have been tested and/or screened. They remain a vintage part from a Soviet source.

    https://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/DME10P22K-F?qs=sGAEpiMZZMv1cc3ydrPrFxxB%252B0gnJhYeEKGyj6VY858%3D

    Would cover it in my book. There are many others as well.
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    Roy Mottram
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    • Post n°6

    VTA ST 70 Russian PIO cap discoloration Empty Re: VTA ST 70 Russian PIO cap discoloration

    Post by Roy Mottram Tue Jul 09, 2019 12:48 pm

    Peter . . . .  kind of like tubes, huh ?

    your reference . . . . polyester caps???? no way !!!!!!!!


    Last edited by tubes4hifi on Tue Jul 09, 2019 4:59 pm; edited 1 time in total
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    Peter W.
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    • Post n°7

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    Post by Peter W. Tue Jul 09, 2019 1:11 pm

    tubes4hifi wrote:Peter . . . .  kind of like tubes, huh ?

    I am going to sit on my fingers for at least 24 hours on this one. Serious snark warning.
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    Bob Latino
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    • Post n°8

    VTA ST 70 Russian PIO cap discoloration Empty Re: VTA ST 70 Russian PIO cap discoloration

    Post by Bob Latino Tue Jul 09, 2019 1:14 pm

    Peter W. wrote:I am sorry, but installing a Russian-made part, very nearly 60 years old, in an expensive amp, where spectacular failure may result in spectacular costs just does not make sense in what passes for my mind. I do not care how often they may have been tested and/or screened. They remain a vintage part from a Soviet source.

    https://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/DME10P22K-F?qs=sGAEpiMZZMv1cc3ydrPrFxxB%252B0gnJhYeEKGyj6VY858%3D  

    Would cover it in my book. There are many others as well.

    PeterW. > I have used over 6000 pieces of the Russian K40-Y and K42-Y paper in oil (PIO) caps in the past 13 years. In that time I only know of only ONE case where a Russian PIO cap failed. The Russian caps are VERY reliable. When used as coupling caps they are 500 volt rated caps looking at about 325 volts. For you to insinuate that they may not be reliable is an "off hand" remark by you that is not backed up with any proof or a generalized indictment of Russia's ability to make quality gear for their military. Your statement makes no sense.

    Bob
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    Jim McShane
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    • Post n°9

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    Post by Jim McShane Tue Jul 09, 2019 4:22 pm

    I agree with Bob 100%! I have used 100s of the Russian K40 PIO caps and I have had ZERO failures. I have absolute confidence in them.

    These caps are very well made. Hermetically sealed PIO caps like these have a VERY long life - so unless it is a cap that was defective at the time of manufacture the chances of failure are quite remote. i use them under more difficult conditions than Bob does in the VTA amps (higher voltages and operating temperatures) and I can vouch for them being superb performers under those conditions.

    For me (and Bob also apparently) to change my opinion of these caps would take a large body of verified evidence. I'm not aware of ANY such evidence.
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    audiobill
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    • Post n°10

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    Post by audiobill Wed Jul 10, 2019 6:36 am

    Agree with Bob and Jim - I've built over 100 VTA amps, without one cap failure.  Peter, you again need to think about basing your comments on facts rather than "one man's opinion".


    Bill
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