The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


5 posters

    Fuse - ST120 Fast Slow..

    penguinpages
    penguinpages


    Posts : 9
    Join date : 2021-04-26
    Age : 53
    Location : Atlanta

    Fuse  - ST120  Fast  Slow..  Empty Fuse - ST120 Fast Slow..

    Post by penguinpages Thu Dec 30, 2021 4:56 pm


    I was being obtuse... and long story short flipped power via an automation of 12v trigger system of my ST-120 about 4 times in 30 seconds... It popped fuse.

    I pulled it.

    Reading on it is F (fast) 5Amp 250v

    To me this is a fast blow 5amp 250v rated 6 x 30 mm glass fuse.


    But to double check.. Figured I would see if / what is for sale if I were to order part via website: https://www.dynakitparts.com/shop/fuses/

    All options on site .. are listed as :

    Max 3amp all Slow blow. 250v.

    So either I am reading this wrong... but seems to me it should always be fast blow fuse.

    I ordered these: https://www.amazon.com/gp/product/B0957XKR8Z/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1


    https://i.servimg.com/u/f48/20/33/91/19/st120-10.png" alt=""/>


    avatar
    New2Tubez


    Posts : 123
    Join date : 2018-03-20
    Location : NY

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by New2Tubez Thu Dec 30, 2021 5:06 pm

    IIRC, its a 5A slow-blow. Do a search on the forum, it's there.
    Hopefully the rectifier didn't get killed...
    penguinpages
    penguinpages


    Posts : 9
    Join date : 2021-04-26
    Age : 53
    Location : Atlanta

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by penguinpages Thu Dec 30, 2021 10:45 pm

    I got two 5amp slow blow fuse.. It blew another one after about 30 seconds after powering on. I think this is the symptom of damaged rectifier. There are "white"

    I have the Tung-Sol 5AR4



    Question:
    1) Replacement of Rectifier.. any other things to check?
    2) Is there a higher / better quality rectifier upgrade option?



    penguinpages
    penguinpages


    Posts : 9
    Join date : 2021-04-26
    Age : 53
    Location : Atlanta

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by penguinpages Thu Dec 30, 2021 11:13 pm

    PS: Just to post.. if I had RTFM... Step 1 of startup sequence:

    5amp SLO-BLO fuse...



    Fuse  - ST120  Fast  Slow..  Fusety10


    diamondblue911 likes this post

    avatar
    New2Tubez


    Posts : 123
    Join date : 2018-03-20
    Location : NY

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by New2Tubez Thu Dec 30, 2021 11:50 pm

    penguinpages wrote:I got two 5amp slow blow fuse..  It blew another one after about 30 seconds after powering on.  I think this is the symptom of damaged rectifier.  There are "white"

    I have the Tung-Sol  5AR4



    Question:
    1) Replacement of Rectifier.. any other things to check?
    2) Is there a higher / better quality rectifier upgrade option?




    The cause of the blown fuse is above my skill level. The (modern Russian) Tung Sol seems to hold up better than other modern tubes. That's what I used.

    #1 have you done the yellow sheet diode mod? It keeps power from going from the caps back into and blowing the rectifier in event of a quick restart. I'm sure someone else can better explain this. Do a search, it's there. Rapid restarts are a bad thing. Some ppl use a GFCI outlet that trips and can prevent the amp from restarting. These aren't supposed to be used w/ a variac- I learned most of this by searching for the answers. This amp's been made for years and there's a lot to be gained by doing a little homework. Sorry to beat a dead horse.
    #2 the older rectifiers are better regarded here but are expensive. Again, let the forum search tool be your friend.
    corndog71
    corndog71


    Posts : 822
    Join date : 2013-03-19
    Location : It can get windy here

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by corndog71 Fri Dec 31, 2021 1:33 am

    Sadly, the forum search function isn’t the greatest.

    Tube amps like these should never be cycled on and off repeatedly. I usually let mine sit minimum 15 minutes before turning it on again. Remember, it has a high voltage power supply. It’s very stressful on the rectifier. The yellow sheet mod is highly recommended as it will protect the tube rectifier. Your tube rectifier is likely damaged and needs to be replaced. It only takes one bad thing to go wrong to kill it. I went through a bunch of them before I learned about the yellow sheet mod. Haven’t blown a tube rectifier since.

    Dynaco style tube amps all use slow blow fuses. ST120 uses 5 amp slow blow. Don’t use anything else or you’re just asking for trouble. In some ways tube amps can be very robust devices and in other ways they can be fragile and require more care than solid state electronics. Expect to blow a tube from time to time. Sometimes it’s just a bad tube. Respect your amp and you might go years with no problems. But you have to treat them right.

    Always have speakers connected when powered on. When it starts distorting it’s likely a power tube that needs to be replaced. Toss the bad one (you can tell by the bias which will probably drift way up) keep the other one from that channel as a spare and buy a new pair of matched tubes.. if it’s distorting but bias is good and stable then it’s likely the small center tube going bad. Replace it. If your not sure what’s wrong with it come back here or shoot an email to Roy at info@tubes4hifi.com

    MechEngVic likes this post

    Brap
    Brap


    Posts : 171
    Join date : 2013-11-28
    Age : 66
    Location : Plainfield, illinois

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by Brap Mon Jan 03, 2022 3:49 pm

    Echo the last two statements -- do the yellow sheet diode mod.
    penguinpages
    penguinpages


    Posts : 9
    Join date : 2021-04-26
    Age : 53
    Location : Atlanta

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by penguinpages Mon Jan 03, 2022 4:28 pm

    Yellow page mode was done. But it can’t prevent stupid. Once I pulled and tested with my backup solid state rectifier. I knew for sure that was issue. New rectifier on order. Thx for responding
    Brap
    Brap


    Posts : 171
    Join date : 2013-11-28
    Age : 66
    Location : Plainfield, illinois

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by Brap Tue Jan 04, 2022 11:09 am

    Its amazing to me how often during my years of troubleshooting how the analysis brings out my stupidity! Welcome to the club of humanity. Glad electronics helps us with that!
    tubes4hifi
    tubes4hifi
    Admin


    Posts : 1729
    Join date : 2008-11-30

    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by tubes4hifi Fri Jan 07, 2022 1:47 am

    I wouldn't be surprised if this has happened to about half of everyone that has a tube amp.
    Blowing the fuse on a fast power cycle (ALWAYS wait at least 2-3 minutes after power down to power up again - how many dozens of posts have mentioned this??)
    is almost a sure signal and sign that you've also blown the rectifier tube. Do the yellow sheet mod and that cuts your chances of blowing the rectifier tube
    by about 70%. But making this mistake, you've bound to lose a rectifier tube at some point !!

    Sponsored content


    Fuse  - ST120  Fast  Slow..  Empty Re: Fuse - ST120 Fast Slow..

    Post by Sponsored content


      Current date/time is Thu Jan 20, 2022 9:52 am