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    Issue with sylvania 6ca7

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    bktheking

    Posts : 51
    Join date : 2011-12-20

    Issue with sylvania 6ca7

    Post by bktheking on 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
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Issue with sylvania 6ca7

    Post by Bob Latino on 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

    bktheking

    Posts : 51
    Join date : 2011-12-20

    Re: Issue with sylvania 6ca7

    Post by bktheking on 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.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Issue with sylvania 6ca7

    Post by tubes4hifi on 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

    bktheking

    Posts : 51
    Join date : 2011-12-20

    Re: Issue with sylvania 6ca7

    Post by bktheking on 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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