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    Weird Behavior ST-120

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    WntrMute2

    Posts : 101
    Join date : 2010-11-21

    Weird Behavior ST-120

    Post by WntrMute2 on Mon Mar 26, 2012 5:54 am

    I had a very strange experience this weekend with my ST-120. I was checking the bias and the rear left tube was running a bit high at 0.60mv. As I was trying to set the bias down a bit, the tube began glowing WHITE, almost flourescent, I shut things down for a few minutes. When I restarted the amp, the bias was way, way off, left tubes somewhere around 1.2mv. and the right ones down around 0.2. As I tried to bring them into line, the right front tube did the same glowy white thing. I shut things down again and fired off an e-mail to Bob (thanks for the response) who suggested a bad tube (new ones ordered).
    I re-read the manual about biasing and decided to switch tubes side to side but only put the left side ones in per the start-up instructions. I turned the pots all the way down and fired it up. Blam went the tube rectifier and took out the fuse. Next I placed the other side tubes in, these were the ones with the light show, installed my SS rectifier and tried one more time. No hysterics, bias around 0.2mv. on all tubes. I slowly tweaked that up a little on each pot until the bias was 0.5-0.51mv. and turned up the preamp. Humm, sounds came out, no weird crackling or flashy stuff in the tubes, Switched from talk radio to CD; sounded fine, tried a few LPs; OK here too. So I ran it all day and checked the bias a few times. A-OK. Someone explain this to me please. I have a set of KT-120s on the way from Jim McShane but they will take a week to get to me. The tubes in use are Sovtek KT-88s. Any help would be appreciated. I don't really trust things that fix themselves.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Weird Behavior ST-120

    Post by Bob Latino on Mon Mar 26, 2012 9:20 am

    Hi Dave,

    From reading your entire post, it is possible that you have either a bad solder connection on the left channel output tube sockets OR a bad "contact" issue where the tubes are not making full contact with all the pins. Let us know how you make out when you install the new tube set ?

    Bob

      Current date/time is Fri Dec 09, 2016 1:42 am