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    Burnt tubes- questions

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    fredg

    Posts : 4
    Join date : 2010-10-01

    Burnt tubes- questions

    Post by fredg on Sun Feb 06, 2011 11:16 pm

    On start up of my ST-120 today I got a little popping noise through my right speaker. One of the EH EL-34's was flashing red/orange then settled down and played. At first it seemed like both of the El 34's on that side were not glowing quite as red at the tip as the other side then they became equally bright. I checked the bias and it was right where I had set it weeks ago.

    I swapped the front and back tubes on that side and the next time I started it up the same thing happened. After playing for a while the sound on the right faded out and the entire interior of one tube was glowing cherry red.

    I'm new to tubes. What is going on?

    I realize that EL34's will have a foreshortened life in a ST 120. I only run the amp in triode.

    Please advise. Thank you,

    Fred

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Burnt tubes- questions

    Post by Bob Latino on Mon Feb 07, 2011 6:13 am

    Hi Fred,

    Just because you run the amp in triode and the output power is lower doesn't mean that the tube is not looking at the same high voltage as it would be in pentode ultralinear mode.

    I don't recommend EL34 tubes for use in the ST-120. Also not recommended in the ST-120 are 6L6, KT66 and KT77 tubes. The only tubes I recommend for use in the ST-120 are KT88, 6550 and the Tung-Sol KT120.

    That tube that turned cherry red on the plates should be replaced. Continuing to use that tube in your ST-120 could lead to something more serious like taking out the output transformer on that side.

    Bob

    fredg

    Posts : 4
    Join date : 2010-10-01

    Re: Burnt tubes- questions

    Post by fredg on Mon Feb 07, 2011 9:13 am

    To make sure it was just the tube(s) can I pull the tubes from the good right side place them in the left leaving the right tubeless while I test?

    Will KT 90's work in a ST 120?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Burnt tubes- questions

    Post by Bob Latino on Mon Feb 07, 2011 9:18 am

    Fred,

    Yes - you can run just one channel at a time on the ST-120. You will have to readjust the bias though. When you pull the output tubes on one side the other remaining tubes will pull too much current with the bias setting for 4 output tubes being used.

    You can use KT90 tubes in there - bias them the same as a KT88.

    Bob

    fredg

    Posts : 4
    Join date : 2010-10-01

    Re: Burnt tubes- questions

    Post by fredg on Mon Feb 07, 2011 9:01 pm

    It turns out that the fuse blew so I opened the amp up to check for anything unusual. Nothing burnt, loose or out of place.
    I will replace the fuse and proceed. Yes?

    Fred

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Burnt tubes- questions

    Post by Bob Latino on Mon Feb 07, 2011 9:02 pm

    Yes

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