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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


    Just had a 6550 Red Plate on a VTA-120

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    Hops

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    Post by Hops on Tue Aug 11, 2020 2:09 am

    Hello,
    I just had a tube Red Plate on my VTA-120. Music was replaced by a noise; looked and saw a Red Tube. I have a spare 6550 -- what checks should I perform for other damage, either before or after replacing the tube. Was using a Variac, I think the Variac fuse blew ( shows Variac at 0 volts). Killawatt shows 123V on another outlet. Rightside tube fathest from the transformers / closest to the bias test points.
    Mr C
    Mr C

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    Post by Mr C on Tue Aug 11, 2020 3:09 am

    When one of my 6550 did the same thing it turned out to be the 10ohm resistor off of pin 8 to ground.
    MechEngVic
    MechEngVic

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    Post by MechEngVic on Tue Aug 11, 2020 7:35 am

    How many hours on the tubes? On the amp?
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    Hops

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    Post by Hops on Tue Aug 11, 2020 11:51 am

    Tubes are the original to the Amp. First powered up in September '19. Blew a 10 ohm resistor in the spring -- Bob said to replace the tube with the blown 10 ohm resistor, but I hadn't marked the tube locations on the tubes when I took the tubes off the flip the amp over.... . I marked the locations on the tubes after that & bought a spare 6550. I suspect it is the same tube, and that the 10 ohm is probably fried ( I do have some spare 10 ohm resistors). I will search the forum for other red-plates and see what checks other people have done. Probably wait awhile --- I need to complete a ST-35 for a daughter, want to get that done first. Listen to the ST-35 while examining the 120...

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    Hops

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    Post by Hops on Sat Aug 15, 2020 2:22 am

    Decided to just check what would happen if I turned all the bias pots to zero bias and replace the tube.
    Everything biased nicely , and music is playing. I have an email out to Jim McShane asking for a matched quad of either Tung Sol or Sovtek 6550WE tubes. Not in a rush, will order when he responds. The Sovteks supplied with the VTA ST-120 Amp kit sounded amazing to me ( although I do not have other tubes or tube amps to compare it to. My first Tube Amp. ). Lot's of other things wrong with my stereo that can be fixed before I need to worry about different tubes ( speaker locations, replace cheap input switch box with a preamp, adjust subwoofer level and cutoff frequency ...).
    Bob Latino
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    Post by Bob Latino on Sat Aug 15, 2020 2:12 pm

    Hops wrote:The Sovteks supplied with the VTA ST-120 Amp kit sounded amazing to me

    The Sovtek 6550WE output tubes are a real bargain in the VTA ST-120 amp .. They sound great and they are noticeably less expensive than the Gold Lion KT88's or the Tung-Sol KT120's. The GL KT88 and the TS KT120 are considered premium tubes.

    As to the loss of one of the 10 ohm 2 watt resistors. When this happens almost all the time it is NOT the fault of the resistor itself. It is either a bad or worn tube that shorts internally (and draws too much current) or it could be loss of tube pin #5 contact with socket pin #5. Pin #5 on the socket provides the negative DC bias voltage that controls that output tube. If you lose control of the tube, the tube will draw too much current and will take out the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground.

    Bob

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    MarcVBelgium
    MarcVBelgium

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    Post by MarcVBelgium on Wed Sep 02, 2020 12:11 pm

    Been using TAD 6550's for many years until one blew a 10 Ohm resistor ..... a few days ago I put Electro Harmonix KT-90 in my ST-120.... sounds GREAT and even more powerful ;-) ;-) ;-)




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    rjpjnk

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    Post by rjpjnk on Thu Sep 03, 2020 6:34 am

    It sounds like those 10 Ohm 2 Watt resistors are working as fuses in this application and protecting the amp from further damage. Why not add a real fuse in the B+ supply? 2W in 10 Ohms is about 440ma, right? Not sure how much over current it is taking to light up these resistors. Maybe a 1 amp fuse?

    I'm guessing there's a real good reason why such a seemingly simple fix isn't done already that you guys will probably tell me about Wink
    peterh
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    Post by peterh on Thu Sep 03, 2020 7:36 am

    rjpjnk wrote:It sounds like those 10 Ohm 2 Watt resistors are working as fuses in this application and protecting the amp from further damage. Why not add a real fuse in the B+ supply? 2W in 10 Ohms is about 440ma, right? Not sure how much over current it is taking to light up these resistors. Maybe a 1 amp fuse?

    I'm guessing there's a real good reason why such a seemingly simple fix isn't done already that you guys will probably tell me about  Wink
    fuses will take longer time to burn off then a surface film resistor.

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