Latino ST-70 Fuse stuck - Help!

K

k1creeker

Active Member
Hi all,
My approximately 1 year old ST-70 started consuming rectifier tubes (2 in 2 weeks) and recently refused to power up at all. Thinking it could be a fuse, I tried to remove the old fuse form the rear of the unit but it won't budge. The plastic cap came off, but the fuse feels like it's welded in place.

Any thoughts on removal would be appreciated.
 
Rectifiers do not fail that fast if things are right. Usually they die from high current flow, guessing either bad filter cap or possibly a problem with the output tubes. Much too big of a first cap won't help it either.

Pull the bottom off the amp and see if you can push on the rear center terminal of the fuse holder. Its typically spring loaded and will push inward when the fuse is in. Pushing it back out will pop the fuse out if its stuck. Worst case, fuse holders are not that spendy.


unplugged of course
 
thanks for the replies guys. I'll look into a new fuse holder.

It's weird that all these issues are grouped together as it ran fine for about a year without so much as a hiccup.

Gold Lion GZ34 dies (had some use and wear but certainly not end of life)
Vintage Westinghouse 5U4G dies after a week (used, could have been near end of life)
Brand new 5AR4 from the Tube Store (lasted about 6 hours) Sounds cuts out while I'm listening. I turn off and remove the amp from my system. Replace rectifier, turn on unit and no power at all.
Now the stuck fuse issue.

I ordered new fuses and will look for a suitable fuse holder. I hope it's it's an easy swap.
 
tonyg88 said:
Seems to be a common problem on here with Latino st-70 eating rectifiers.
Click to expand...
Really, only if the amp is driven hard, a lot. Or there is something else wrong. A GZ-37 would be a better rectifier to use, but they aren't made anymore and old stock are $$. My ST70 has been running fine for 6 or 7 years now on the original JJ 5AR4.
 
k1creeker said:
Hi all,
My approximately 1 year old ST-70 started consuming rectifier tubes (2 in 2 weeks) and recently refused to power up at all. Thinking it could be a fuse, I tried to remove the old fuse form the rear of the unit but it won't budge. The plastic cap came off, but the fuse feels like it's welded in place.

Any thoughts on removal would be appreciated.
Click to expand...

How is the bias adjusted ?
If this is a dynaco ST70 the bias according to the manual should be adjusted to 1.56Volt. I suggest
you adjust it to 1.45Volt. Too high will kill the gz34, to low will make sound harsh. Another
reason for blowing gz34 is the big can-cap ( the electrolytic storage capacitor). Exact replacement
is available at dynakitparts.com
 
It killed a 5U4?

Those things are built like the proverbial brick you-know-what-house. It's EXTREMELY hard to make one of those not work. I've seen 5U4s flash blue, several times, and still work afterward. They are some of the toughest rectifiers I know of.

Something is causing way too much current somehow, if one of those got killed...

Regards,
Gordon.
 
There's a problem with that amp. I wouldn't replace anything until I got it on the bench for a look. If you want new rectifiers buy the Sovtek. They are every bit as good as any of the Russian products. I've switched back and forth between Sovtek and vintage Amperex and the voltage drop is really close.
As others have mentioned, it sounds like you have abnormally high current draw. It also sounds like you have a "hot " connection in the primary fuse. I've seen this. If the end cap is wrong or the fuse is a bit short it will arc under load as it tries to maintain the current flow. Now I could be all wet, but it wouldn't hurt to look under the hood and replace the fuse holder.
 
You don't want to run a 5U4GB if it's has the original power transformer. It puts more current demand on the 5 volt supply and this becomes heat. The 5AR4 gives you a controlled start up and uses less current. It should run that amp fine.
 
I had a ST-120 and had bad luck with GZ34s and KT88s. I was told that it could be ratty line voltage, so I plugged in my DVM and saw that it was well over 120V. Not good. I even made the switch to KT120s, and had one of those blow too.
 
Thanks for all for your feedback. I really appreciate it!
I've ordered 2 new 5AR4 tubes, a half dozen 3A slo-blo fuses, and a new fuse holder from Dynakit parts.

To answer the bias questions, I'm running KT-66 (Gold Lion) tubes and bias them at .450V per the manual. I did need to turn the bias pots up when I inserted the 5U4G Westinghouse (I expected this), and turned the pots back down when I installed the last 5AR4.

I'm hoping that when I open it up to replace the fuse holder, I'll have a better idea of the problem. I tried to remove the fuse again last night with pliers and it wouldn't budge (in, out or sideways).

Thanks again, and I'll keep you posted on the progress..
 
k1creeker said:
Thanks for all for your feedback. I really appreciate it!
I've ordered 2 new 5AR4 tubes, a half dozen 3A slo-blo fuses, and a new fuse holder from Dynakit parts.

To answer the bias questions, I'm running KT-66 (Gold Lion) tubes and bias them at .450V per the manual. I did need to turn the bias pots up when I inserted the 5U4G Westinghouse (I expected this), and turned the pots back down when I installed the last 5AR4.

I'm hoping that when I open it up to replace the fuse holder, I'll have a better idea of the problem. I tried to remove the fuse again last night with pliers and it wouldn't budge (in, out or sideways).

Thanks again, and I'll keep you posted on the progress..
Click to expand...
I'm hoping that is a typo, but I think the bias should be .045V or 45mV. If you were to bias them as .450V, they would be pulling so much current there is no doubt you would fry the 5AR4 rectifier tube.

Also, I'm assuming 45mV = 45mA of current through the tube. i.e. I'm not sure how you are getting that mV reading, across which resistor. Because with a vintage ST-70, 1.56V = 50mA per tube.
 
I'd figure 0.45v is correct Most of these things use a 10 ohm resistor just because of meter resolution. 0.45 volts is 0.045 amps, or 45ma, which sounds about right.

still not convinced you don't have deeper problems here, amps do not eat rectifier tubes like candy for no reason. Yes the ST-70 is hard on the rectifier tube by design, but its not that hard on it.
 
Have you contacted Latino for assistance. I have always heard he was responsive to requests for help.

That is one of the first things I would do even if I was not the original owner.

Good luck.
 
As gadget just said, LOWER ALL the bias pots and adjust UP TO the level recommended for your output tube. Somewhere in the 0.45-0.5v range. Any higher and yes, you will be eating rectifier tubes.

As for the fuse holder, the plastic cap came off the metal rimmed cup holding the fuse in. Push IN the metal cup and gently turn to see if you can unhook the two metal tabs holding the fuse cap in place.
 
For the VTA ST-70, Bob recommends .45VDC when running KT-66 tubes.
I have reached out to Bob for assistance as well and he was quick to respond, but I'm on hold until the replacement fuse holder arrives since I can't power it up at all. The amp was built by him personally, so it's not like this unit has a sketchy past. I'm sure it'll work out.

I realize you need to push in on the fuse holder and turn, but mine is locked up. It wouldn't push in, or turn. The best I could do was remove the outer cap to get a look at one end of the fuse. Pulling with pliers, I thought for sure I'd break the fuse without releasing it. I'll try accessing the tabs, but am probably just going to begin removal of the stock fuse holder while I wait on the replacement.

Thanks for the support!
 
My 1962 ST-70 still has a Dynaco-labeled 5AR4, so I know they can last a good while. I had a cheapo one while I was working on the amp... good thing, too, as I power cycled it too quickly and had a nice flash from the rectifier and blew the fuse. Hopefully yours is something simple.

For those who know, how long does an ST-70 need to sit, after powering it off, before it's safe to turn it back on?
 
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