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    ST-70: One tube's glow is intermittent

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    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    ST-70: One tube's glow is intermittent

    Post by Aljaheejus on Mon Feb 21, 2011 3:27 am

    One of my ST-70's four EL34 (output?) tubes was not glowing yesterday. I moved it around in the socket, and it lighted up, somewhat, but soon faded again. This behavior continued today after I put some de-oxit on and mechanically tightened the contacts. (I thought it was just a loose connection.) Same thing happens in other sockets - so I guess it is a problem in just this one tube.

    I am puzzled by the fact that I did not notice the sound change when the tube is not glowing.

    What are the options? Is replacing one tube all I should need to do? What advantage might a "matched quad" offer?

    The tubes are EL34WXT SOVTEK. I have the original driver board, and I'm going to stick with that for now.

    Recommendations to try another brand of output tubes for sonic change are welcome.

    Thanks in advance!

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST-70: One tube's glow is intermittent

    Post by GP49 on Mon Feb 21, 2011 3:36 am

    Heat the filament pins on the bottom of the tube socket with a hot soldering iron. That may melt enough solder to make a proper connection but I'd feed some more fresh solder into the pins once it flows.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70: One tube's glow is intermittent

    Post by Aljaheejus on Mon Feb 21, 2011 3:50 am

    Thank you. I can certainly give that a try. How can I determine which pins are for the heater - or should I just work all the pins?

    I've been running these tubes for about 2 years, so I hope they have some more life in them.

    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: ST-70: One tube's glow is intermittent

    Post by anbitet66 on Mon Feb 21, 2011 9:53 am

    Heat up pins 2 and 7. Hold the tube with the pins up. Have the key in the center facing you. The pins adjacent to it are 1 and 8, so heat the pins next to them. Sometimes the solder used to attach the wires to the pins loosens with age, then they may not conduct enough current. And with 1.5 amps flowing through them, you can see why the tube goes dark.

    Tony

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70: One tube's glow is intermittent

    Post by Aljaheejus on Mon Feb 21, 2011 10:36 am

    Tony, GP49,
    Thank you both for the suggestion and details - just what I needed. I'll have a go at it. Among the other three EL34s there was one that did seem dimmer than the other two. For good measure I'll reflow each of them, hopefully tonight. Thanks again for the prompt responses! - David

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70: One tube's glow is intermittent

    Post by Aljaheejus on Tue Feb 22, 2011 1:31 am

    This worked! I went ahead 'round all the pins on two of the EL34s. All four tubes appear steadily aglow to equivalent color/brightness as far as I can see. I reset bias adjustments to 1.5V and put on some music.

    I love it when it is something simple that can be remedied with tools and materials at hand. Thanks again to the responders and the forum host.

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