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ST-70 Custom Died - Where To Start?

I would just check the fuse first. Don't replace it, just check it.

If there is a fuse and it is open, chances are good that another one might cure it.
If the fuse is blackened, chances are that there is a short somewhere and another fuse will just blow until you find the short - rectifier / tube / capacitor / transformer.
 
FlaCharlie said:
If the fuse is blown and it's a normal fast blow fuse I'd try replacing it with a slow blow. But since it died while in use, not on startup, I'm guessing something else failed. Surface mount solid state rectifiers?? Hmmm.
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I was thinking that too, I recently had a silicon rectifier short and take out an amp in a very similar fashion to the way this one died. Found it by using a variac to troubleshoot.
 
Anyway don't consider having a tube amp a negative as it's much easier to troubleshoot and repair as you will see!
SS can have many parts die due to more delicate nature and chain reaction affects.
 
If nobody else pointed it out. Dead things are generally easier to troubleshoot and fix than intermittent or weird things. I suspect fuse like many have stated . The rectifier would be my second guess . I'd also like to repeat that your issue isn't because it's a tube amp. There are ST 70s that are untouched and still working today 60 years later (admittedly on borrowed time). My point is its a solid dependable design.
 
Sorry to hear of your troubles. Since this is a custom design, if you can get them, some pics of the back panel, top and internal bottom layout will help all of us understand and help you figure out where things are on the amp chassis, and where to start looking or checking. Importantly, if you can find the fuse, you'll need some magnifiers to see the markings, they're quite small and with very little contrast, hard to read for anyone's eyes.
 
nj pheonix said:
If nobody else pointed it out. Dead things are generally easier to troubleshoot and fix than intermittent or weird things.
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Oh man, ABSOLUTELY. At least you have a consistent subject to test. That makes things pretty straight-forward.

It's those "it doesn't sound right anymore" or "it goes on and off by itself" that can drive you batty...

It's not impossible that a fuse could give up while its running- especially if it's a fast-blow. Usually, amps need slow-blow fuses, just due to the turn-on transient current to charge up the power supply, each time the amp is turned on. A fast-blow that's seen a lot of turn-on "hits" could just fail, anytime.

If the fuse is blown, and it's a fast-blow fuse, I'd put in a slow-blow fuse, of the value that's recommended in a stock ST70. If THAT blows, then do NOT put another one in- it will then be time to find out what's causing the excessive current draw (short, bias problem, etc).

However- like was said above- if the fuse was flatly "nuked" inside (it looks like the molten metal sprayed onto and covered the inside of the fuse with black or silver haze), then it would be a good idea to check it over completely, before putting another fuse in. That sort of fuse failure means BIG TIME overload (dead short, etc).

Regards,
Gordon.
 
I started reading that schematic - holy cow it's amazing that amp worked as long as it did.

Want some good tube advice - take out that driver board, SS rectification and any other modification and just bring it back to near stock. Get yourself a decent "working" driver board, proper caps, parts from tubes4hifi.

Just my two cents but I've found over the years the further away you get from the original plan and the more complicated you make an amp, the more you'll be repairing it than listening to it.

Those plans are for tinkerers that aren't using it as they're main setup. Again, just my two cents.
 
Step One accomplished - amp is now on dining table where I can handle it. I can pick it up, seated, but have fallen a couple times in the last two months while using a walker, LOL - carrying a heavy amp across the room isn't in the cards. After ablutions and breakfast, will check again for a fuse - I didn't realize one might be included as part of the power cord inlet - thanks to gadget73 for pointing out that possibility, and to all, as I'll probably still need help and supervision - not counting on being lucky enough that a fuse alone will solve the issue, just hoping.
 
I don't see the point of replacing that circuit PCB with a tubes4hifi version as it's very similar in design with the same Mullard
circuit and Solid State CCS added to the tail end.

As for the PS that amp is still using the rectifier tube and just has the " diode mod" that's even on the tubes4hifi site.
 
Okay, yes, there was a cover with a pull-out fuse-holder containing a blown GFE 250V 5A Fuse, which is a fast-blow type. Of course I have none on hand, but found a 5-pack on Amazon that will arrive Friday. I'd never have even seen the holder - even after you guys told where me to look - if my wife hadn't surprised me with an LED-lit magnifying glass three days ago, as an early stocking stuffer. Should I also acquire the slo-blo type? Any consensus on that? I'm always hesitant to make such changes, and Amazon didn't offer one Prime eligible, ie, with the free two-day shipping, so I ordered the fast-blow.

I realize that there must've been a reason why the fuse blew. I noticed that it takes a good bit of pressure to snap the IEC cord fully seated, and it sits in a spot I can't access, so it was Christine who connected it. Now she recalls a few days ago the cat brushing up against that cable and the amp shutting down, but coming back on with the switch flipped off and back on. Now she tells me. That also seems to indicate the possibility that the cord wasn't plugged in fully. So I'm hopeful that a fuse replacement and ensuring the plug snaps in will get function restored, but we shall see Friday.
 
musichal said:
Okay, yes, there was a cover with a pull-out fuse-holder containing a blown GFE 250V 5A Fuse, which is a fast-blow type. Of course I have none on hand, but found a 5-pack on Amazon that will arrive Friday.
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Surely there is a store in Norman that would have fuses? Heck, even in our small town of 3200 people I can find 5 x 20 fuses. The Fleet/Farm store of all places, LOL.
 
I usually use slow-blow fuses on any tube amp that doesn't have a thermistor to soft-start it. The inrush current tends to pop the fast-blow types.

5 amps seems a lot to me for an ST70. My fairly comparable Pilot amplifier runs I believe a 2.5 or 3 amp fuse.
 
5 amps does seem like a high value for a st70 amp. If that fuse blew then that would indicate a large surge.A quick cycling of the AC can blow the rectifier tube which if it did might take out the main fuse. Still you should check if the 5A is the correct value for your amp.
 
maxhifi said:
does the blown fuse look destroyed inside, all blackened and vaporized fuse element, or does it just look like a burned out light bulb?
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No, the glass is clean and clear as a bell. The filament is open at the midpoint, with tags from either end about equal in length - as clean a failure as you are likely to see. Or, hopefully, a success, as in nothing else affected.

I'll try to contact rustnrot and ask if that is the intended fuse value - I guess the first owner could have replaced it with what he had on hand, if he had one blow.

And, yes, I'm sure I could find a fuse in town if I still drove a car and got around okay, or wait until Saturday when Christine is off work - she works hard at home and at work, and I already ask a lot from her. Amazon Prime is often my fastest way to get things I need without scheduling others to run errands; a godsend, actually, for things I once took for granted. Besides, with a spare Emotiva amp in service, I'm not without; I can wait two days. With fingers crossed.

petercapo said:
I just knew a cat had something to do with this!
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Jumper denies all, and resents being called a cat. He says he is a person, too.
 
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