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    Bias not stopping to zero

    peterh
    peterh

    Posts : 1317
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh on Wed Apr 22, 2020 5:12 pm

    B&WTube wrote:I hate to bring it up again, but I am having issues with the same back right socket again. I was quietly listening to music, and heard heavy distortion and the tube was glowing red hot. I cut the amp off for a while, turned the bias all of the way down, and then fired it back up. No problems after 30 minutes. So I added the bias, but it will not go high enough- with the highest bias pot position i can only get to .240 Other GL kt77's are at .450).

    After the last round of issues put in a new tube, tested all the resistors around this socket as well as the ground- everything was within spec. I also re-soldered everything related to the socket. Life has been good for the last almost 3 months. Amp had been on for 2 hours before this. Anyone have any advice?
    New power tubes might be what's needed. Don't reuse a tube that has redplated.
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    B&WTube

    Posts : 35
    Join date : 2019-08-19

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    Post by B&WTube on Wed Apr 22, 2020 5:45 pm

    peterh wrote:
    B&WTube wrote:I hate to bring it up again, but I am having issues with the same back right socket again. I was quietly listening to music, and heard heavy distortion and the tube was glowing red hot. I cut the amp off for a while, turned the bias all of the way down, and then fired it back up. No problems after 30 minutes. So I added the bias, but it will not go high enough- with the highest bias pot position i can only get to .240 Other GL kt77's are at .450).

    After the last round of issues put in a new tube, tested all the resistors around this socket as well as the ground- everything was within spec. I also re-soldered everything related to the socket. Life has been good for the last almost 3 months. Amp had been on for 2 hours before this. Anyone have any advice?
    New power tubes might be what's needed. Don't reuse a tube that has redplated.

    I swapped it out with a new tube, and got the bias set back where it should be.

    The concerning part is that this is the 3rd red plating of this socket since I built it in August of 2019. 3 Tubes in 7 months in only the back right socket indicates I have a problem.. BUT, if the resistors and the grounds are fine, and everything on that socket has been re-soldered- I don't know where to look for the issue.
    peterh
    peterh

    Posts : 1317
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Bias not stopping to zero  - Page 2 Empty Re: Bias not stopping to zero

    Post by peterh on Wed Apr 22, 2020 5:54 pm

    B&WTube wrote:
    peterh wrote:
    B&WTube wrote:I hate to bring it up again, but I am having issues with the same back right socket again. I was quietly listening to music, and heard heavy distortion and the tube was glowing red hot. I cut the amp off for a while, turned the bias all of the way down, and then fired it back up. No problems after 30 minutes. So I added the bias, but it will not go high enough- with the highest bias pot position i can only get to .240 Other GL kt77's are at .450).

    After the last round of issues put in a new tube, tested all the resistors around this socket as well as the ground- everything was within spec. I also re-soldered everything related to the socket. Life has been good for the last almost 3 months. Amp had been on for 2 hours before this. Anyone have any advice?
    New power tubes might be what's needed. Don't reuse a tube that has redplated.

    I swapped it out with a new tube, and got the bias set back where it should be.

    The concerning part is that this is the 3rd red plating of this socket since I built it in August of 2019. 3 Tubes in 7 months in only the back right socket indicates I have a problem.. BUT, if the resistors and the grounds are fine, and everything on that socket has been re-soldered- I don't know where to look for the issue.

    If this is a VTA-70 the grid resistors for power tubes are 270k in my documentation. That is fine
    ( or at least within the book ) for EL34. But it's outside spec for KT77, the KT77 in fixed bias and >25w
    stipulates at most 250k as grid resistor. This might explain the tendency to have the tubes go red.

    Get EL34 or reduce the grid resistors to 100k.

    /PS
    as a security i would also recommend "screen stoppers" installed, soldered at the tube socket pin4.
    Carbon comp 1/2w 1kohm will be fine. This will prevent parasitical high frequence oscillations that
    is very hard on tubes.
    /DS
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    B&WTube

    Posts : 35
    Join date : 2019-08-19

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    Post by B&WTube on Wed Apr 22, 2020 6:10 pm

    peterh wrote: If this is a VTA-70 the grid resistors for power tubes are 270k in my documentation. That is fine
    ( or at least within the book ) for EL34. But it's outside spec for KT77, the KT77 in fixed bias and >25w
    stipulates at most 250k as grid resistor. This might explain the tendency to have the tubes go red.

    Get EL34 or reduce the grid resistors to 100k.

    /PS
    as a security i would also recommend "screen stoppers" installed, soldered at the tube socket pin4.
    Carbon comp 1/2w 1kohm will be fine. This will prevent parasitical high frequence oscillations that
    is very hard on tubes.
    /DS

    Interesting. Bob Latino is the one who said KT77's are ok in his VTA-70, and then provided the recommended bias of .450 . However, I am showing 270k resistors on my VTA board instructions (if I am looking at the right thing). If it is beyond spec, I wonder if I am pushing it even further beyond the envelope with my Weber SS rectifier and 12BH7A signal tube. Great insight- thank you!
    avatar
    B&WTube

    Posts : 35
    Join date : 2019-08-19

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    Post by B&WTube on Sat May 23, 2020 2:50 pm

    I gave up on the amp for a while, and now I am back at it. I was just measuring things (not totally knowing what I am doing), and I discover that Pin 6 on the VTA board for the V6 tube socket (front right), doesn’t show any resistance. All other Pin 6’s at the board and on the socket measure around .295 @ the 2M setting in my multimeter.

    I re-soldered a new wire and still no reading. I’m not sure what this indicates, but it seems something isn’t right. Thoughts?

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