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    ST-120 power tube red plate issue

    calauver1
    calauver1

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    Post by calauver1 on Wed Jan 15, 2020 2:16 pm

    Good morning,

    Yesterday while listening to my VTA ST-120, I suddenly noticed a drop in volume and looked to see one of the KT88's glowing red. I immediately shut down and headed to this forum. I found a thread that indicated some things I should check. http://dynacotubeaudio.forumotion.com/t3871-serious-red-plate-event?highlight=kt88+red+plate

    These are the things I checked.
    A. Socket is fine
    B. All 10-ohm resistors from pin 1 & 8 to ground read correctly
    c. All 1K-ohm resistors between pins 5 & 6 read correctly
    d. R29 - 32 (mine are 150K) all read fine
    e. Checked the grid voltage at pin 5 with all tubes removed and voltages are in line and smooth change when the pot is rotated.
    f. Checked the tube for any shorts and found none. ( I don't have any way to test the tubes ).

    My amp is exactly 3 years old and the only issue I have had is the 5AR4 rectifier tube went out about 1 1/2 years ago. At that time I did the Yellow sheet Mod and put in a new rectifier tube.

    About two or three weeks ago I notice a little spark inside the rectifier tube when I powered it on, but it did not go out and the amp worked fine and has worked fine up until yesterday.

    Since everything checked out OK and the only thing I could think might be the problem was the rectifier, I put in a new rectifier, the same KT88 tubes, and powered up as a new start up. Everything seems to be working fine although it does seem like it takes a while for the bias to stabilize, like 10 min. or so. Also I have checked my voltage from my wall outlet and it varies between 121-124 vac.

    My questions are.
    1. Can the rectifier tube cause this issue?
    2. Is there anything else I should check?
    3. Should I change the R29-32 resistors to 100k as they are in the newer kits.
    4. Also thinking now about adding the auto bias board. Yes or No?

    Thanks for advice.
    peterh
    peterh

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    Post by peterh on Wed Jan 15, 2020 4:00 pm

    calauver1 wrote:Good morning,

    Yesterday while listening to my VTA ST-120, I suddenly noticed a drop in volume and looked to see one of the KT88's glowing red. I immediately shut down and headed to this forum. I found a thread that indicated some things I should check. http://dynacotubeaudio.forumotion.com/t3871-serious-red-plate-event?highlight=kt88+red+plate

    These are the things I checked.
    A. Socket is fine
    B. All 10-ohm resistors from pin 1 & 8 to ground read correctly
    c. All 1K-ohm resistors between pins 5 & 6 read correctly
    d.  R29 - 32 (mine are 150K) all read fine
    e.  Checked the grid voltage at pin 5 with all tubes removed and voltages are in line and smooth change when the pot is rotated.
    f.  Checked the tube for any shorts and found none. ( I don't have any way to test the tubes ).

    My amp is exactly 3 years old and the only issue I have had is the 5AR4 rectifier tube went out about 1 1/2 years ago. At that time I did the Yellow sheet Mod and put in a new rectifier tube.

    About two or three weeks ago I notice a little spark inside the rectifier tube when I powered it on, but it did not go out and the amp worked fine and has worked fine up until yesterday.

    Since everything checked out OK and the only thing I could think might be the problem was the rectifier, I put in a new rectifier, the same KT88 tubes, and powered up as a new start up. Everything seems to be working fine although it does seem like it takes a while for the bias to stabilize, like 10 min. or so. Also I have checked my voltage from my wall outlet and it varies between 121-124 vac.

    My questions are.
    1. Can the rectifier tube cause this issue?
    2. Is there anything else I should check?
    3. Should I change the R29-32 resistors to 100k as they are in the newer kits.
    4. Also thinking now about adding the auto bias board. Yes or No?

    Thanks for advice.
    I'd say: get a new matched pair kt88 ( or a quad). 3 years is some time.
    Power tubes might by time acquire debris on the grid that causes current runaway, especially
    if the grid resistor is at or above recommendation. GEC recommend < 100kohm.
    A tube that has redplated cannot be trusted.
    The flash seen in the rectifier might been triggered by a flash in the kt88, not the other way
    around. But you might get a new GZ34 and have handy whenever a new is needed.
    MechEngVic
    MechEngVic

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    Post by MechEngVic on Wed Jan 15, 2020 6:39 pm

    I would listen to Peterh on this one, his comments mirror my experience also. A tube can red-plate for several reasons, but once it does, it is damaged. It will more easily short and arc in the future even though it seems to be working fine at the moment. It will also start taking other components out with it if you leave it in. Once you replace them, I would re-do my voltage checks. Hopefully, no resistors or capacitors are damaged. Three years is a good amount of time for a set of tubes.
    tubes4hifi
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    Post by tubes4hifi on Sat Jan 18, 2020 1:57 pm

    also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.
    calauver1
    calauver1

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    Post by calauver1 on Sat Jan 18, 2020 2:40 pm

    tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

    Best to change them out then, correct?
    calauver1
    calauver1

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    Post by calauver1 on Sat Jan 18, 2020 3:53 pm

    calauver1 wrote:
    tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

    Best to change them out then, correct?

    Are these 1/2 watt metal film??

    Thanks
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi on Sat Jan 18, 2020 4:55 pm

    yes, 1/2w metal film
    avatar
    TN Allen

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    Post by TN Allen on Sun Jan 19, 2020 10:45 am

    Does this apply to the Mk3 PCB as well? If so, should I change the #30 & 32 150K resistors on the Version 11 board I have?
    corndog71
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    Post by corndog71 on Sun Jan 19, 2020 12:37 pm

    tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

    Does this also apply to M125's? What about using KT120's?
    Bob Latino
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    Post by Bob Latino on Sun Jan 19, 2020 12:47 pm

    corndog71 wrote:
    tubes4hifi wrote:also MOST KT88s will not hold bias well with R29-32 at 150K, all amps and kits shipped in the past year or two have been supplied with 100K resistors in those positions.

    Does this also apply to M125's?  What about using KT120's?

    VTA M-125's have always used 100K resistors in R30 and R32.

    Bob
    corndog71
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    Post by corndog71 on Sun Jan 19, 2020 4:16 pm

    Ah, yes. That’s what I have in my M125s.

    Looking over my VTA120 docs from 5 years ago I got 150k. Just ordered some 100k resistors.

    I thought I read a while ago that KT120s should use 50k.
    peterh
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    Post by peterh on Sun Jan 19, 2020 4:27 pm

    corndog71 wrote:Ah, yes.  That’s what I have in my M125s.  

    Looking over my VTA120 docs from 5 years ago I got 150k.  Just ordered some 100k resistors.

    I thought I read a while ago that KT120s should use 50k.
    That is correct, the documentation stated 51k


    http://www.tungsol.com/specs/kt120-tung-sol-specs-curves.pdf
    Bob Latino
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    Post by Bob Latino on Sun Jan 19, 2020 6:07 pm

    peterh wrote:
    corndog71 wrote:Ah, yes.  That’s what I have in my M125s.  

    Looking over my VTA120 docs from 5 years ago I got 150k.  Just ordered some 100k resistors.

    I thought I read a while ago that KT120s should use 50k.
    That is correct, the documentation stated 51k


    http://www.tungsol.com/specs/kt120-tung-sol-specs-curves.pdf

    Yes - Tung Sol says use 50K resistors there BUT > In the VTA ST-120 and the VTA M-125 monoblocks the KT120 output tubes are being used well below their maximum plate voltage and as such > 100K resistors work fine in R29 - R32 and allow the needed current to bias these tubes.

    Bob
    markeby
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    Post by markeby on Sun Feb 02, 2020 5:47 pm

    MechEngVic wrote:I would listen to Peterh on this one, his comments mirror my experience also. A tube can red-plate for several reasons, but once it does, it is damaged. It will more easily short and arc in the future even though it seems to be working fine at the moment. It will also start taking other components out with it if you leave it in. Once you replace them, I would re-do my voltage checks. Hopefully, no resistors or capacitors are damaged. Three years is a good amount of time for a set of tubes.

    Ditto for me. If I have a power tube red-plate, I replace them all, toss the pair of the one that red-plated, and keep the other pair as an emergency set. Also, I always replace my rectifier tube as, it is likely damaged and they aren't that expensive. I have had spectacular reliability with the Telefunken GZ34-TK rectifiers with my KT88 power tubes

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