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    Possible Blown KT120??

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    vego99

    Posts : 2
    Join date : 2010-12-22

    Possible Blown KT120??

    Post by vego99 on Fri Jan 30, 2015 6:47 pm

    Hello everyone,

    About a few weeks ago I installed a new set of Tung Sol KT-120's in my Latino ST-120 amp and things have been nothing but perfect.

    Tonight, I fired up the my stereo and I started hearing a flutter coming from my right channel.
    As soon as this started happening, I looked at the amp and i noticed one of the KT120s started emitting a blue plasma color which would appear and disappear.

    I quickly shut down the amp pulled the tube ( very hot compared to the other three) plugged back in thinking it may be a loose socket. Then reinstalled and tried again. No luck. I then removed the Weber Copper cap and installed a good GZ34 Rectifier tube to see if the weber went bad. No such luck. The tube started doing the same thing.

    Can i assume I just experienced my first power tube failure? Is this normal for a new tube with less than 20 hours on it?

    Typically will tube suppliers warranty tubes? I bough them from one of the common online stores.

    Thanks for the input.

    Erik

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Possible Blown KT120??

    Post by Jim McShane on Fri Jan 30, 2015 8:18 pm

    Erik,

    The pulsing blue is often the sign of a tube oscillating. You can verify that by swapping the suspect tube with one of the other 3 in the amp and seeing if the problem moves with the tube or stays with the socket. Do this check as quickly as you can then shut things down since oscillating is TOUGH on a tube. If the problem moves with the tube then the tube is the likely offender. If it doesn't move and a different tube acts up when installed in that socket, your amp is causing the issues. Get it repaired promptly and don't use it until it is fixed.

    No, it is not "normal" to have this occur, but it is not unheard of.

    The manufacturer warrants the KT-120s for 90 days, so if the tube is bad I would assume the vendor will stand behind it. If the tube is part of a matched set be sure to remind them of that so they can be sure the tube they send you matches your existing tubes properly. If they sell "factory matched" tubes it's a bit of a crapshoot since the "factory" matching is pretty lousy.

    Good luck! I hope it all works out for you.

    vego99

    Posts : 2
    Join date : 2010-12-22

    Re: Possible Blown KT120??

    Post by vego99 on Fri Jan 30, 2015 9:30 pm

    Jim,

    Thanks for the info.

    I have not tried swapping into another socket. I did however remove all the KT120s and installed my previous set of 6550s. With these installed I did not notice any oscillation.

    Can I install the suspect KT120 in a different socket with the 6550s installed?

    It would be nice to be certain that its a bad tube or something faulty with the amp.

    Thanks again,

    Erik

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Possible Blown KT120??

    Post by Jim McShane on Fri Jan 30, 2015 10:42 pm

    vego99 wrote:Jim,

    Thanks for the info.

    I have not tried swapping into another socket.  I did however remove all the KT120s and installed my previous set of 6550s.  With these installed I did not notice any oscillation.  

    Can I install the suspect KT120 in a different socket with the 6550s installed?

    It would be nice to be certain that its a bad tube or something faulty with the amp.

    Thanks again,

    Erik

    You can, but since you will have altered the conditions under which the "suspect" tube is operating you won't be able to be 100% sure of the results. It probably won't make any difference but I just can't say with 100% certainty.

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