Fuse - ST120 Fast Slow..

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=""/>

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

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?

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

5amp SLO-BLO fuse...

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.

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

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

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

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!

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 !!

I have a custom built power controller to sequence the power amp and preamp/sources so that the speakers don't get popped on power down but this also prevents power from being turned on for 4 minutes after you turn it off.