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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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    Issue with sylvania 6ca7

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    bktheking


    Posts : 51
    Join date : 2011-12-20

    Issue with sylvania 6ca7 Empty Issue with sylvania 6ca7

    Post by bktheking Wed Jul 25, 2012 10:38 am

    in my amp, with v11 VTA board running the SDS cap board, I have a 6ca7 - sylvania that biases up perfectly then after a minute runs away. It tests at 78 percent in my tube tester without issue. I've read a few sites that talked about these tubes being superior to anything new yet so many of them have this thermal runaway issue and it's more predominant in St-70's. Is there anything that can be done like changing out a grid resistor or is this tube simply destined for the trash? My jj's operate flawlessly.

    Many thanks
    Bob Latino
    Bob Latino
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    Issue with sylvania 6ca7 Empty Re: Issue with sylvania 6ca7

    Post by Bob Latino Wed Jul 25, 2012 10:59 am

    If that Sylvania 6CA7 has runaway bias no matter what tube socket you place the tube in, then the tube is bad. If the tube is new, I would send it back to the vendor and request a replacement.

    Bob
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    bktheking


    Posts : 51
    Join date : 2011-12-20

    Issue with sylvania 6ca7 Empty Re: Issue with sylvania 6ca7

    Post by bktheking Wed Jul 25, 2012 11:06 am

    Any socket it goes nuts, it came in a box of tubes I bought at a hamfest, probably why it was in the box.
    Roy Mottram
    Roy Mottram
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    Issue with sylvania 6ca7 Empty Re: Issue with sylvania 6ca7

    Post by Roy Mottram Wed Jul 25, 2012 12:50 pm

    try changing R29 - R32 to 150K rather than 220K or 270K as was original, that has helped me in the past with tubes with this type of problem
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    bktheking


    Posts : 51
    Join date : 2011-12-20

    Issue with sylvania 6ca7 Empty Re: Issue with sylvania 6ca7

    Post by bktheking Tue Jul 31, 2012 9:36 am

    Without changing anything- I got another sylvania 6ca7 and put it in with the tube that was causing the issue. Last night I biased it up and it held stable for 15 minutes.

    Strange.



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    Issue with sylvania 6ca7 Empty Re: Issue with sylvania 6ca7

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