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    VTA ST - 120 Power fuse question

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    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    VTA ST - 120 Power fuse question

    Post by Analog Man on Sun Sep 22, 2013 3:03 pm

    I notice that the VTA ST - 120 power transformer is very stable but runs a little warm. This may be because I am using KT - 120 output tubes that require more current. My question is, I blow a 5a slo - blo after running the amp for 90 min. in triode mode. Why is this? Should I try a 6a due to the KT - 120 current draw? Or do I have a different problem?Shocked 

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST - 120 Power fuse question

    Post by Bob Latino on Sun Sep 22, 2013 5:09 pm

    The 5 amp slo-blo fuse should be fine with either KT88 or KT120 output tubes in either pentode ultralinear or triode mode. The power transformer will get warm ... That is normal .. Sometimes a power spike on your AC line or a transient issue inside a power tube can take out the 5 amp fuse. If the amp blows a fuse again or repeatedly after being on an hour or two look for an issue in the build of the amp or try different output tubes.

    As an aside > before I recommended the use of KT120 output tubes in the VTA ST-120 I ran them in my own amp for 5 1/2 hours one day, playing music at fairly loud levels with each KT120 biased @ .750 VDC (much higher than the recommended value) The amp did get warmer than normal but never blew a fuse.

    Bob

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: VTA ST - 120 Power fuse question

    Post by Analog Man on Mon Sep 23, 2013 7:41 pm

    I found the problem. One of the KT - 120's failed. I ran the amp for the first time on this past Sunday At 8:30 am and at 2:30 pm the fuse blew in the amp. I replaced the fuse and did all of the voltage measurments. I found that the bias voltage on one of the KT - 120 output tubes fell to .207 volts. I switched the tubes around and got the same readings. This tells me that a short occurred. I bought these tubes from Tube Depot and according to their web site, this should be under warranty.
    Has anybody out there had to deal with Tube Depot on a tube warranty issue? If so, please let me know how it went. Thanks!

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: VTA ST - 120 Power fuse question

    Post by Analog Man on Tue Sep 24, 2013 4:11 pm

    The guys at Tube Depot are great. There sending me out a new tube no problem, no hassle, no need to send the bad one back to them. WOW!

    Go Tube Depot!
    http://www.tubedepot.com/?gclid=CKnQvOHr5LkCFY3m7AodVgIA9g

    ixe13

    Posts : 18
    Join date : 2012-07-11

    Re: VTA ST - 120 Power fuse question

    Post by ixe13 on Wed Sep 25, 2013 10:55 am

    Analog Man wrote:The guys at Tube Depot are great. There sending me out a new tube no problem, no hassle, no need to send the bad one back to them.   WOW!

    Go Tube Depot!
    http://www.tubedepot.com/?gclid=CKnQvOHr5LkCFY3m7AodVgIA9g
    Hi,

    I just saw your post. Otherwise,  I would have answered it before.

    I Agree with you, I buy most of my tubes from tubeDepot and I can say they're great. Last time I had a tube dying on me , (the filament went dead),  they replaced it, no fuss.
    I briefly explain the tests I did and they agreed to send another one right away.

    Very nice to deal with.

    And I can say the same for Jim Mchane.


    Yves

    jimmytamp

    Posts : 7
    Join date : 2013-06-24

    Re: VTA ST - 120 Power fuse question

    Post by jimmytamp on Sun Sep 29, 2013 3:12 am

    Hi,

    Few weeks ago I bought KT120s & some driver tubes from TubeDepot.

    I have the same problem with of the KT120, it sparked & made the rectifier (Hitachi) which also i got from them, sparked & made a loud "popped" noise.
    Immediately I switched off the amp and luckily the fuse is still in good condition.
    I changed back to the stock KT88 from Bob and did some driver tubes rolling which i got from other merchant with no problem at all till now.
    I contacted TubeDepot and they refunded me with no hassles after they received back the tubes. The drivers tubes from them are just fine so far.

    I think i just got one of the tube from the bad batch.

    Cheers,
    Jimmy

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