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    ST-120 What Kind of Fuse Is This ?

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    EddieGnz1

    Posts : 23
    Join date : 2013-10-16

    ST-120 What Kind of Fuse Is This ?

    Post by EddieGnz1 on Tue Dec 31, 2013 2:44 am

    Which of the following three kinds of fuses do I need to use on the ST-120;

    1) fuse1

    2)  fuse2

    3)  which other kind do you recommend if the above two are not the right kind?
    ______________________________________________________________________

    I think the amp is fixed now, hopefully maybe anyway.  I have not yet heard any music out of it.  I unscrewed the driver board and snipped off several of the wires that might have been a little too long (not sure if that was it tho).  Then I turned the amp open side down and used cans of compressed air to blow it out of any possible bits of wire or anything in there.  I replaced the rectifier tube with a JJ GZ34.  So then I turned it on and everything was fine for 5 to 7 minutes as I read the instructions to bias the amp.  All eight tubes lit up perfectly and no noise, all seemed great.  Then I biased the first tube to 0.550 and then as I was turning up the bias on the second tube the fuse blew.  I think it might have blown because I needed an extra hand to hold the ground probe on the chassis and so I didn't get a good meter read on the bias and so i think i went too high.  So now I need to find a source for this fuse, which is definitely blown.

    Kind thanks,
    Eddie

    EddieGnz1

    Posts : 23
    Join date : 2013-10-16

    Re: ST-120 What Kind of Fuse Is This ?

    Post by EddieGnz1 on Tue Dec 31, 2013 2:57 am

    fuse3

    this one looks a little different and maybe a better match as far as looks more like the one that Bob shipped with the ST-120, just don't know if it should be a 5-20 or a 1 1/4 x 1/4. The glass measures 5 x20 but the overall is 1 1/4 x 1/4

    Jim McShane

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

    Re: ST-120 What Kind of Fuse Is This ?

    Post by Jim McShane on Tue Dec 31, 2013 9:47 am

    Measure the entire length of the old fuse end-to-end, it's likely 1/4 x 1 1/4. Get the Littelfuse 5 amp slow blow. It's a high quality product at 1/2 the price of the Cooper/Bussman.

    And I strongly suggest you lose the JJ 5AR4/GZ34. They are not very reliable. If the budget allows get a new production Tung-Sol 5AR4 or Genalex GZ34. Both will be far better in the application. I'd also suggest you do the diode mod that Bob recommends - just use UF4007 diodes instead of the noisy 1N4007s.

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 What Kind of Fuse Is This ?

    Post by sKiZo on Wed Jan 01, 2014 4:06 pm

    #2

    Amp calls for a 3AG fuse type, which describes the body style and size. An AGC also works ...

    PS ... make sure you get the correct rating for your amp and NOT slow blows or fast acting ... Good old fashioned "standard" is the way to go.

    CORRECTION > > The VTA amps do use slow blow fuses. My bad ...  Embarassed 

    Little trick for the ground probe ... Prop the chassis up a bit with a chunk of scrap wood or whatever that's non-conductive, and just slip the probe tip between that and the chassis. Voila ... you just grew an extra hand ... use it wisely.  clown


    Last edited by sKiZo on Wed Jan 01, 2014 5:59 pm; edited 1 time in total

    Jim McShane

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

    Re: ST-120 What Kind of Fuse Is This ?

    Post by Jim McShane on Wed Jan 01, 2014 4:22 pm

    sKiZo wrote:#2

    Amp calls for a 3AG fuse type, which describes the body style and size. An AGC also works ...

    PS ... make sure you get the correct rating for your amp and NOT slow blows or fast acting ... Good old fashioned "standard" is the way to go.

    Little trick for the ground probe ... Prop the chassis up a bit with a chunk of scrap wood or whatever that's non-conductive, and just slip the probe tip between that and the chassis. Voila ... you just grew an extra hand ... use it wisely.  clown 

    According to Bob Latino's post on 9/22 of 2013 "The 5 amp slo-blo fuse should be fine with either KT88 or KT120 output tubes in either pentode ultralinear or triode mode."

    So your advice to not use a slow blow fuse contradicts the amp's designer. Are you saying Bob was not correct with his recommendation?

    Bob Latino
    Admin

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

    Re: ST-120 What Kind of Fuse Is This ?

    Post by Bob Latino on Wed Jan 01, 2014 4:41 pm

    Jim McShane's post is correct. All the VTA amps are supposed to use SLO-BLO fuses. Tube amps have a power surge when they start up. A SLO-BLO fuse will withstand the power surge but will blow if the current exceeds the rated value for a period of time. Sometimes electric motors will also be fused with a slo-blo fuse since the current demands are higher than normal as the motor comes up to speed. The VTA ST-70 and the original Dynaco ST-70 are supposed to use a 3 amp slo-blo fuse. The VTA ST-120 is supposed to use a 5 amp slo-blo fuse. Each M-125 monoblock amp is supposed to use a 5 amp slo-blo fuse.

    Bob

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 What Kind of Fuse Is This ?

    Post by sKiZo on Wed Jan 01, 2014 5:56 pm

    My bad ... never had the privilege of blowing a fuse in mine and didn't bother to check.  Embarassed 

    Not sure as I'd still need one, having an inrush limiter on mine?


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