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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


    Bias not stopping to zero

    peterh
    peterh

    Posts : 1364
    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 5:12 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 : 36
    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 : 1364
    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 : 36
    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!
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    B&WTube

    Posts : 36
    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?
    tubes4hifi
    tubes4hifi
    Admin

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    Join date : 2008-11-30

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    Post by tubes4hifi on Wed Jun 03, 2020 9:00 pm

    sorry I only check in here about once a week . . .
    So, there is only one connection to pin 6, it goes to the driver board.
    Since all the other output tubes measure a certain amount from pin 6, so should the one that reads bad.
    So the problem would either be a bad solder connection on the wire (either end) or something like a bad solder connection
    on the driver board, for that output only. Simple to check and compare.
    MechEngVic
    MechEngVic

    Posts : 71
    Join date : 2019-01-16

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    Post by MechEngVic on Sat Jul 25, 2020 1:52 am

    B&WTube,

    Any progress on your issue? Had recent trouble similar to yours with my Dynaco ST-70 Series II, was causing me trouble for months and several tubes, then just settled into the rare half second buzz. No more arcing or red-plating. I have several mods waiting in the wings if the problem comes back including lowered value coupling caps, carbon comp screen resistors, carbon comp control grid resistors (solder directly to pin 5), and two cl-80 thermistors for a softer start up.

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